14.1

14

The Chemistry ofAlkynes

An alkyne is a hydrocarbon containing a carbon-carbon triple bond; the simplest member ofthis family is acetylene, H-C-C-H. The chemistry of the carbon-carbon triple bond issimilar in many respects to that of the carbon--carbon double bond; indeed, alkynes andalkenes undergo many of the same addition reactions. Alkynes also have some unique chem-istry, most of it associated with the bond between hydrogen and the triply bonded carbon, the:C-H bond.

NOMENCTATURE OF ATKYNESIn common nomenclature, simple alkynes are named as derivatives of the parent compoundacetylene:

H3C-C:C-Hmethytacetylene

HrC-C:C-CH:dimethylacetylene

CH3CH2-C:C-CH:ethylmethylacety-lene

Certain compounds are named as derivatives of the propargyl group, HC:C-CH'-, inthe common system. The propargyl group is the triple-bond analog of the allyl group.

HC Q-CH,-Cl H,C-CH-CH,-Cl

644

propargyl chloride allyl chloride

14.1 NOMENCLATURE OF ALKYNES 645

We might expect the substitutive nomenclature of alkynes to be much like that of alkenes,and it is. The suffix ane in the name of the corresponding alkane is replaced by the suffixyne, and the triple bond is given the lowest possible number.

HgC-C-Q-Hpropyne

cH3cH2cH2cH2-Q-C-CH:2-heptyne

HrC-CH2-Q-C-Hl-butfne

HEC-CH-C-Q-CHr"lI

CH:4-methyl-2-pentyne

HC-Q-CH z-CHz-C-Q-CH:1,S-heptadiyne

HC-C-CH2-2-propynyl group

Substituent groups that contain a triple bond (called alkynyl grcups) are named by replac-ing the final e in the name of the corresponding alkyne with the suffix yl. (This is exactly anal-ogous to the nomenclature of substituent groups containing double bonds; see Sec. 4.2A.) Thealkynyl group is numbered from its point of attachment to the main chain:

HC-C-ethynyl group(ethynf + yl)

t2cH2c-cH3 - (2- propyny')cyclohexanolf\I \position of triple bond within the substituentI

position ofthe 2-propynyl group on the ring

As with alkenes, groups that can be cited as principal groups, such as the -OH group inthe following example (as well as in the previous one), are given numerical precedence overthe triple bond. (See Appendix I for a summary of nomenclature rules.)

OH

HC:C-CHz-CH-CH35 4 ' 2\ r

,{-pentyn-2-ol \ OH group receivesnumerical priority

When a molecule contains both double and triple bonds, the bond that has the lower num-ber at first point of difference receives numerical precedence. However, if this rule is ambigu-ous, a double bond receives numerical precedence over a triple bond.

646 CHAPTER 14 O THE CHEMISTRY OF ALKYNES

ttl45HC-C-CH:CHCHT

I -pentyn-3-ene

j-l rllcH3c -c-cH-cHr

1-penten-3-yne

tli45HzC:CHCH2C-CH

1-penten-4-yne

precedence is given to the double bondwhen numbering is ambiguous

14.1 Draw a lrwis structure for each of the following alkynes.(a) isopropylacetylene (b) cyclononyne (c) 4-methyl-1-p€ntyne(d) l-ethynylcyclohexanol (e) 2-butoxy-3-heptyne (f) l,3-hexadiyne

14.2 hovide the substitutive name for each of the following compounds. Also provide cofllmonnames for (a) and (b).(a) cHrcHrcHrcHrc-cH (b) cH3cHrcHrcHrc-ccH2cH2cH2cH3

precedence is given to the bond that haslower number at first point of difference

(c) OHIH:C-A-Q-C-CH:"t

CH:

(d) HC- CCHCH2CHzCHTl"r.\ iC:C/\H CH2OCH3

(e) oHIHC-C-CH- CH:CH,

14.2 STRUCTURE AND BONDING IN ATKYNESBecause each carbon of acetylene is connected to two groups-a hydrogen and another car-bon-the H-C:C bond angle in acetylene is 180o (Sec. 1.3B); thus, the acetylene moleculeis linear.

il.ro4 iH\cT9-\tV i;A,

The C:C bond, with a bond length of t.ZO A, is shorter than the C:C and C-C bonds,which have bond lengths of 1.33 A and 1.54 A, respectively.

Because of the 180" bond angles at the carbon--carbon triple bond, cis-trans isomerismcannot occur in alkynes. Thus, although 2-butene exists as cis and trans stereoisomers, 2-bu-tyne does not. Another consequence of this linear geometry is that cycloalkynes smaller thancyclooctyne cannot be isolated under ordinary conditions (see Problem 14.3).

The hybrid orbital model for bonding provides a useful description ofbonding in alkynes.We learned in Secs. 1.9B and 4.1A that carbon hybridization and geometry are correlated:tetrahedral carbon is sp3-hybridized, and trigonal planar carbon is sp2-hybridized. The lineargeometry found in alkynes is characterized by a third type of carbon hybridization, called sphybridiTation Imagine that the 2s orbital and one 2p orbital (say, the 2p, orbital) on carbonmix to form two new hybrid orbitals. Because these two new orbitals are each one part s andone partp, they are called sp hybrid orbitqls. Two of the 2p orbitals (2prand2pr) are not in-cluded in the hybridization.

14.2 STRUCTURE AND BONDING IN ALKYNES 647

nn.rftcctrrr,l br. hvlt rid i zirti on

2p)n-f--r

)6 )6 t :*t '-t',

^, \ ^ f I hl"hricl/(spl --f- j.,ibitul,2s -f$-

ls

atomic carbon:(1s)?(2s)t(2p*)(2py)

rrrraffcctcd L-r'hi'Lrn riiz;rtion r- tlls Tf-

carbon in acetylene

An sp hybrid orbital, then, is an orbital derived from the mixing of one s orbital and a p or-bital of the same principal quantum number.

An sp orbital has much the same shape as an sp2 or sp3 orbital (Fig. 14.1; compare withFigs. 1.16a and4.4a). However, electrons in an,1p hybrid orbital are, on the average, somewhatcloser to the carbon nucleus than they are in sp2 or sp3 hybrid orbitals. In other words, qp or-bitals are more compact than sp2 or qp3 hybrid orbitals. The reason is that an sp orbital con-tains a greater fraction ofs character than an sp2 or ansp3 orbital, and 2s electrons are, on theaverage, closer to the nucleus than 2p electrons. An ,qp-hybridized carbon atom, shown in Fig.14.1c, has two sp orbitals at a relative orientation of 180". The two remaining whybidized2porbitals lie along axes that are at right angles both to each other and to the sp orbitals.

The o bonds in acetylene result from the combination of two sp-hybridized carbon atomsand two hydrogen atoms (Fig. 14.2, p. 648). One bond between the carbon atoms is a o bondresulting from the overlap oftwo sp hybrid orbitals, each containing one electron. This bondis an qp-^1p o bond. The remaining ,sp orbital on each carbon overlaps with a hydrogen ls or-bital to form a carbon-hydrogen obond. These bonds are sp-1s o bonds. Because electrondensity in an sp hybrid orbital is closer to the nucleus than electron density in other hybrid or-bitals, the C-H bond in acetylene is shorter (1.06 Al than the C-H bonds in ethylene (1.08A) anA ethane (l.ll A). Table 5.3 (p.213) shows that the C-H bond in acerylene, with abond dissociation energy of 558 kJ mol-r (133 kcal mol-r), is also stronger than the C-Hbonds of ethylene (463 kJ mol-r, I I I kcal mol-r; or ethane (423kl mol-r. l0l kcal mol-r).

2p orbitals

/peak

sp orbitalsPo

,/

Figure 14.1 (a) A perspective representation of an sp hybrid orbital. (b) A more common representation of ansp hybrid orbital used in drawings. (c) The two sp hybrid orbitals shown together.The "leftover" (unhybridized) 2porbitals are shown with dashed lines. Notice that the ip hybrid orbitals are oriented at 180'.The blue and greencolors represent wave peaks and wave troughs.

rbiralwave trough---_\

nodalsurface \_/

(c)(b)(a)

548 CHAPTER 14 . THE CHEMISTRY OF ALKYNES

2p orbitals

sp-ls tr bond

sp-sp a bond

Fltwe 14.2 The o-bond framework of acetylene (shown in blue). Overlap of carbon sp hybrid orbitals gives thecarbon-carbon o bond, and the overlap of carbon sp hybrid orbitals with hydrogen ls orbitals gives thecarbon-hydrogen o bonds.Two 2p orbitals on each carbon, shown as dashed lines, do not participate in o bond-ing. (5ee Fig. 1a3.)

This bond-strength effect occurs because the C-H bond in acetylene contains a greater per-centage of the lower-energy 2s orbital than the bonds derived from sp: or sp3 hybrid orbitals.which, in contrast, contain progressively more high-energy 2p character. Notice that the lineargeometry of acetylene results from the 180o orientation of the sp orbitals on each carbon.Again: hybridization and geometry are correlated.

