ALKYNESGeneral Methods of Preparation:

1. Dehydrohalogenation of Vicinal and Geminal Dihalides:(A) Vic- Dihalides:* E2 elimination will occur two times in a vicinal or geminal dihalide to form alkyne. Firstone molecule of HX is eliminated with fist mole of the base to form vinyl halide and with 2ndmole of base, one more HX molecule is eliminated to form alkyne. So it is a double elimination.* Alcoholic KOH or KOEt gives a slow reaction, particularly in the second step hencestronger base like NaNH2 in liq. NH3 is used for the purpose. Liq. NH3 is merely the solvent.Moreover, the former reagents will require higher temperature at which the terminal alkyne willisomerise to internal alkyne. 1,2-dibromobutane with the former reagents will give but-2-yne instead of but-1-yne. The latter reagent(NaNH2 in liq. NH3) uses lower temperature, which doesnot catalyse the isomerisation process. We get but-1-yne as expected from the reactant.* For sythesizing terminal alkynes three moles of NaNH2 are required, as after the formationof some alkyne, the second mole of NaNH2 will immediately engage in removing the acidic Hatom from the terminal alkyne and hence further elimination to complete the reaction will notbe possible. Hence the third mole is used which solves the purpose. In such the alkynide saltsis produced at the end which needs aqueous work-up as the additional step to get neutral alkyne.But for preparing internal alkynes two moles of NaNH2 are used.

R CH

Br

CH

H

Br

NaNH2 R C CH

Br

H

NaNH2

R C CH-HBr

-HBr

R CBr

CH

H+(I)

(II)

NaNH2R C C-Na+

H2O

R C CH

In the first step, all possible vinyl halides are formed. In the above example, (I) is major, asterminal –CH2Br hydrogen is more acidic than the intenal –CHBr– hydrogen. I have not shownanother vinyl halide in the other side(to the side of R), because that will not give alkyne, insteadit will form allene(which is very unstable).In the 2nd step the vinyl chloride undergoes the second dehydrohalogenation to form alkyne.* For 2,3-dihalobutane, two moles of NaNH2 are required to get but-2-yne, as there is noacidic H atom here to change the track of the reaction. Alkynes are best prepared from thecorresponding alkenes, as vicinal dihlaides are easily obtained form alkenes, rather than alkanes.

CH3 CH CH CH3Br2/ CCl4 CH3 C CH3C

Br

Br

H

H

NaNH2(2 moles)

l iq. NH3

CH3 C CH3C

NH3 NaBr2 2

+

+

Alkynes

S S Tripathy 1

N.B: From 2,3-dihaloalkanes, conjugated diene is the coproduct of alkyne.* Vinylic halides can eliminate by E2 mechanism by NaNH2 in liquid ammonia to form

alkynes.

CH2 CH

ClNaNH2(two moles)/l iq. NH3

H2O

(i)

(ii)CH CH

vinyl chloride

(B) Gem- dihalides:* This is mostly an academic exercise, as we cannot prepare a gem- dihalide easily

like vic-dihaldies. One of the methods of preparing gem-dihalides is to add two moles of HXwith alkyne. In such case, there is no point in getting back the same alkyne by double elimiationprocess. The other method is the reaction of aldehyde and ketone with PCl5. In such case, thegem-dihalide can be used to prepare alkyne.

* The two step elimiations occur in gem- dihalide too, when react with NaNH2 inliquid ammonia. in the same way as we found for vic-dihalides.

(2) Double Dehalogenation of tetrahaloalkanes:* This is mostly for academic exercise, because a tetrahaloalkane is primarily prepared

from the addition of two moles of X2. Then what is the fun of getting back the same alkyne bydehalogenation process. However, let us study the process for fun.

R C

X

X

CH

X

X

R C CH + 2 X2Zn/EtOH

heat

tetrahaloalkane

When tetrahaloalkane is distilled in ethanolic zinc, we get alkyne.(3) Kolbe’s Electrolytic Method:

* This is mostly to prepare ethyne(acetylene) from the electrolysis of aq. potassiummaleate(HOOC–CH=CH–COOH), which is an unsaturated dicarboxylic acid.

* Like the Kolbe’s electrolytic process for alkanes and alkenes, at anode we get ethyne.Already a C=C is present in maleate ion and one more bond joins between C=C to make it atriple bond.

