Chemistry 125: Lecture 54February 21, 2011

AcetylenesAllylic Intermediates & DienesLinear and Cyclic Conjugation

(4n+2) Aromaticity This

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Generalization to

Acetylenes

e.g. J&F Sec. 10.6-10.11 pp. 444-455

Stepwise / Markovnikov

“Keto-Enol Tautomerism”Regioselection

Addition of HBr

Addition of H2O

Addition of H2 Stepwise / Stereoselection

Acidity and base-catalyzed isomerization

Stepwise Addition of HBr to Alkyne

1-Hexyne + HBr 2-Bromo-1-hexene

FeBr3

15°C

with “inhibitor”to trap radicals isolated in 40% yield

100 to 1000x slower than comparable ionic addition to alkene, because vinyl cation is not so great.

CH3-CH2-Cl CH3-CH2+ + Cl-

Gas Phase Ionization

193 kcal/mole

CH2=CH-Cl CH2=CH+ + Cl-225 kcal/mole

Stepwise Addition of HBr to Alkyne

1-Hexyne + HBr 2-Bromo-1-hexene

FeBr3

15°C

with “inhibitor”to trap radicals isolated in 40% yield

HBr can add again to the bromoalkene (obviously more slowly) to give a second Markovnikov addition

If the bromo substituent slows addition to an alkene, why is there Markovnikov orientation?

2,2-Dibromohexane

Br is a “schizophrenic” substituent: both electron withdrawing (), and electron-donating ().

Hydration and Hydrogenation

of Alkynes

+Hg(OAc)2

H+ / H2OHC CR +

HC CR

HgOAc HgOAc

CO

R

CHH2O -H+H+

NaBH4

CO

R

CH H

H

H

Markovnikov EnolH

+

H

Ketone

an easy allylicrearrangement

“Keto-Enol Tautomerism”

+

(favors ketoneCf. Lecture 37)

ve Bond Energies

Can one sum bond energies to getaccurate"Heats of Atomization"?

H C

O

H

CCH H

H

H

H C

O

H

CCH H

H

HKetone "Enol"

C

O

CH

C

O

C

H

C=O 179

C-C 83

C-H 99

sum 361

C-O 86

C=C 146

O-H 111

sum 343

Kcalc = 10-(3/4) 18 = 10-13.5

Kobs = 10-7 = 10-(3/4) 9.3

Bonds that change(the others should cancelin taking the difference)

H C

O

H

CCH H

H

H

H C

O

H

CCH H

H

HKetone "Enol"

H

Why is Enol9 kcal/mole

"Too" Stable?

O

C=O 179

C-C 83

C-H 99

sum 361

C-O 86

C=C 146

O-H 111

sum 343

Kcalc = 10-(3/4) 18 = 10-13.5

Kobs = 10-7 = 10-(3/4) 9.3

••

C(sp2)-Hstronger than

C(sp3)-H

(they shouldn’t actually cancel)

IntramolecularHOMO-LUMO

Mixing

H C

O

H

CCH H

H

H+"ResonanceStabilization”

from

Markovnikov Enol

+Hg(OAc)2

H+ / H2OHC CR +

HC CR

HgOAc

CO

R

CH

H2O

-H+H +

H

Ketone

R’2B-HHC CR C

R

R’2B

CH

H

Anti-Markovnikov Enol

Aldehyde

HOOH

HO-

CR

HO

CH

H

H

vinylborane(hindered R’2BHadds only once)

BH3 + 2

e.g. “disiamylborane”

Hydration with Either Regiospecificity

(what is R’?)

R-C C-R

Hydrogenation with Either Stereospecificity

( Pd / CaCO3 / Pb )

H2

Lindlar CatalystC

R

H

C

H

R

deactivate Pd to stop at alkene

n-Pr-C C-n-PrNa / NH3

C

n-Pr

H

CHn-Pr

“dissolving metal reduction”

syn addition

H H

anti addition

H

H

97% forR = (CH2)3CO2CH3

80-90%

solvated electronNa

NH3

e-(NH3)n+ Na+

R-C C-R

First H+

R-C C-R

R-C C-R e-First e-

C C

R

RH

C C

R R

H

Vinyl radicals are sp2

but they invert easily

H

NH2NH2

Second H+ e-H

NH2

NH2

C C

R

RHVinyl anions are sp2

and invert very slowly(remember XH3)

Second e-

C C

R

RH

C C

R R

H

Vinyl radicals are sp2

but they invert easily

C C

R

RH

H

anti addition(because of radical isomerism)

H

H

Alkyne Acidity and Isomerization

e.g. J&F Sec. 12.4 pp. 516-518

Approximate “pKa” Values

CH3-CH2CH=CHH ~ 44

CH3-CH2C CH ~ 25

CH3-CH=C=CHH

CH3-C C-CH2H ~ 38

sp3 C_

sp2 C_ (no overlap)

sp C_ (no overlap)