The leftover 2p orbitals on each carbon overlap to form z'bonds. Because each carbon ofacetylene has two 2p orbitals, two rbonds are formed. Like the 2p orbitals from which theyare formed, they are mutually perpendicular. The two bonding z'molecular orbitals that resultfrom this overlap are shown in Fig. 14.3. Notice that the acetylene molecule is literally sur-rounded by z'electrons. The total electron density from all of the z'electrons taken togetherforms a cylinder, or barrel, about the axis of the molecule (Fig. 14.3c). This cylinder of z-elec-tron density is particularly evident in the electron potential map (EPM) of acetylene. Comparethis with the EPM of ethylene, which has a'-electron density above and below the plane of themolecule.

ring of rr-electron densi6.

zr-electron clensityilbove ancl belou'the plane of the rnolecLlle

EPM of acetylene

The following heats of formation

EPM of ethylene

show that alkvnes are less stable than isomeric dienes:

HzC-CH-CH ?- CH:CHzH-Q-C-CH2CHzCH:l-pentyne

+ 144 kJ mol-t(34.5 kcal mol-l)

H:C-C-Q-CHzCH:2-pentyne

+ I29 kl mol-t(30.S kcal mol-l )

1,4-pentadiene+ 106 kJ mol-l

(25.4 kcal mol-t )

2p orbitals

/

AHT

14,3 PHYSICAL PROPERTIES OF ALKYNES 649

Figure14.3 Thetwobondingz'molecularorbitalsinacetylene.(a) Aperspectiveview.(b) An"end-on"viewasindicated by the eyeball in (a). The blue and green colors in (a) and (b) represent wave peaks and wave troughs.(c) The total r-electron density in acetylene. Acetylene is completely surrou nded by zr electrons.

+

(c)(b)(a)

In other words, the sp hybridization state is inherently less stable than the sp' hybridizationstate, other things being equal. These heats of formation also show that a triple bond, like adouble bond, is more stable in the interior of a carbon chain than at the end.

@l4.3(a)Attempttobuildamodelofcyclohexyne.Explainwhythiscompoundisunstable.(b) Build a model of cyclodecyne. Compare its subility qualitatively to that of cyclohexynrc;

explain your answer.

14.3 PHYSICAL PROPERTIES OF ALKYNES

A. Boiling Points and solubilitiesThe boiling points of most alkynes are notalkanes:

HC-C(CHr):CH:1-hexfne

boiling point: 71.3 "Cdensity: 0.7155 g ml-t

very different from those of analogous alkenes and

HzC-CH(CHz):CH:1-hexene63.4 "C

0.6731 g ml-l

H:C-CHz(CHz):CH:hexane68.7 "C

0.6603 I ml-t

Like alkanes and alkenes. alkvnes have much lower densities than water and are also insolu-ble in water.

B. lR spectroseopy of AlkynesMany alkynes have a C:C stretching absorption in the 2100-2200 cm-r region of the infraredspectrum. This absorption is clearly evident, for exapple, at2l20 cm-t in the IR spectrum of l-octyne (Fig. 14.4, p. 650). However, this absorption is very weak or absent in the IR spectra ofmany symmetrical, or nearly symmetrical, alkynes because of the dipole moment effect (Sec.12.38). For example, 4-octyne has no C:C stretching absorption at all.

650 CHAPTER 14 . THE CHEMISTRY OF ALKYNES

2.6 2.8 3 3.5 4 4.5wavelength, micrometers5 5.5 6 7 8 910 13 14 15 16

100

3800 3400 3000 2600 2200 2000 1800 1600 1400 1200 1000 800 600wavenumber, cm-l

Figule 14.4 The lR spectrum of l-octyne.The two key absorptions indicated are absent in the spectrum of+octyne.

The C:C sftetching absorption (2120 cm-') lies at considerably higher frequency thanthe C:C stretching frequency (I&C-1,675 cm-r). This is a clear manifestation of the bond-strength effect on absorption frequency. (See Sec. 12.34 and Srudy Problem 12.1, p. 5a6.)

A very useful absorption of l-alkynes is the :C-H stretching absorption, which occursat about 3300 cm-I. This strong, sharp absorption, very prominent in the spectrum of l-octyne(Fig.14.4), is well separated from other C-H stretching absorptions. Because alkynes otherthan l-alkynes lack the unique :C-H bond, they do not show this absorption.

C. NMR Spectroscopy of AllqnesPnoton NMR Spectloscopy Compare the typical chemical shifts observed in the protonNMR spectra of alkynes with the analogous shifts for alkenes:

q.)

H80(\t

H60(sL{

q)(Jb20Or

- C-Q-H acefylenic protons6 1.7-2.5

I

-C -f, -q - H propargylic protons| 6 r.8-2.2

-H

H\/f-fU-U/\\/

l^-l^\--\,/\ (-./YI

vin1,lic protons6 4.s-5.5

alll4ic protons6 1.8-2.2

Although the chemical shifts of allylic and propargylic protons are very similar (as might beexpected from the fact that both double and triple bonds involve n'electrons), the chemicalshifts of acetylenic protons are much smaller than those of vinylic protons.

The explanation for the unusual proton chemical shifts observed in alkynes is closely re-latedtotheexplanationforthechemicalshiftsofvinylicprotons(Fig.13.I4,p.613),althoughthe effect is in the opposite direction. An alkyne molecule in solution is tumbling rapidly, butalkyne chemical shifts are dominated by the effects resulting from one particular orientationof the alkyne molecule relative to the magnetic field, as shown in Fig. 14.5. When an alkynemolecule is oriented in the applied field Bo as shown in this figure, an induced electon circu-

14.3 PHYSICAL PROPERTIES OF ALKYNES 651

-- _-\\l \// r .-. \. / -/ / \.\ / / -\i / \,\ /,/ \i t \,\ l,/ \

- I z^t \\ ll /-''.

:i*?fff J ,/ ",',,' \ ti !' l' ".circulation i\ \\\ liili I i\c\ llqii i)IrrllllrlI rlzrl | |s\f

r'i'lH\,',\, I

B1 (indr-rc-ecl fielcl)

\ \ lr.t "',\ \\ 1 t ll l\'. /, \ \"/ ll \1 t-,/t. \ p/ \r /tr \, ti \. ,'t\ \'-

-/r/ tt't

- -\r--t' \--l

+I 8,, t aprprlie 11 field I

Figure 14.5 Explanation of the chemical shift of acetylenic protons.The induced field B, of the circulating z'electrons (red) opposes the applied field Bo (b/ue) from the spectrometer in the region of space occupied byacetylenic protons. As a result, the local field at an acetylenic proton is reduced. Hence, acetylenic protons haveNMR absorptions at relatively small chemical shift.The same effect accounts for the chemical shifts of acetylenicand propargylic carbons in the r3C NMR spectra of alkynes.

lation is set up in the cylinder of zr electrons (Fig. 14.3c) that encircles the molecule. The re-sulting induced field B, opposes the applied field along the axis of this cylinder. Because theacetylenic proton lies along this axis, the local field at this proton is reduced. Consequently, byEq. 13.4, p. 583, acetylenic protons have NMR absorptions at smaller chemical shift than theywould have in the absence of this effect.

f, Carbon NMR Spectroscopy Chemical shifts of alkynes in 13C NMR are subject to thesame influences as proton chemical shifts. Although carbons involved in double bonds havechemical shifts in the 6 100-145 range, carbons involved in triple bonds absorb at consider-ably lower chemical shift, in the 6 65-85 range. Propargylic carbons, like acetylenic hydro-gens, also have smaller chemical shifts, typically by 5-15 ppm. The chemical shifts in 2-heptyne are typical:

propargyilc carDons

----_------r-5 3.3 75.2 79.2 r8.7 31.7 22.3 13.8H3C - C = C - CH2 - CH 2- CH2- CH3\/

:etvler-ric carbirns' 2-heptyne

Compare, for example, the chemical shift of the propargylic methyl carbon (6 3.3) with that ofthe other methyl carbon (5 13.8), which is much like that of an alkane methyl group.

The explanation for these chemical-shift effects is the same one (Fig. 14.5) discussed forthe proton chemical shifts in alkynes.

652 cHAprER 14 . THE cHEMrsrRy oF ALKyNES

wavelength, micrometers2.62.83 3.5 4 4.5 5 5.5 6 7 8 910 11 12 l3t4 15t6

100 T1 n mi{^: Il l,{ li. II.x1,ilj-,].'|-L..lJilI,[,,j{ilf-, ,.i i, Ii- lf.[ llf i \t. il; ,iti, r t

3800 3400 3000 2600 2200 2000 1800 1600 1400 1200 1000 800 600wavenumber. cm-l

Figure 14.6 The lR spectrum for Problem 14.4.

c.)

H80(s.EH60cd

:40a,)(,b20

0

@ 14.4 Identify tbe compound with a molecular mass of 82 that has the IR spectn,* shown inFig. 14.6 and the following NlvtR spectmm: 6 1.X) (|If, s); 6l.2l (9H, s)

f?|F (a) March each of the following r3C NIvIR spectra to either 2-hexyne or 3-hexyne. Explain.\ SpearumA.' 63.3, 13.6,21.1,22.9,75.4,79.1

Speannt B: 6 12.7, 14.6, 81.0

O) Assign each of the resonances in the two spectra to the appropriate carbon atoms.

14.6 A student consult€d arwell-known compilation of reference spectra for the proton NMR spec-trum of propyne and was surprised o find ttrat ttris $pectrum consists of a single unsplit res-onan@ at 61.8. Believing this to be an error, he comes to you for an explanation. Explain tohim why it is reasonable that propyne could have this spectum.