Alkynes

S S Tripathy 2

HC CH C

O

O - K+C

O+K -O

potassium maleate

electrolysis

HC CH C

O

O-C

O-O

At anode:

HC CH + 2 CO2 + 2 e

At cathode, as usual, H2 is evolved.

(4) Acetylene from calcium carbide :Calcium carbide is an ionic compound with an inbuilt triple bond in carbide ion(C2

2–). When itreacts with water at room tempeature, vigourous reaction takes place to produce acetylene gas.This gas is responsible for artificially ripening of the fruits. We often say, repined by cardies.

Ca2+C

C

calcium carbide

+ 2 H2O

H+OH -

H+OH -

HC CH + Ca(OH)2

(5) Acetylene from Iodoform:* Iodoform reacts with moist and finely divided silver to form acetylene. Heating is not

required.

HC CH

HC

I

II + 6 Ag CH

I

II+

heat

+ 6 AgI

(6) Higher alkynes from Acetylene:Acetylene is treated with Na in liq. NH3 to form sodium acetylide(see later) which makes

SN2 reaction with 10-halide(bromide is preferable) to form a higher acetylene homologue.

CH CH Na(l iq. NH3)

CH C-Na+ R BrCH CR

Na / liq. NH3

Na+ -C CRBr R'

R'C CR

By this method we can prepare propyne by reacting sodium ethynide(acetylide) with CH3Br. Wecan prepare but-1-yne, by reacting the ethynide salt with Et–Br. With two successive salt formationand reaction with two moles of CH3Br will form but-2-ene, as the above scheme shows.

Alkynes

S S Tripathy 3

PROPERTIES:

Physical:* C2–C4 are gases, C5–C12 are liquids and higher memebers are solids.* Like alkanes and alkenes, they are sparingly soluble in water but highly soluble in nonpolaror weakly polar organic solvents.* MP/BP: They have higher bp/mp than the respective alkanes. This may be due to greaterpolarisability of the triple bond. While alkenes have lower bp than alkanes, alkynes have higherbp than alkanes.

– BP/MP increase with increase in the nmber of carbon atoms. In BP, the increasingtrend is bit systematic(approx. linear), but in MP, the trend is erractic.

Alkyne MP(0C) BP(0C)Ethyne – 82 –75Propyne –101.5 –23But-1-yne –122 +9But-2-yne –24 +27Pent-1-yne –98 40Pent-2-yne –101 55Hex-1-yne –124 72Hex-2-yne –92 84Hex-3-yne –51 81Hept-1-yne –80 100Oct-1-yne –70 126Non-1-yne –65 151Dec-1-yne –36 182

You can see in the MP column, alkynes longer than C4 have lower values than but-2-yne(–240C).The trend of MP is highly erratic. But BP shows the increasing trend nearly systematically.* Polarity: The symmetrical alkynes like ethyne, but-2-yne are nonpolar having zero dipolemoment. However unsymmetrical alkynes like propyne, but-1-yne, pent-1-yne etc are polar,though weakly. They are more polar than their alkene counterparts. That is the reason for theirrelatively greater bp/mp than alkanes and alkenes.

But-1-yne has a dipole moment of 0.80D as against 0.37D for but-1-ene. That is thereason why alkynes have greater MP/BP than the corresponding alkenes and alkanes.

CHEMICAL:(A) Addition Reactions:

Addition of electrophilic reagents to C C of alkynes is slower than that to C=C in

alkenes. This is because of (a) much higher C C bond energy and (b) lower stability of vinylcarbocation as compared to alkyl carboncation.

(1) With H2:[For details of mechanism. refer GOC-Part-III(mechanism)](a) Uncontrolled hydrogenation: In presence of Ni or Pd or Pt, hydrogen can add

in a controlled way to form alkane, even with one mole of H2.Pd/C can be used at lower temperature for the same purpose.

R C C R'H2

Ni/ 3000CR CH2 CH2 R'

Alkynes

S S Tripathy 4

(b) Controlled Hydrogenation:SYN Addition: Internal alkynes react with H2 in presence of Lindlar’s catalyst(Pd-

BaSO4 or Pd-CaCO3 poisoned by quinoline-lead acetate) to form Z-alkene.P-2 catalyst(Ni2B) can also catalyse Syn hydrogenation.

CH3 C C CH3 H2Pd CaCO3

quinoline/lead acetate

- CH3 CH3

H H

Z(cis)-but-2-ene

+ 36.7 Kcals+

ANTI Addition: Na/liq. ammonia carries out anti hydrogenation to form E-alkene.

CH3 C C CH3

CH3 H

H CH3

E(trans)-but-2-ene

Na/liq.NH3

[2H]

[N.B: If you are impatient to look to all the mechanisms, then halt here and refer GOC-III(mechanism)]

(2) With X2(Halogenation):* Cl2/Br2 in CCl4 or CH2Cl2 add up to alkyne first to form vic. dihalo alkene and then

tetrahalo alkane.* The first step addition is ANTI stereoselective, meaning anti addition product gives the

major product.

CH3 C CH X2CH2Cl2

CH3

C CX

X

H

X2CH3 C C

X

X

H

X

X anti -stereoselectivityvicinal dihaloalkene tetrahaloalkane

+

* Aq. X2 adds analogous to HOX, as H2O competes with halide ion in the 2nd step ofmechanism.

* Mechanism is similiar to that found in alkene halognetion, i.e formation of bromoniumion cyclic intermediate. Only difference is that the reaction in case of alkenes was stereospecificwhile here it is stereoselective. Partly it goes via the formation of vinyl carbocation.

(Please refer the mechanism in GOC-III)*Even, ethyne adds with one mole of Br2 in CCl4 to form E-1,2-dibromoboethene.

Br Br

- Br BrH

CBr

C

Br

HE-alkene

CH CH CH CH

Br

HOOC C C COOH Br2 C C

Br

COOH

HOOC

Br(70 % trans product)

+

but-2-ynedioic acid with one mole of Br2 gives 70% E-alkene.

Alkynes

S S Tripathy 5

(3) With HX :* Addition of HX to alkynen is Markonikoff regioselective and ANTI stereoselective.

R C CH HX R C

X

CH2HX R C

X

X

CH3+

Markonikoff Regioselectivity

CH3 C C CH3 HCl C CCH3

Cl

H

CH3

+

Anti- stereoselectivity

HCl CH3 C

Cl

Cl

CH2 CH3

Markonikoff RegioselectivityZ-alkene

The mechanism goes predominantly via the hydronium ion cyclic intermeidate(like bromoniumion), though one can argue in favour of formation of vinyl carbocation.

( For more on mechanism, refer GOC-III)(4) with H2O :* Markonikoff addtion of H2O to a triple bond gives first an enol, which immediately

tautomerizes to its stable keto form. Acetylene only gives aldehyde(acetaldehyde) and otheralkynes give ketones.

* This reaction is catalysed by dil. H2SO4 along with Hg2+ salt(HgSO4) on mildwarming conditions( 60–800C)

R C CH H OH HgSO4/H2SO4

800CCH3 C

OH

CH2 R C

O

CH3

enol form keto form

H C CH H OH HgSO4/H2SO4

800CH C

OH

CH2 H C

O

CH3

enol form keto form(acetaldehyde)

It comes under electrophilic addition reaction as a +ve ion is formed in the rate determining state.(For details of mechanism, refer GOC-III please)

(5) With HOX :* Marknokoff addition of HOX with alkyne takes place in two steps followed

dehydration to form gem. dihalo ketone.* With one mole of HOX, an enol is formed which tautomerises to monohalo

ketone.

Alkynes

S S Tripathy 6

With 2 moles of HOX:

R C CH R C CH

OH

R C CH

OH

O H

+ HO XMarkonikoff

addn.

+- HO X+-X

X

X

- H2OR C CH

O Cl

Clgem. dihaloketone

With 1 mole of HOX:

R C CH HOX R C

OH

CH

X

R C

O

CH2

X

+

Acetylene with HOCl: Acetylene reacts with two moles of HOCl to form dichloroethanal.Shown below the step-wise mechanism to justify as to why the second OH group is to join thesame carbon to which the frist OH group had joined. The carbocation form adjacent to -OH willbe more stable due to +M effect than carbocation adjacent to -Cl. Due to larger size of Cl atom,its +M effect is dominated by –I effect. If you restrict to one mole of HOCl, then we shall get2-chloroethanal after tautomerisation. See below.