INTRODUGTION TO ADDITION REACTIONS71.4 OF THE TRIPLE BOND

In Chapters 4 and 5 we learned that the most common reactions of alkenes involve additionsto the double bond. Additions to the triple bond also occur, although in most cases they aresomewhat slower than the same reactions of comparably substituted alkenes. For example,HBr can be added to the triple bond.

CH:(CHz)IC:CH * HBr TEHII- CH3(CH2)3C:CHz (I4.I)Il-hexyne Br

2-bromo-l-hexene

The regioselectivity of the addition is analogous to that found in the addition of HBr to alkenes(Sec. 4.7A): the bromine adds to the carbon of the triple bond that bears the alkyl substituent.As in alkene additions, the regioselectivity is reversed in the presence of peroxides becausefree-radical intermediates are involved (Sec. 5.6).

14.4

CHr(CHz):C-CHl-hexFne

+HBr €(excess)

+HBr ffi

BrIH:C-C:CH-CH:

(not isolated)

CHr(CHz)iCH:CHBrI -bromo- I -hexene;

stereochemistry not determined(7 4o/o yield)

HBr+Br

IHrC-9-CHzCH: (14.3a)I

Br

2,2-dibromobutane(600/o yield)

INTRODUCTION TO ADDITION REACTIONS OF THE TRIPLE BOND 653

Because addition to an alkyne gives a substituted alkene, a second addition can occur in manycases.

(r4.2)

( 14.3b)

HsC-C-C-CH:2-butyne

The regioselectivity of this addition reaction is determined by the relative stabilities of the twopossible carbocation intermediates. One of the two possible carbocations (A in the followingequation) is stabilized by resonance. By Hammond's postulate (Sec. 4.8D), this carbocation isformed more rapidly.

: Br:IH:C-C -CHCHT

t 1:B'r, *Br, ll\l lt I

Ltr.-9-cHzcH: € H:c-c-cHrcH, lresonance- stabil ized carbo cation A

I

I

vrBr t

IH:C- C -CHzCHi,J,,

ob*.**d product

:Br:l*H:C- CH-CHCH:less stable carbocation B

I

I

v: Br:

IHrC- CH -CHCHTI

'P1,,

not formed

In the addition of a hydrogen halide or a halogen to an alkyne, the second addition is usu-ally slower than the first. The reason is that the halogen that enters the molecule in the first ad-dition exerts a rate-retarding polar effect (Sec. 3.6C) on carbocation formation in the secondaddition. In other words, both carbocations A and B in Eq. I4.3b are destabilized by the polareffect of bromine, and this polar effect is only partially counterbalanced by the resonance sta-bilization in carbocation A. Because the second addition is slower, it is possible to isolate theproduct of the first addition if one equivalent of HBr is used, as in Eq. 14.1.

654 cHAprER 14 . THE cHEMrsrRy oF ALKyNES

CONVERSION OF ALKYNES INTO14.5 ALDEHYDES AND KETONES

14.7 Give the product that results from the addition of one equivalent of Br, to 3-hexyne. Whatare the possible stereoisomers that could be formed?

14.8 The addition of HCI to 3-hexyne occurs as an anri-addition. Give the structure, stereochem-istry, and name of the product.

A. Hydration of AlkynesWater can be added to the triple bond. Although the reaction can be catalyzed by a strong acid,it is faster, and yields are higher, when a combination of dilute acid and mercuric ion (Hg2+)catalysts is used.

oHs2+, Hrso, (dilute) lL -( )-C-CH + H2o "o r"Z""+'\""-.'' > ( f C-Cu, 14.4)\__J \__J

cyclohexylacetylene cyclohexyl methyl ketonei9lo/o yreld)

The addition of water to a triple bond, like the corresponding addition to a double bond, iscalled hydration. The hydration of alkynes gives ketones (except in the case of acetyleneitself, which gives an aldehyde; see Study Problem 14.7, p. 656).

Let's contrast the hydration reactions of alkenes (Sec. 4.9B) and alkynes. The hydration ofan alkene gives an alcohol.

R-cH:cH2 + H2o HzSo+ t R-cH-cH3 (14.5a)

an alkene OHan alcohol

Because addition reactions of alkenes and alkynes are closely analogous, it might seem that analcohol should also be obtained from the hvdration of an alkvne:

OH

R-c:c-H + H2o HzSOa' Hs2+ t R-C:CH 2 o4.5b)an alloTne an enol

An alcohol containing an OH group on a carbon of a double bond is called an enol (pronouncedEnl6l). In fact, enols are formed in the hydration of alkynes. However, most enols cannot beisolated because most enols are unstable and are rapidly converted into the corespondingaldehydes or ketones.

oH9R-C:CH z T> R-C-CU: (14.5c)

an enol a ketone

14.5 CONVERSION OF ALKYNES INTO ALDEHYDES AND KETONES 655

Most aldehydes and ketones are in equilibrium with the corresponding enols, but the equilib-rium concentrations of enols are in most cases minuscule-typically, one part in 108 or less.The relationship among aldehydes, ketones, and enols is explored in Chapter 22.T\e impor-tant point here is that, because most enols are unstable, if an enol isformed as the product of areaction, it is rapidly converted into the corresponding aldehyde or l<etone.

The mechanism of alkyne hydration is very similar to that of the oxymercuration of alkenes(Sec. 5.4A). In the first part of the mechanism, mercuric ion reacts as an electrophile with thezr electrons of the triple bond to form a carbocation, which could be in equilibrium with acvclic mercurinium ion:

-:oH,R-C-CH1

I

Hg*

Hl^,o*fH| 'Ou,R-C-CH-

I

Hg*

tOHIR-C-CH +

I

Hg*

(r4.6a)

Hro* (14.6b)

R-Q-CH ----+I1' Hgr*

R-C-CH _ R-C-CH\ I \/\'Hg* Hgt*

The carbocation is formed at the carbon of the triple bond that bears the alkyl substituent. (Re-call that alkyl substitution stabilizes carbocations; Sec. 4.7C.) This carbocation reacts withwater, and loss of a proton to solvent water gives the addition product. As a result, the oxygenfrom water ends up on the carbon with the alkyl substituent.

In the oxymercuration of alkenes, the reducing agent NaBI{o is the source of hydrogen thatreplaces the mercury. However, the use of NaBHo is unnecessary in the hydration of alkynes.The reason is that the presence of a double bond makes possible removal of the mercury by aprotonolysis reaction. This protonolysis occurs under the conditions ofhydration; a separateprocedure is not required. The first step in the mechanism of this protonolysis reaction is pro-tonation of the double bond. This protonation occurs at the carbon bearing the mercury be-cause the resulting carbocation is resonance-stabilized.

.\:ou i,Hr0r,t/t(R-C:CH +

I

Hg*

/tOH *OH I\'l ll IR-C-CHz <+ R-C-CHzl+ | - | -lHs* I. r.T;nce-stabilized carbocation

(Recall that formation of a resonance-stabilized carbocation also explains the position of pro-tonation in HBr addition; Eq. 14.3b, p. 653.) Dissociation of mercury from this carbocationliberates the catalyst Hg2+ along with the enol.

OHIR-Q;CHz ++Ll

Hg*

OHIR-C-CHz + Hgt*

an enol

(14.6d)

656 CHAPTER 14 o THE CHEMISTRY OF ALKYNES

Conversion of the enol into the ketone is a rapid, acid-catalyzeddouble bond gives another resonance-stabilized carbocation:

process. Protonation of the

n,Qnrr r--tiu,t(R-C-CHz < -+

This carbocation is also theproduct.

a resonance*stabilized carbocation

conjugate acid of a ketone. Loss of a proton gives the ketone

f>H '9H,:O'tlR-C-CH:

:oll*.F:- R-C-CH: + Il-'--LlHl ( 14.6f)

The hydration of alkynes is a useful way to prepare ketones provided that the starting ma-terial is a l-alkyne or a symmetrical alkyne (an alkyne with identical groups on each end ofthe triple bond). This point is explored in Study Problem 14.1.

@Whichoneofthefollowingcompoundscouldbepreparedbythehydrationofalkynessothatitisuncontaminated by constitutional isomers? Explain your answer.(a) o (b)

llCH3CH

acetaldehyde

oll

cH3cH2ccH2cH33-pentanone

HO CH2CH3

\-al H:o+ t/\HrC Hg*

Hgl CH2CH3

\:a' Hro+ t/\

otl

cH3ccH2cH2cH32-pentanone

o Q4'7)tl

cH3cH2ccH2cH33-pentanone

SOlution First, what alkyne starting materials, if any, would give the desired products? Theequations in the text show that the two carbons of the triple bond in the starting material corre-spond within the product to the carbon of the C:O group and an adjacent carbon. Thus, for part(a), the only possible alkyne starting maierial is acetylene itself, HC-CH. For part (b), the onlypossible alkyne starting material is 2-pentyne, CI{,C-CCH'CH,.

Nexg it remains to be shown whether hydration of these alkynes gives ozly the products in theproblem. Remember, a good synthesis gives relatively pure compounds. The hydration of acetyleneindeed gives only acetaldehyde. (In fact, acetaldehyde is the only aldehyde that can be prepared bythe hydration of an alkyne.) However, hydration of 2-pentyne gives a mixture consisting of compa-rable amounts of 2-pentanone and 3-pentanone , because the carbons of 2-pentyne both have onealkyl substituenl. Thus, there is no reason that the reaction of water at either carbon should bestrongly favored.