CH CH Cl OH CH CHCl

OH-

CH CH

Cl

OH

Cl OH

CH

OH

CH

Cl

Cl

HO-

CH

OH

CH

Cl

Cl

OHH2OC

O

H CHCl2-

OH-

OH-

-

-

Res. stabi lisedcarbocation

Dichloroacetic acid(Cl2CHCOOH) is also formed by the oxidation of some dichloroacetaldehydeby HOX.N.B: Cl2/H2O can be used for the purpose.

Alkynes

S S Tripathy 7

(6) With O3(Ozonolysis):

* Alkyne reacts with ozone to form an unstable ozonide which further reacts withwater to form -dicarbonyl compound and H2O2. The former is oxidised by the latter to form twomoles of carboxylic acid.

* Acetylene, exceptionally, gives a mixture of -dicarbonyl compound(gyloxal) andits oxidation product formic acid.

C CR + O3 C CR

O O

Oozonide

H2O

C CR

O O

R' R'

R'

-diketone

+ H2O2

OH OH

C OHR

O

C OHR'

O

+

carboxylic acids

C CH HO3

HC CH

O

O OH2O

H C

O

C

O

H H2O2

HCOOH2

+glyoxalacetylene

acetyleneozonide

(7) Anti-Markonikoff Addition of HBr (Peroxide Effect) :* HBr adds on to a terminal alkyne in anti-Markonikoff way like alkenes in presence of aperoxide.* This goes via the free radical mechanism in the same way as we found in alkenes,excepting first the vinyl free radical is formed and then alkyl free radical.

R C CH + H Brperoxide+-

RHC CH

Br H Br

+-

RH 2C CH

Br

Brgem . dibrom ide

(8) Controlled Addition to Acetylene:(A) Electrophilic Additions:

(i) Acetylene adds onto one mole of HCl in presence of Hg2+ salt catalyst to form vinylchloride, which is an important monomer to prepare PVC.

HC CH + H Cl+ -

H2C CH650C

Hg 2+Cl

vinyl chloride

Alkynes

S S Tripathy 8

(ii) Acetylene reacts with HCN in presence of Cu2Cl2 in presence of HCl to form animportant monomer, acrylonitrile.

HC CH + H CN+ -

H2C CH

CN

vinyl cyanide(acrylonitrile)

Cu2Cl2

(iii) It reacts with acetic acid in presence of Hg2+ salt catalyst to form vinyl acetate,another important monomer. In this case some, ethylidene acetate is also formed, as the additionto alkene also occurs partly in the second step. Hence vinyl acetate is not manufactured by thismethod.

HC CH+-

H2C CH

OCOCH 3

vinyl acetate+ CH3COO H

Hg2+

CH2 CH

OCOCH3

CH3COO H+-

CH3 CHOCOCH3

OCOCH3

+Hg2+

ethylidene acetate

(N.B: Ethylidene acetate on heating gives acetic anhydride and acetaldehyde).(B) Nucleophilic addition:Alkynes can undergo AdN because vinyl carbanion it produces is more stable than alkyl

carbanion obtained from alkenes.

CH CH HCN CH2 CHCN

+acrylonitri le

NaCN

A small and catalytic amount of NaCN is added to intiate the nucleophilic attack. SubsequentlyHCN produces CN– after donating the proton to the carbanion.

CH CH HOEt CH2 CHOEt

+KOEt

1500C ethyl vinyl ether

Here also a small KOEt is added to inititate the attack.

(9) Hydroboration-Oxidation:* Terminal alkynes react with sterically hindered disiamylborane [(Sia)2BH} followed

by oxidation by H2O2/OH– to form aldehyes. This is very special reaction to form aldehyde froma terminal alkyne as Hg2+ catalysed hydration gives ketones.Sia2BH is preferred to BH3 as theaddtion will occur once, not second time for steric reasons.

* Unsymmetrcial internal alkynes give a mixture of two ketones in this reaction.Only symmetrica alkynes give single ketone. This is not popular reaction as Hg2+ catalysedhdyration gives same products more easily.

Alkynes

S S Tripathy 9

CH3 CH

CH3

CH

CH32

BH

disiamylborane

Siamyl = sec-isoamyl

R C CH (Siamyl)2BH R C CH

H B(Siamyl)2H2O2/OH-

R C CHH OHOH

OH

HO B(Siamyl)2-

R CH2 CHO

tautomerisation

The addition of the borane takes place via a cyclic TS and hence addition mechanism is SYN.But this has no significance here as we get an aldehyde

(10) With aq. KMnO4 or cold dil. alkaline KMnO4(Baeyer’s Reagent):* Alkyne reacts with cold aq. KMnO4 or with Baeyer’s reagentn to form an -diketone viaan unstable tetrahydroxy compound.

R C C R'aq. KMnO4

or Baeyer's reagent

R C C

OH

OH

OH

OH

R' R C

O

C

O

R'H2O-2

Acetylene will first give gyloxal(dial) which is further oxidised to oxalic acid.

HC CH + 2 [O]dil. alk.

KMnO 4HC CH

O O

glyoxal

[O]

C C

O O

HO OHoxalic acid

N.B: SeO2 converts alkyne to -dicarbonyl compound. Aldehyde group not undergo furtheroxidation to -COOH group in this method.

R C C HSeO2

R C

O

C

O

H

Alkynes

S S Tripathy 10

Oxidative Cleavage by hot acidified or alkaline KMnO4 :

Alkynes udergo oxidative cleavage at the triple bond analogous to akenes, to form two carboxylicacids with hot acid/basic KMnO4. Formic acid further oxidises to CO2.

C C

C OHR

O

C OHR'

O

+

carboxylic acids

R R' + H2O + 3[O]hot acid

KMnO4

Acetylene gives only CO2 and H2O.(B) ACIDIC PROPERTIES (SUBSTITUTION Reactions):

* Alkynes are weak acids. H-atom bonded to sp-hybrid carbon is acidic, which ismore acidic than NH3 but less acidic than alcohols(ROH). Hence acetylene(ethyne) and allterminal alkynes(propyne, but-1-yne, pent-1-yne etc.) have acidic H atoms. Acetylene has twoacidic H atoms while all terminal alkynes have one acidic H atom.

H C C H CH3 CH2 C C H

acidic acidic* Such acidic H atoms are displaced by active metals to form ALKYNIDE salts.(i) Formation of Sodium alkynides:

HC CHNa/liq. NH3 HC C Na

monosodium acetylide

Na/liq. NH3

Na C C Nadisodium acetylide

+ 1/2 H2

+ 1/2 H2

Ethyne reacts with one mole of Na in liq. NH3 to form monosodium ethynide(acetylide)and hydrogen gas. With 2nd mole of Na, it forms disodium ethynide(can be called sodiumcarbide).

* Other terminal alkynes need NaNH2 for the formation of alkynide salt.Na/liq NH3 is notused for the purpose.

* Alkynide ions can be reprotonated back to alkyne by H2O.

Alkynes

S S Tripathy 11

You know that a stronger acid(H2O) can displace a weaker acid (alkyne) from the salt of weakeracid. Conversely you can say that a stronger base, the alkynide ion can displace a weaker baseOH– from water.Some Reactions of Alkynide Salts:

(1) SN2 reaction with 10 halide:

C C NaR + H2C X

R'

C CR CH2 R'

+ Na X

Being a strong base and nucleophile, alkynide ion cann undertake SN2 in 10-substrates to givehigher alkynes. Note that with 30-halide it will undertake -elimination reaction(E2) to formalkene from the alkyl halide and itself converted to the original alkyne(discussed in Haloalkanechapter)

(2) AdN reaction with carbonyl compounds:

C C NaR + R' C

O

R''

C CR C R'

R''

O NaH2O/H+

C CR C R'

R''

OH

alkynol

Alkynide ion adds onto aldehyde or ketone to form alkynol. This is nucleophilic addition(AdN)reaction to be discussed later.

(3) AdN reaction with CO2:

C C NaR + O C

C CR C ONa

OH2O/H+

O

C CR C OH

O

alkynoic acid

Alkynide ion adds onto CO2 followed by hydrolysis to form alkynoic acid.(4) AdN with epoxides:

C C NaR +

C CR CH 2H 2O /H+

H2C CH

O

R'

CH

ONa

R'

C CR CH 2 CH

OH

R'

Alkynes

S S Tripathy 12

Alkynide ion adds onto the less hindered position of alkene oxides(epoxy alkanes) to formalkynols.

(ii) Formation of Copper and Silver alkynides:(a) With ammoniacal cuprous chloride (Cu2Cl2/NH4OH)

HC CHCu2Cl2/NH4OH

C CCu Cu(Cu2C2)

cuprous acetylide or dicopper acetylide(red ppt.)