H:C- C-C-CHzCHr

one alkyl substituenton each carbon H:C OH

14.5 CONVERSION OF ALKYNES INTO ALDEHYDES AND KETONES 657

I{ence, hydration would give a mixture of constitutional isomerr that would have to be separated,and the yield of the desired product would be low. Consequently, hydration would nar be a goodwey tb Fepare 3-pentanone. (However, 2.pentanone could be preparod by hydration of a differentalkyne; see Problem 14.9a).

ilt1(cH3)3c-c-cHr

(c) oou"trotrVor)A-*o2eH2cH,. H3

14.10 Thc hydration of an alkyne is not a reasonabfe prepar*ive rrcnoA for eacb of the folloudngcompoxnds, Explain yhy. . ,

{a)CF{sCHtCH-g'(b) " O (c}ll',,,(cHj)5f, ;;' 5 c(cH3)3

{4f ii li;) #; ue'srmqys or 1rj epqtfe{n*ior ur*eu@ i, fi!ffi *;*;cHrcHr-c-cH(cH3L

O) Would alkyne hydration te a gooA preparative method for this coryound? Explain.

o

/[cH,c*ro\,I

EEEil r+.> ;ffJ" auq/ne oourq e,acn or rr* rorowrng compouxls Dc pr€pe€u Dy a",o-c$aryzoo

(nl" $', " (b) Otl

cH3ccH2cH2cH3

Hydroboration-oxidation of Al kynesThe hydroboration ofalkynes is analogous to the same reaction ofalkenes (Sec. 5.4B).

/ CH,CH\icr"cu" I \

3CH3CH2C-CCH2CH3 + BH3 -THF* t " --gr"\J t'o.rul\tls

\H/3

As in the similar reaction of alkenes, oxidation of the organoborane with alkaline hydrogenperoxide yields the corresponding "alcohol," which in this case is an enol. As shown in Sec.14.5A^, enols react further to give the corresponding aldehydes or ketones.

CH,CH\l--'---'\ cH3crHz cH2cH3 o

tlt\ | H':o'?loH-> b:a1 ----> cHrcH2cH2-c-cH2cH3\/\ INHOH/3 an enol (14.8b)

6s8

STUDY GUIDE LINK 14.1Functional Group

Preparations

CHAPTER '14 . THE CHEMISTRY OF ALKYNES

Because the organoborane product of Eq. 14.8a has a double bond, a second addition ofBH, is in principle possible. However, the reaction conditions can be controlled so that onlyone addition takes place, as shown, provided that the alkyne is not a 1-alkyne.

If the alkyne is a I -alkyne (that is, if it has a triple bond at the end of a carbon chain), a sec-ond addition of BH, cannot be prevented.

R-C:CH + BH3 ---------> multiple addition reactions

a l-a$me

Howeveq the hydroboration of l-alkynes can be stopped after a single addition provided thatan organoborane containing highly branched groups is used instead of BHr. One reagent de-veloped for this purpose is disiamylborane, represented with the skeletal structure shown inEq. 14.9. (How would you synthesize disiamylborane? See Sec. 5.4B.)

B-H represented as (+) BH\ | tz

( 14.e)

disiamylborane

The disiamylborane molecule is so large and highly branched that only one equivalent canreact with a l-alkyne; addition of a second molecule results in severe van der Waals repul-sions. In many cases, van der Waals repulsions, or steric effects, interfere with a desiredreaction; in this case, however, van der Waals repulsions are used to advantage, to prevent anundesired second addition from occurring:

/ cH, (./llI u -(.-c\ll\ CHt H

f!-) rrH

cH:(cHz)5-Q-CH \l /z tTHF

cH:(cHz)s\ iC:CH/ t-(+)\ | lz

L)H

Hr( )r+ oH-

cH:(cHz)s H\/t^-f +U-U cHr(cHz)s-cHz-cH-L) (14.10)

octanal(an aldehyde; 70o/o yield)

H(an enol)

Notice from this example that the regioselectivity of alkyne hydroboration is similar to thatobserved in alkene hydroboration (Sec. 5.4B): boron adds to the unbranched carbon atom ofthe triple bond, and hydrogen adds to the branched carbon.

Because hydroboration-oxidation and mercury-catalyzed hydration give different productswhen a l-alkyne is used as the starting material (why?), these are complementary methods forthe preparation of aldehydes and ketones in the same sense that hydroboration-oxidation andoxymercuration-reduction are complementary methods for the preparation of alcohols fromalkenes.

14.6 REDUCTION OF ALKYN ES 6 5 9

otlcH:(cHz)scHz-c-H

cH:(cHz)s-c-c-H

14.6 REDUCTION OF ATKYNES

Notice that hydroboration--oxidation of a l-alkyne gives an aldehyde; hydration of any l-alkyne(otherthan acetylene itself) gives aketone.

14.12 Compare the results of hydroboration-oxidation and mercuric ion-catalyzed hydratioa for(a) cyclohexylacdylene and @) 2-butyne. ,, .,,,

otlcH:(cHz)s-c-cH:

(14.1l)

2) HzOz /OH-

A. Catalytic Hydrotenation of AllrynesAlkynes, like alkenes (Sec. 4.9A), undergo catalytic hydrogenation. The first addition of hy-drogen yields an alkene; a second addition ofhydrogen gives an alkane.

H2, Hz,

R-c:c-R catalvst. R-cH:cH-R catalvst > R-cH2-cHr-R (r4.r2)

The utility of catalytic hydrogenation is enhanced considerably by the fact that hydrogena-tion of an alkyne may be stopped at the alkene stage if the reaction mixture contains a cata-lyst poison: a compound that disrupts the action of a catalyst. Among the useful catalyst poi-sons are salts of Pb2+, and certain nitrogen compounds, such as pyridine, quinoline, or otheramines.

quinoline

These compowds selectively block the hydrogenation of alkenes without preventing the hy-drogenation of alkynes to alkenes. For example, a Pd/CaCO, catalyst can be washed withPb(OAc), to give a poisoned catalyst known as Lindlar catalyst. In the presence of Lindlarcatalyst, an alkyne is hydrogenated to the corresponding alkene:

\rpyriaine

Lindlar catalyst orPd/C, pyridineH, + CH3CH2CH2C-CCH2CH2CH3

cH3cH zc.Hz cH2cHzcHi\- /C:C. ( 14.13)/\HHcis-4-octene

4-octFneethanol

660 CHAPTER 14 o THE CHEMISTRY OF ALKYNES

As Eq. 14.13 shows, the hydrogenation of alkynes, like the hydrogenation of alkenes(Sec. 7.9E), is a stereoselective syn-addition. Thus, in the presence of a poisoned catalyst, hydro-genation of appropriate alkynes gives cis alkenes. In fact, catalytic hydrogenation of allcynes isone ofthe best ways to prepore cis alkenes.

In the absence of a catalyst poison, two equivalents of H, are added to the triple bond.

Pd/c

2Hz + cH3cH2cH2c-ccH2cH2cH3 noPoison t cH3cH2cH2CH2cH2cH2cH2cH3 04.14)4-octyne octane

The catalytic hydrogenation of alkynes can therefore be used to prepare alkenes or alkanes byeither including or omitting the catalyst poison. How catalyst poisons exert their inhibitory ef-

'fect on the hydrogenation of alkenes is not well understood.

2Na+ -Nn,

(14.1s)

\\ Hrlpoisoned catalyst

\ (Sec. 14.6A)

\RR\/f-fU-U/\HHcis alkene

(14.16)

The stereochemistry of the Na/NH, reduction follows from its mechanism. If sodium orother alkali metals are dissolved in pure liquid ammonia, a deep blue solution forms that con-tains electrons complexed with ammonia (solvated electrons).

l@I.**,nr, w*o*i*,"r***o* .r*nofthefol@ins.$astiw/-\{.) C*n(CdjsC= +' ' ft) snne as part (a)'with no poison

I-S$[!P! ,

CH3CH zCHz-C-Q-CH zCHzCHr * 2Na + zNn,

/Na/NHy'

/rRH\/l^-fU-U/\HRtrans alkene

:, . .:: :

C:-CH

Reduction of Allqnes wlth Sodlum in tiquid AmmonlaReaction of an alkyne with a solution of an alkali metal (usually sodium) in liquid ammoniagives a trans alkene.

cH3cHzcHz HL

\' /+C:C+ /\H CH2CHzCHT(97o/o yield)

The reduction of alkynes with sodium in liquid ammonia is complementary to the catalytichydrogenation of alkynes, which is used to prepare cis alkenes (Sec. 14.6A).