+ NH4Cl + H2O

Acetylene reacts with ammoniacal solution of cuprous chloride to form the red precipitate ofcuprous acetylide. Both the acidic H atoms leave in this case together.

(b) Reaction with ammoniacal AgNO3(Tollen’s reagent)

HC CH C CAg Ag(Ag2C2)

silver acetylide or disilver acetylide(white ppt.)

+ NH4Cl + H2OAgNO3/NH4OH

Acetylene reacts with ammoniacal AgNO3 solution to form a white ppt of silver acetylide. Herealso both the acidic H atoms are removed.

* All terminal alkynes also give these tests. In these cases, substituion takes place at onlyone place.

CH3 C C HCu2Cl2NH4OH

CH3 C C Cucuprous propynide

CH3 C C HNH4OH

CH3 C C Agsilver propynide

AgNO3

* Internal alkynes do not give these tests as they do not have acidic H atoms.

Distinction between Alkenes and Alkynes:Any of the above tests in which acidic H atom of alkynes is substittuted can be used to

distinguish it from alkene, which do not give these tests.(iii) Reaction with Grignard Reagent and Alkyl Lithium.

HC CH + R MgX-

RH HC C MgX+-

Since alkyne CH is more acidic than alkane CH, GR abstracts a proton from acetylene and allterminal alkynes to form alkanes and alkynyl magnesium halide(GR).Alkyl lithium also behaves in the same way to form alkane and alkynyl lithium.

R- Li+ HC CR' R H R'C C- Li++ +

Alkynes

S S Tripathy 13

(C) Polymerisation Reactions:(i) Linear di and trimerisation:When acetylene is passed through Cu2Cl2 dissolved in NH4Cl solution, di- and trimerisation

produce a mixture of vinyl acetylene and divinyl acetylene. The two can be separated easily byany physical methods.

HC CH HC CH+Cu2Cl2

NH 4ClH2C CH C CH

vinyl acetylene

H2C CH C CH HC CHCu2Cl2

NH 4Cl

H2C CH C C CH CH 2

divinyl acetylene

+

Synthetic application of vinyl acetylene:Vinyl acetlene reacts with one mole of HCl to form a very important monomer called

chloroprene(2-chlorobuta-1,3-diene) which on polymerisation gives neoprene rubber(refer polymerchemistry for details).

CH C CH CH2 HCl CH2 C

Cl

CH CH2+vinyl acetylene chloroprene

(ii) Cyclic trimerisation:When acetylene is passed through red hot copper tube(6000C), benzene is formed due to

cyclic trimerisation.

3 HC CHCu

heat

benzene

CHHC

HCCH

CH

CH

hot Cu

Similarly propyne on cyclic trimerisation gives 1,3,5-trimethylbenzne(mesitylene).

CH3 C CH3 Red hot Fe

6000C

CH3

CH3CH3

(mesitylene)

Alkynes

S S Tripathy 14

N.B: Ni(CN)2, Ph3P in THP at 700C can be used to undertake the above reaction quantitatively.

In presence of above catalyst and with high pressure of acetylene gas, tetramerisation occurs togive cyclooctatetraene.

CH CH4 Ni(CN)2/Ph3P(THF)12-15 atm (cyclooctatetraene)

(iii) Linear Polymerisation:Acetylene undergoes linear polymerisation with high degree of polymerisation to give

polyacetylene. Ziegler Natta catalysts such as Ti(OPr)4 and Et3Al can be used for carrying outthe polymerisation.

HC CHncatalyst

HC CHnacetylene

polyacetylenePolyacetylene is a conductive polymer as it has conjugated C=Cs. On doping with p-type

of dopants like I2, Br2, Cl2, AsF5 its conductivity increases manyfold. It is used in microelectronics.

(D) Isomerisation:Internal alkynes on treatment with alcoholic KOH or NaNH2 isomerise to terminal

alkynes.

C CH3C CH3alc. KOH

or NaNH2

H2C CH3C CH

Terminal alkynes having -CH2- group isomerise in presence of the same reagents tointernal alkynes as well as conjugated dienes.

C CH3C CH 3alc. KOH

or NaNH2

H2C CH3C CH

HC

HCH2C CH 2

+

conjugated diene

Alkynes

S S Tripathy 15