R-Q-C-R

14.6 REDUCTION OF ALKYNES 561

Na' + nNH3 (liq) --* Na+ + e-(NH3)osolvated electron

(r4.r1)

The solvated electron can be thought of as the simplest free radical. Remember that free radi-cals add to triple bonds (Eq. 14.2, p.653). The reaction of solvated elecffons with the alkynesbegins with the addition of an electron to the triple bond. The resulting species has both anunpaired electron and a negative charge. Such a species is called a radical anion:

e- Na+(\1nR-C:=C-R

H' NHra"

'-JR-C-C-R --+

.iR-C-C-R Na+a radical anion

cis vinylic radical

(r4.1 8a)

( 14.18d)

The radical anion is such a strong base that it readily removes a proton from ammonia to givea vinylic radical-a radical in which the unpaired electron is associated with one carbon of adouble bond. The destruction of the radical anion in this manner pulls the unfavorable equilib-rium in Eq. 14.18a to the right:

( 14.18b)

Na+a vinylic radical

The vinylic radical, like the unshared electron pair of an amine (Sec. 6.10B), rapidly under-goes inversion, and the equilibrium between the cis and trans radicals favors the trans radicalfor the same reason that trans alkenes are more stable than cis alkenes: repulsions between theR groups are reduced.

(14.1 8c)

Na+ Na+solvatedelectron

This step of the mechanism is the product-determining step of the reaction (Sec. 9.68). Therate constants for the reactions ofthe cis and trans vinylic radicals with the solvated electronare probably the same. However, the actual rate of the reaction of each radical is determinedby the product of the rate constant and the concentration of the radical. Because the trans

R

C

I

\_

H. / .oR-C:C + -NHr Na+\*

RH\p /c-cL-Uffi\*.ZR

frl HV/R-C:CA\\JR

AHq -c//w \RRtrans vinylic radical(strongly favoredat equilibrium)

Next, the vinylic radical accepts

transition statefor inversion

an electron to form an anion:

H/-r \

R

RH,\/t.\

R

662 cHAprER 14 . THE cHEMtsrRY oF ALKYNES

vinylic radical is present in much higher concentration, the ultimate product of the reaction,the trans alkene, is derived from this radical.

The anion formed in Eq. 14.18d is also more basic than the amide anion and readily re-moves a proton from ammonia to complete the addition.

RH\ / ..+ c:c. + -JJH, Na+ (14.18e)/\HR

trans alkenepK" - 42

Because ordinary alkenes do not react with the solvated electron (the initial equilibriumanalogous to Eq. 14.18a is too unfavorable), the reaction stops at the trans alkene stage.

The Na/NH, reduction of alkynes does not work well on l-alkynes unless certain modifi-cations are made in the reaction conditions. (This is explored in Problem 14.39.) However, thisis not a serious limitation for the reaction, because the reduction of l-alkynes to l-alkenes iseasily accomplished by catalytic hydrogenation (Sec. 14.64).

RH\/Na+ C:C(-\I\R\

H,/\..I NHzV

pK. - 35

@l4.14Whatproductisobtainedineachcasewhen3-hexyneistreatedineachofthefollowingways? (Hint: The products of the two reactions are stereoisomers.){a} with sodium in liquid ammonia and the product of that reaction with D, over Pd/C(b) with H, over Pd/C and quinoline and the product of that reaction with D, over Pd/C

14.7 ACIDITY OF I.ALKYNES

A. Acetylenic AnionsMost hydrocarbons do not react as Br@nsted acids. Nevertheless, let's imagine such a reactionin which a proton is removed from a hydrocarbon by a very strong base B:-.

-p-H + B:- -------> -C:- + B-H (14.19)//

a carDanlon

In this equation, the conjugate base of the hydrocarbon is a carbon anion, or carbanion. Recallfrom Sec. 8.8B that a carbanion is a species with an unshared electron pair and a negativecharge on carbon.

The conjugate base of an alkane, called generally an allql anion,has an electron pair in ansp3 orbital. An example of such an ion is the 2-propanide anion:

O * sP3 orbitalV_C

,,,''b("cHjH -tCH,

Z-propanide anion(an alkyl anion)

14.7 ACIDITY OF 1-ALKYNES 653

The conjugate base of an alkene, called generally avinylic anion, has an electron pair in an sp2orbital. An example of this type of carbanion is the l-propenide anion:

H:C,, (:) * sp2 orbital\ _t-/C:C

/ d\

l-propenide anion(a vinylic anion)

The anion derived from the ionization ofa l-alkyne, generally called an acetylenic anion,hasan electron pair in an sp orbital. An example of this type of anion is the l-propynide anion:

sp orbital

_tcH3c-c€l-propynide anion

(an acetylenic anion)

The approximate acidities of the different types of aliphatic hydrocarbons have been mea-sured or estimated:

HH

R:C-H

alkane>55

alkene42

R/R'C-C.\H

R-Q C-H ( 14.20)

alkyne25

type of hydrocarbonapproximate pK.

These data show, first, that carbanions are extremely strong bases (that is, hydrocarbons are veryweak acids); and second, that alkynes are the most acidic ofthe aliphatic hydrocarbons.

Alkyl anions and vinylic anions are seldom if ever formed by proton removal from the cor-responding hydrocarbons; the hydrocarbons are simply not acidic enough. However, alkynesare sufficiently acidic that their conjugate-base acetylenic anions can be formed with strongbases. One base commonly used for this purpose is sodium amide,.or sodamide, Na* -:NHz,dissolved in its conjugate acid, liquid ammonia. The amide ion, -:NHz, is the conjugate baseof ammonia, which, as an acid, has a pK" of about 35.

B:-+:I.{Ha:-:NHr+B-HSTUDY GUIDE LINK 14.2Ammonia, Solvated

Electrons, andAmide Anion

(r4.2r)pK. - 35

amide ion

Because the amide ion is a much stronger base than an acetylenic anion, the equilibrium forremoval of the acetylenic proton by amide ion is very favorable:

n-c:C\u'lNH, 11u* --N!4!L R-C:e: Na+ + iriH, (14.22)

In fact, the sodium salt of an alkyne can be formed from a l-alkyne quantitatively withNaNHr. Because the amide ion is a much weaker base than either a vinylic anion or an alkylanion, these ions cannot be prepared using sodium amide (Probleml4.l7).

The relative acidity of alkynes plays a role in the method usually used to prepare acetylenicGrignard reagents, which are reagents with the general structure R-C:C-MgBr. Recallfrom Sec. 8.8A that Grignard reagents are generally prepared by the reactions of alkyl halides

664 cHAprER 14 . THE cHEMlsrRy oFALKyNES

with magnesium. The "alkyl halide" starting material for the preparation of an acetylenicGrignard reagent by this method would be a l-bromoalkyne-that is, R-C:C-Br. Suchcompounds are not generally available commercially and are difficult to prepare and store.Fortunately, acetylenic Grignard reagents are accessible by the acid-base reaction between al-alkyne and another Grignard reagent. Methylmagnesium bromide or ethylmagnesium bro-mide are often used for this purpose.

CH1CH2CH2CH2-C -C- H + CH-ICH 2-MgBr ffi CH3CH2CH2CH2-C-C-MgBr + CH3CH3an acetylenic Grignard reagen,

gthane(a gas)

(t4.23)

H-C:C-H + CH3CH2-MgBr -T"F* H-C:C-MgBr + CH3CH3 04.24)ethynylmagnesium bromide

This reaction is extremely rapid and is driven to completion by the formation of ethane gas(when CH,CHTMgBT is used as the Grignard reagent). This reaction is an example of a trans-metallation: a reaction in which a metal is transferred from one carbon to another. However.it is really just another Br0nsted acid-base reaction:

,"-._.---r-{_\BrMg+ :CH.,CH1 -R -4 ii-('Hr('Hr + BrMg+ -r-:R (11.25)

Although Grignard reagents are covalent compounds, the two Grignard reagents in this equa-tion are represented as ionic compounds to stress the acid-base character of the equilibrium.This reaction is similar in principle to the reaction of a Grignard reagent with water or alco-hols (Eq. 8.27,p.363). Like all Brgnsted acid-base equilibria, this one favors formation of theweaker base, which, in this case, is the acetylenic Grignard reagent. The release of ethane gasin the reaction with ethylmagnesium bromide makes the reaction irreversible and at one timewas also a useful test for l-alkynes. Alkynes with an internal triple bond do not react becausethey lack an acidic acetylenic hydrogen.

What is the reason for the relative acidities of the hydrocarbons? Sec. 3.6,4. discussed twoimportant factors that affect the acidity of an acid A-H: the A-H bond strength and theelectronegativity of the group A. Bond dissociation energies show that acetylenic C-Hbonds are the strongest of all the C-H bonds in the aliphatic hydrocarbons:

RI

(r4.26)R -C-C-Hacetylenic C - H

(548 kJ mol-l,l3 l kcal mol-1)

vinylic C - H(460 kl mol-l,110 kcal mol-l )

R:C-H

alkyl C - HeA2-41S kl mol-l,96-100 kcal mol-l)

If bond strength were the major factor controlling hydrocarbon acidity, then alkynes would bethe least acidic hydrocarbons. Because they are in fact the most acidic hydrocarbons, the elec-tronegativities of the carbons themselves must govern acidity. Thus, the relative electronega-tivities of carbon atoms increase in the order sp3 < sp2 < sp, and the electronegativity differ-ences on aciditv must outweish the effects of bond strensth.

14.7 ACIDITY OF 1-ALKYNES 555

This trend in electronegativity with hybridization can be explained in the following way.The electrons in sp-hybridized orbitals are closer to the nucleus, on the average. than sp2 elec-trons, which in turn are closer than spr electrons (Sec. 14.2). In other words, electrons in or-bitals with larger amounts of s character are drawn closer to the nucleus. This is a stabilizingeffect because the interaction energy between particles of opposite charge (electrons and nu-clei) becomes more strongly negative (that is, favorable) as the distance between them de-creases (the electrostatic law; Eq.3.40, p. 113). Thus, the stabilization of unshared electronpairs is in the order sp3 I sp2 ( sp. In other words, unshared electron pairs have lower energywhen they are in orbitals with greater s character.

14.15 Each of the following compounds protonates on nitrogen. Draw the conjugate acid of each.which compound is m:l:!3el1ft:rtut

"ta ^ -:ti-i?rt rr ^ ^-\r.H:c-cH-NH "'t -t;*t. nrri,LA B lg,trl ,,rr.

14.16 (a) Ion A is more acidic than ion B in the gas phase. Is this the acidity order predicted byhybridization arguments? Explain.

++H:C-OHz H:C-CH:OH

(b) Ion .B is less acidic because it is stabilized by resonance, whereas ion A is not. Show theresonance structure for ion B, and, with the aid of an energy diagram, show why stabi-lization of ion B should reduce its acidity.

(c) In aqueous solution, ion A is less acidic than ion B. Explain.14.17 (a) Using the pK" values of the hydrocarbons and ammonia, estimate the equilibrium con-

stantfor(l)thereactioninEq. 14.22and(2)theanalogousreactionofanalkanewithamide ion. (Hint: See Study Problem 3.6, p. 91)

(b) Use your calculation to explain why sodium amide cannot be used to form alkyl anionsfrom alkanes.

B. Acetylenic Anions as NucleophilesAlthough acetylenic anions are the weakest bases of the simple hydrocarbon anions, they arenevertheless strong bases-much stronger, for example, than hydroxide or alkoxides. They un-dergo many of the characteristic reactions of strong bases, such as S*2 reactions with alkylhalides or alkyl sulfonates (Secs. 9.4, 10.34). Thus, acetylenic anions can be used as nucle-ophiles in S*2 reactions to prepare other alkynes.

/-;----\' | ..CH3CH2CH2CHrIB.I: f Na+ :C:CH -"il'r)- CH3CH2CH2CH2-C:CH * Na+

I -bromobutane sodium acetylide l-hexFne(640/o yielcl)

(14.21)

- zt-.------'- n.CH3CH2CH2CH2-C:C :' Na+ + H.,t I f B-r: -------> CH3CH2CH2CH2- C:C- CH r * Na+ ( 14.28)

The acetylenic anions in these reactions are formed by the reactions of the appropriate 1-alkynes with NaNH, in liquid ammonia (Sec. 14.7A). The alkyl halides and sulfonates, as in mostother S"2 reactions, must be unhindered primary compounds. (Why? See Secs. 9.4D,9.5G.)

666 cHAprER 14 . THE cHEMtsTRy oF ALKyNES

The reaction of acetylenic anions with alkyl halides or sulfonates is important because i/ lsanother method of carbon-carbon bond formation. Let's review the methods covered so far:

l. cyclopropane formation by the addition of carbenes to alkenes (Sec. 9.8)2. reaction of Grignard reagents with ethylene oxide and lithium organocuprate reagents

with epoxides (Sec. 11.4C)3. reaction ofacetylenic anions with alkyl halides or sulfonates (this section)

14. I 8 Give the structures of the products in each of the following reactions.(a)cHrc:e: Na+ + cH3cH2-I ------->(b)butyltosylate + Ph-C:C: Na+ ------->(cl CH3C:C-MgBr * ethylene oxide -------> Hro+ t(d) Br(CHz)sBr + HC:C: Na+(excess) ------->

14.19 Explain why graduate student Choke Fumely, in attempting to synthesize 4,4-dimetfil-2-pentyne using the reaction of H.C-C-C: Na+ with tert-bvtyl bromide, obtained none ofthe desired product.

14.20 Propose a synthesis of 4,4-dimethyl-2-pentyne (the compound in Problem 14.19) from analkyl halide and an alkyne.

14.21 Outline two different preparations of2-pentyne that involve an alkyne and an alkyl halide.14.22 Propose another pair of reactants that could be used to prepare 2-heptyne (the product in Eq

14.2$.

14.8 ORGANIG SYNTHESIS USING ATKYNESLet's tie together what we've learned about alkyne reactions and organic synthesis. The solu-tion to Study Problem 14.2 requires all of the fundamental operations of organic synthesis: theformation of carbon--carbon bonds, the transformation of functional groups, and the establish-ment of stereochemistry (Sec. 11.9).

Notice that this problem stipulates the use of starting materials containing five or fewer car-bons. This stipulation is made because such compounds are readily available from commer-cial sources and are relatively inexpensive.

@outlineasynthesisofthefollowingcompoundfromacetyleneandanyothercompoundscontain-ing no more than five carbons:

cH3(cH2)6\ /cr{2crl2c}l(cH)2C:C/\HH

cis-2-methvl-5-tridecene

Solution As usual, we stafi with the target molecule and work backward. First, notice the stere-ochemistry of the target molecule: it is a cis alkene. We've covered only one method of preparingcis alkenes free oftheirtrans isomers: the hydrogenation ofalkynes (Sec. 14.6.4). This reaction,then, is used in the last step of the synthesis:

Lina#LtahstCH3(CH z)e-C-f, -CH2CH2CH(CH3)2

-

14.8 ORGANIC SYNTHESIS USING ALKYNES 667

cH3(cH2)6. cH2cH2cH(cH3)2J\

-."\

/

-

C:C/\HH(target molecule)

(14.29a)

2-niethyl-S -tridegrne

t1t" **1task is to preparc *rc alkyne used as the sarting naferial in Eq. 14.29a. Because the desiredalkyne contains 14 carbons and the problem stiprlates the use of compormds with five q fsw€r carbons,we'll have to use several reaptions that forrn carbon<a6on bonds. Ttrerc are two pimary alkyl groupson the tiple bond; the oderin which they ae intoduced is abitrary. I€t's infrodrce the fue+arbonfiagment on tlre right-hand side of this alkyne in tbe las step of the alkyne qmfuis. This is accom-plisbed by forming tlrc conjugate.base aoetylenic anion of l-noryrne and alowing it to rcact with the apprcpriate commercially available €ve-cabon alk;f halide, l-bromo3-meflrylbutane (Sec. 14.78):

--.

CH3(CHz)o-C:C-H ffi CHI(CHa)5C-C:1-nonlare

Br-CHzCHzCH(CHE)z1 -bromo-3-methylbutane

H-c-c-H ffi Na+ :e-c-H cHr(cHz)oBr t cH3(cHr6-c-c-H(rarge excess

relative to NaNHu)

CH3(CH2)6-C:-C-CHzCHzCH(CH3)2 04.29b)

The staning material for this reaction, l-nonyne, is prepared by the reaction of l-bromohoptanewith the sodium salt ofacetvlene itself.

(14.29c)

(14.29d)

The large excess ofacetylene relative to sodium amide is required to ensure formation ofthemonoanion-thatis, the anion derived from the removal of only otu acetylene p'roton. If therewere more sodium amide than acetylene, some dianion:f,:f,: could fonn, and other reactionswould occur. (What are they?) Because acetylene is cheap and is easily separated ftom the prod-ucts (it is a gas), use of a large oxcess presents no practical problem.

Because the l-bromoheptane used in Eq. 14.29c haq 1161p than five carbons, it must be pre-pared as well. The following sequence of reactiols will accomplish this objective.

AcH3cH2cH2cH2cH2-Br #

Hrc-cH:t H:o+t

l-bronopentanecoac HBr

CH3CH2CH2CH2CH2GH2CH2-oH Hzsor > CH3CH2CH2CH2CH2GH2CH2-B1

1-heptanol

The synthesis is now complste. To summarize:

HC-CH (excess) ffi cHr.(94r)eBr(Pqrl+tzgd) t CH3(CHz)'C-CH

H2

cH3(cHz)oc-ccHzcH2cH(cHr)z Lindlar catalvst t

l-bromoheptane

NaNHr- L

NHr (liq)

cH3(cH2)q cH'cH2cH(cH3)2\/C:C/\HH

BTCHzCHzCH(CHr)z

(14,29e)

568 CHAPTER 14 o

14,9 PHEROMONES

THE CHEMISTRY OF ALKYNES

14.23 Outline a synthesis of each of the following compoundspounds containing five or fewer carbons.(a) CHTCH'CH'CH2CH2CH2CH2CH2CH2-OH (b)

l-nonanol

from acetylene and any other com-

otl

HrC-C-(CHz)sCH:2-undecanone

As Problems 14.24 and 14.25 on p. 664 illustrate, the chemistry of alkynes can be applied tothe synthesis of a number of pheromones--rchemical substances used in nature for communi-cation or signaling. An example of a pheromone is a compound or group of compoundsthat the female of an insect species secretes to signal her readiness for mating. The sex attrac-tant of the female Indian meal moth (Plodia interpunctella, a common pantry moth in theUnited States) is such a compound:

?H. 6Hz, CH2CH2CH2CH2CH2CH2CH2CH2-O-C-CU'\/\/C:C C:C/\/\H:C HH H

(9 Z,l2E) -9,12 -tetradecadienyl acetate(mating pheromone of the female Indian meal moth)

Pheromones are also used for defense, to mark trails, and for many other purposes. It was dis-covered not long ago that the traditional use of sows in France and Italy to discover buriedtruffles owes its success to the fact that truffles contain a steroid that happens to be identical toa sex attractant secreted in the saliva ofboars during premating behavior!

About three decades ago, scientists became intrigued with the idea that pheromones mightbe used as a species-specific form of insect control. The thinking was that a sex attractant, forexample, might be used to attract and trap the male of an insect species selectively without af-fecting other insect populations. Alternatively, the males of a species might become confusedby a blanket of sex attractant and not be able to locate a suitable female. When used success-fully, this strategy would break the reproductive cycle of the insect. The harmful environmen-tal effects and consequent banning of such pesticides as DDT stimulated interest in suchhighly specific biological methods.

Although experimentation has shown that these strategies are not successful for the broadcontrol ofinsect populations, they are successful in specific cases. For example, local infesta-tions of the common pantry moth can be eradicated with commercially available traps that uti-lize the female sex attractant (Fig. A.7).In large-scale agriculture, traps employing matingpheromones are useful as "early-warning" systems for insect infestations. When this approachis used, conventional pesticides need be applied only when the target insects appear in thetraps. This strategy has brought about reductions in the use of conventional pesticides by asmuch as 70Vo in many parts of the United States. Sex pheromones, aggregation pheromones(pheromones that summon insects for coordinated attack on a plant species), and kairomones(plant-derived compounds that function as interspecies signals for host plant selection) areused commercially in this manner. Attractants for more than 250 different species of insectpests are now available commercially.

14.9 PHEROMONES 669

Figure 14.7 An infestation of the Indian meal moth, a common pantry moth. is controlled with a commerciallyavailable trap.The pad on the trap is impregnated with the female mating pheromone. Males attracted to thepheromone are immobilized and die on the sticky surface of the trap.The mating cycle of the moth is thus bro-ken.Through use of these traps, fumigation ofthe pantry with insecticides is unnecessary.

l4.U ln the course ofthe synthesis ofthe sex athactant ofthe grape berry moth, both the cis andtrans isomers of the following alkene were needed.

^I

CH3CH2CH: CHCH2CH2CH2CH2CH2CH2CH zCHz-O { O)(a) Outline a synthesis of the cis isomer of this alkene from the following alkyl halide and any

other organic compounds.

Br - CHZCH2CH2CH2CH2CH2CH2CH z-." -l- ,-, )

(b) Outline a synthesis of the trans isomer of the same alkene from the same alkyl halide andany other organic compounds.

14.25 The following compound is an intermediate in one synthesis of the mating pheromone of thefemale Indian meal moth (structue on p. 668). Show how this compound can be convertedinto the pheromone in a single reaction.

otlH2- Q -C- CHzCH2CHzCH2CH2CH2CH2CHz- O -C- CH'HC\/C:C/\HrC H

pad impregnated withmating pheromone

67 O cHAprER 14 . THE cHEMtsrRy oF ALKyNES

14. 10 OCCURRENCE AND USE OF ALKYNESNaturally occurring alkynes are relatively rare. Alkynes do not occur as constituents ofpetroleum, but instead are synthesized from other compounds.

Acetylene itself comes from two corlmon sources. Acetylene can be produced by heatingcoke (carbon from coal) with calcium oxide in an electric furnace to yield calcium carbide,CaCr.

cao + 3c heat > cac2 + co (14.30)

*'il gfiffi -T;:r?"Calcium carbide is an organometallic compound that can be regarded conceptually as the cal-cium salt of the acetylene dianion:

Q22+ 3(l:Q;calciurn carbide

Like any other acetylenic anion, calcium carbide reacts vigorously with water to yield the hy-drocarbon; the calcium oxide by-product ofthis reaction can be recycled in Eq. 14.30.

The carbide process is widely used in Japan and eastern Europe, and it may become more im-portant in the United States as the use of coal as a carbon source grows.

The second process for the manufacture of acetylene, and the predominant process used in theUnited States, is the thermal "cracking" (that is, decomposition) of ethylene at temperatures above1400 "C to give acetylene and Hr. (This process is thermodynamically unfavorable at lower tem-peratures.)

The most important general use of acetylene is for a chemical feedstock (starting mate-rial), as illustrated by the following examples:

:ffi: + HCr Hscr:'

::;::: (1431)

' (a monomer uded in the manufactureof poly(vinyl chloride), PVC)

2HC:CH catalvst > HC:c-cH:cHu HCI > Hrc:c-cH:cHz e4.32)

acetylene vinylacetylene Cl

chloroprene

I::ll"',9:'ffi:1T3.:ig$

Oxygen-acetylene welding is an important use of acetylene, although it accounts for a rel-atively small percentage of acetylene consumption. The acetylene used for this pulpose is sup-plied in cylinders, but it is hazardous because, at concentrations of 2.5-80Vo in air, it is explo-sive. Furthermore, because gaseous acetylene at even moderate pressures is unstable, thissubstance is not sold simply as a compressed gas. Acetylene cylinders contain a porous mate-rial saturated with a solvent such as acetone. Acetylene is so soluble in acetone that most of itactually dissolves. As acetylene gas is drawn off, more of the material escapes from solutionas the gas is needed-another example of Le Chdtelier's principle in action!

ADDITIONAL PROBLEMS 67 1

II

T

Alkynes are compounds containing carbon-carbontriple bonds. The carbons of the triple bond are sp-hybridized. Electrons in sp orbitals are held somewhatcfoser to the nucleus than those in spz or sp3 orbitals.

The carbon-carbon triple bond in an alkyne consists ofone o bond and two mutually perpendicular rr bonds.The electron density associated with the rr bonds re-sides in a cylinder surrounding the triple bond.The in-duced circulation of these n electrons in a magneticfield shields acetylenic protons as well as acetylenicand propargylic carbons, and results in the relativelysmall chemical shifts observed in NMR spectra.

The sp hybridization state is less stable than the sp2 orsp3 state. For this reason, alkynes have greater heats offormation than isomeric alkenes.

Alkynes have two general types of reactivity:

1. addition to the triple bond2. reactions at the acetylenic -C-H bond

Useful additions to the triple bond include Hg'*-catalyzed hydration, hydroboration, catalytic hydro-genation, and reduction with sodium in liquid am-monia.

Both the hydration and hydroboration-oxidation ofalkynes yield enols, which spontaneously form theisomeric aldehydes or ketones.

Catalytic hydrogenation of alkynes gives cis alkeneswhen a poisoned catalyst is used. When a poison isnot used, hydrogenation to alkanes occurs. The re-duction of alkynes with alkali metals in liquid ammo-nia, a reaction that involves radical-anion intermedi-ates, gives the corresponding trans alkenes.

1 -Alkynes, with pq values nea r 25,are the most acidicof the aliphatic hydrocarbons. Acetylenic anions areformed by the reactions of 1-alkynes with the strongbase sodium amide (NaNHr). ln a related transmetalla-tion reaction, acetylenic Grignard reagents can beformed in the reactions of 1-alkynes with alkylmagne-sium halides.

Acetylenic anions are good nucleophiles and reactwith alkyl halides and sulfonates in Sr2 reactions toform new carbon-carbon bonds.

I

I

I

I

t

I

F'lii

REACTTON I \ REVTEW{n} For a summary of reactions discussed in this chapter, see Section R, Chapter 14, in the Study

Guide and Solutions Manual.

11.26 Give the principal product(s) expected when 1-hexyneor the other compounds indicated are treated with eachof the following reagents:(a) HBr (b) Hz,Pd/C(c) Hz, Pd/C, Lindlar catalyst(d) product of part (c) + Or, then (CH3)2S(e) product of part (c) + BH, in THF, then HzO?l-OH(f) product of part (c) * Br,(g) NaNH, in liquid ammonia(h) product of part (g) + CH3CH2I(i) Hgt*, H2SO4,Hro(j) disiamyl borane, then HzOzl-OH

1,4.21 Give the principal products expected when 4-octyne orthe other compounds indicated are treated with each ofthe following reagents:(a) Hz, Pd/C catalyst(b) Hr, Lindlar catalyst(c) product of (b) + Or, then H'O'/FI2O(d) Na metal in liquid NH,(e) Hg2+, HrSOo,HrO(0 BH:, then HzOzl-OH

14.28 In its latest catalog, Blarneystyne, Inc., a chemicalcompany of dubious reputation specializing in alkynes,

67 2 cHAprER 14 . THE cHEMtsrRy oF ALKyNES

has offered some compounds for sale under the follow-ing names. Although each name unambiguously speci-fies a structure, all are incorrect. Propose a correctname for each compound.(a ) 2-hexyn-4-ol(b ) 6-methoxy- 1,5-hexadiyne{e } 1-butyn-3-ene(d) 5-hexyne

14.29 In each case, draw a structure containing only carbonand hydrogen that satisfies the indicated criterion.{a) a stable alkyne of five carbons containing a ring(b) a chiral alkyne of six carbon atoms(c,l an alkyne of six carbon atorns that gives the same

single procluct in its reaction either with BH, inTHF followed by HrOr/-OH or withH,o/Hg2*/H.o*

(d ) a six-carbon alkvne that can exist as diastereomers

14.30 On the basis of the hybrid orbitals involved in thebonds, arrange the bonds in each of the following sets

in order of increasing length.(a) C-H bonds of ethylene; C-H bonds of ethane;

C- H bonds of acetylene(b) C-C single bond of propane; C-C single bond

of propyne; C-C single bond of propene

14.31 Rank the anions within each series in order of increas-ing basicity, lowest first. Explain.

tal CH:CHzQ:-, HC-a:, :it:-(b) cH:(cHr)3c - ci cHrqcHr;ncHr,

cH-.(cHz)_,cH-cH

14.32 Using simple observations or chemical tests with read-ily observable results, show how you would distin-guish between the compounds in each of the followingpairs. (Don't use spectroscopy.)(a ) cris-2-hexene and 1-hexyne(b) I -hexyne and 2-hexyne(c ) 4,,4-dimethyl-2-hexyne and 3,3-dimethylhexane(cl) propyne and 1-decyne

14.33 Outline a preparation of each of the following com-pounds from acetylene and any other reagents.(a) CHTCH'CD'CD,CHTCH, (b) 1-hexene(c) 3-hexanol (d) 1-hexyne(e) O

IcH3(cH)t-c-oH

(f) (CH3)2CHCH2CH2CHTCH-O( g ) cls-2-pentene (h) trans-3-decene(il meso-4,5-octanediol ( j) (Z)-3-hexen- 1-ol

14.34 Using 1-butyne as the only source of carbon in the re-actants, propose a synthesis for each of the followingcompounds.(a) CHTCHTC:C-D (b) CHTCH2CDTCD,(c) o

tlcH3cH2cH2coH

(d) 1-butoxybutane (dibutyl ether)(e)

the racemate of (3R,4S) -CHTCH2CHCHCH2CH2CHtCH:ttDD(f ) octane (g) O

tl

cHrcH2ccH3

lrl.35 A box labeled "CuH,n isomers" contains samples ofthree compounds: A, 8,, and C. Along with the com-pounds are th.e IR spectra of A and B, shown in Fig.P14.35. Fragmentary data in a laboratory notebooksuggest that the compounds are 1-hexyne, 2-hexyne,and 3-methyl- 1,4-pentadiene. Identify the three com-pounds.

14.-16 You have just been hired by Triple Bond, Inc., a com-pany that specializes in the manufacture of alkynescontaining five or fewer carbons. The President, Mr.Al Kyne, needs an outlet for the company's products.You have been asked to develop a synthesis of thehousefly sex pheromone, m"uscalure, wrth the stipula-tion that all of the carbon in the product must comeonly from the company's alkynes. The muscalure willsubsequently be used in a household fly trap. You willbe equipped with a laboratory containing all of thecompany's alkynes, requisition forms for otherreagents, and one gross of fly swatters in case you aresuccessful. Outline a preparation of racemic muscalurethat meets the company's needs.

cH:(cHz)z (cHr),rcH:\/C-C/\HHmuscalure

14.37 Outline a preparation of racemtc disparlnre, a

pheromone of the gypsy moth, from acetylene and any

other compounds containing not more than five carbonatoms.

A(- _(-CH3(CH)i'J" -t'(CH2)4CH(CH3)2t

urrnurlure

H

14.38 In the preparation of ethynylmagnesium bromide bythe transmetallation reaction of Eq. 14.24,ethylmagnesium bromide is ad&d to a large excess ofacetylene in THF solution. Two side reactions that can

ADDITIONAL PROBLEMS 67 3

occur in this procedure are shown in Fig. P14.38,(a) Suggest a mechanism for reaction (1), and explain

why an excess of acetylene is important for avoid-ing this reaction.

(b) Suggest a mechanism for reaction (2), and explainwhy an excess of acetylene is important for avoid-ing this reaction.

(c) Tetrahydrofuran (THF) is used as a solvent becausethe undesired by-product, BrMg-C-C-MgBr,is relatively soluble in this solvent. Explain why itis important for this by-product to be soluble ifboth side reactions are to be minimized.

2.6 2.8 3 4 4.5

2.6 2.8 3 3.s 4 4.5

wavelength, micrometers55.s678

wavelength, micrometers5 5.56 7 8

t2 13 t4 1516l

800

ll 12 13 14 1516

800

l1l03.5100

a,)

H80(g

.:860(dLr:40q)Ub20o..

03800 3400 3000 2600 2200 2000 1800 1600 1400 1200 1000

wavenumber, cm- 1

l0

a.)

H80(g.:E60tr(€L{:40q.)(Jb20a

0

100

3800 3400 3000 2600 2200 2000 1800 1600 1400 1200 1000wavenumber, cm- I

FiSure P14.35 lR spectra for Problem 14.35.

(l) H-C-Q-MgBr +

(2) 2H-C-C-MgBrFlgure P14.38

CH3CHz-MgBr ---->rr* H-.-C-H +

CHTCH3 + BrMg-C-C-MgBrBrMg-C-C-MgBr

67 4 cHAprER 14 . THE cHEMtsrRy oF ALKyNES

14.39 (a) When the reduction of alkynes to alkenes by Na inliquid ammonia is attempted with a l-alkyne, everythree moles of l-alkyne give only one mole ofalkene and two moles of the acetvlenic anion:

3RC-(T' NU * RCH-CH, + 2RC=lH -Fff RCH:CHz + 2RC-Q: Na+

Explain this result using the mechanism of this re-duction and what vou know about the aciditv of 1-

alkynes.(b) When (NHo)rSOo is added to the reaction mixture,

the 1-alkyne is converted completely into thealkene. Explain.

14.40 Identify the following compounds from their IR andproton NMR spectra.(a) CuH,oO'

NMR: 63.31 (3lll, s); 52.41 (1F1, s);5 1.43 (6F1, s)

IR: 2110, 3300 cm t (sharp)

(b) C4H.O: liberates a gas when treated withCrHsMgBr

NMR: 62.43 (IH,t, J : ZHz):63.41 (3H, s); 64.10 (2H, d, J - ZHz)IR: 2 125,3300 cm-r

(c) CoHuO:

NMR in Fig. P14.40IR: 2100, 3300 cm ' lsharp), superimposed on abroad, strong band at 3350 cm-r

(d) csH6o

IR: 3300,2102, 1634 cm-INMR: 53.10 (1F1, d, J - 2Hz); 63.79 (3F1, s);64.52 (lH, doublet of doublets,J - 6HzandZHz);66.38 (IH,d,J - 6Hz)

14.41 (a) Identify the compound CuH,n that shows IR ab-sorptions at 3300 cm-r and 2100 cm-r and has thefollowing t'C NMR spectrum: 6 2l .3, 31.0, 66.7 ,

92.8.(b) Explain how you could distinguish between

1-hexyne and 4-methyl-2-pentyne by '3C NMR.

2400 2 100 r 800chemical shift, Hz

1s00 1200 900 600 300

dryA

6.7 Hz

disappearson D2O shake

43chemical shift, ppm (6)

FiSure P'|4.40 The NMR spectrum for Problem 14.40c. A trace of aqueous acid was added to the compoundbefore the spectrum was obtained.The integral (as the number of protons) is shown in red over each absorption.The absorptions labeled "dry"were obtained on a very dry sample before addition of the acid.The absorptions la-beled "wet"were obtained in the presence of aqueous acid.

ADDITIONAL PROBLEMS 67 5

14.42 Propose mechanisms for each of the following knowntransformations: use the curved-arrow notation wherepossible.

(a)

Ph-C-CH +

oBrz Hzo t Ph-A-cH2Br

+ HBr

NaOD,DzO (large excess), ph_e_C_D

THF

ooilllHOC-CHzCHz-COHsuccinic acid

ollH-COH

formic acid

14.43 A compound A (C6H) undergoes catalytic hydrogena-tion over Lindlar catalyst to give a compound B, whichin turn undergoes ozonolysis followed by workup withaqueous HrO, to yield succinic acid and two equiva-lents of formic acid. In the absence of a catalyst poi-son, hydrogenation of A gives hexane. Propose a struc-ture for compound A.

14.44 An optically active alkyne A (Cr'Hr+) can be catalyti-cally hydrogenated to butylcyclohexane. Treatment ofA with C2HsMgBr liberates no gas. Catalytic hydro-genation of A over Pd/C in the presence of quinolinepoison and treatment of the product with O, and thenHrOrgives an optically active tricarboxylic acidCsHr2Oo. (A tricarboxylic acid is a compound with

H"i;|"?'jl il::il1,:i:: the structure orA' and

14.45 Complete the reactions given in Fig. P14.45 usingknowledge or intuition developed from this or previouschapters.

(b)

Ph-Q-C-H

(a) cH3cH2cH zcH2c-cH cHrcHzMgBr t DzO t

(b) CH3CH2CHzCHz-C-Q-H + CH3CH2CH2CHz-Li € (CH3)3SiCl -

-

(Hint: Tertiary silyl halides, unlike tertiary alkyl halides, undergo nucleophilicsubstitution reactions that are not complicated by competing eliminationreactions.)

(c) otlCH3CH2-O-S-O-CHzCHrA

cH3(cH z)o-.c-cH NaNHz t diethylsulfate t(Hint: See Sec. 10.3C.)

(d) ri-c-cH + F-(cHz)s-cl -*( I equivalent)

(e)Ph-cH:cHz + Brz 1 NaNHz t(Hint: See Sec. 9.5)

(f) p'_c-e_ph + HCCI3 K+(cHr)rco- ,(Hint: See Sec. 9.8A)

Figure P14.45

neutralize(H:o+) (a hydrocarbon not

containing bromine)