C_ HOMO - overlap

CH3-CH2CH2CH2H ~ 52

~ 34 H2NH

= 16 HOH

(better E-match N-H)

(bad E-match O-H)

(best E-match C-H)50

40

30

20

10

pKa

*

:

:

(allylic)

(e.g. J&F Acidity of 1-Alkynes Secs. 3.14 p. 129; 12.4 p. 516-518)

H+(aq) +

Equilibrium & Rate

kcal

/mol

40

30

20

10

-10

50

0 CH3-CH=C=CH2

CH3-C C-CH3

CH3-CH2C CH

CH3-CH2C C

CH3-CH=C=CHCH3-C C-CH2

pKa 38

Ka 10-38

G 4/3 38 = 51

pKa 25

Ka 10-25

G 4/3 25 = 33

4.1 4.8

0.1% 0.03%

k 1013 10-38 /sec

t1/2 = 0.69/k 1025 sec = 1017 yrs 104 time since Big Bang

[0]

at equilibrium

H+(aq) +

+ HO-

favors dissn. by 21 kcal

(4/3 16)

Equilibrium & Rate

kcal

/mol

40

30

20

10

-10

50

0 CH3-CH=C=CH2

CH3-C C-CH3

CH3-CH2C CH

CH3-CH2C C

CH3-CH=C=CHCH3-C C-CH2

t1/2 30 yrs @ 300K

-7.20.0001%

2 min @ 150°C + H2N

-

favors dissn. by 45 kcal (4/3 34)

at equilibrium

Trick to obtain terminal acetylene:

Equilibrate with RNH_

base(in RNH2 solvent at room temp)

to form terminal anion.“Quench” by adding water which donates H+ to terminal anion and to RNH_, leaving OH_, which is too weak to allow equilibration.Or add H+, so even [OH

_] is very low.

C C

Conjugation & Aromaticity

Conjugated Pi Systems

OC

Yoke

Jungere

Jugóm

(to Join)

e.g. J&F Ch. 12-13

The Localized Orbital Picture(Pairwise MOs and Isolated AOs)

Is Our Intermediate betweenH-like AOs and Computer MOs

When must we think more deeply?

When does conjugationmake a difference?

Experimental Evidence

Allylic Stabilization:Cation

R-Cl R+ + Cl-(gas phase kcal/mol)

Cl

Cl

Cl

193

172

171

Anion

pKa

OH

OH

16

10

5OHO

Radical

Bond Dissociation

Energy (kcal/mol)

H

H

101

89

Conjugation worth ~ 13 kcal !

as good as secondary

4/3 6 = 8 kcal

Br-

-78°C

20%

80%

85%

15%

Allylic Cation Intermediates:Addition of HX to Butadiene

HBr-78°C

e.g. J&F Sec. 12.9-12.10 pp. 534-541

H

+

+

H

+

H

H

Br

H

BrKineticvs.

Thermodynamic Control

Reason for Kinetic

Distribution?

FeBr3

+17.6

-21.4

HOMO-4HOMOLUMO+1LUMOHOMO-1LUMO+1LUMOHOMO

Butadiene Propenyl CationH+

hyperconjugated C-H

bestoverlap

bestpotential

best productbest overlap

Propenyl Cation

+152+144

+132+99

Surface Potential

bestpotential

best potential

symmetrical(but for D)

p. 1288

3.1 : 1-78°

1.6 : 125°

Cl-

DCl

rapid ion-pair collapse competes with motion

Allylic Transition States:SN1

e.g. J&F Sec. 12.11a,b pp. 541-543

krel for solvolysis in 1:1 EtOH/H2O at 45°C

Cl<< 0.01

Cl0.01

Cl6300

Cl43

Cl0.05

Cl

0.07

Cl[100]

Cl39

++

methylation is effective where charge is (C1,C3)

Allylic Transition States:SN2

e.g. J&F Sec. 12.11a,b pp. 541-543

krel for Displacement by EtO- in EtOH at 45°C

Cl[1]

Cl560

Cl97

Cl37

Cl

33

Cl1.9

Allylic Anion Intermediates:RH Acidity

allylic

benzylic

e.g. J&F Sec. 12.11d pp. 543-544 and Sec. 13.12

pKa ~52

HCH2

pKa 43

HCH2

pKa 41

HCH2

4/3 x 9 ≈ 11 kcal/mole

4/3 x 12 ≈ 16 kcal/mole

Allylic Free-Radical Intermediates:Allylic Bromination

Cf. J&F Sec. 11.8 pp. 497-500, Sec. 12.11c p. 543

N-Bromosuccinimide(NBS)

N

O

O

Br

58% yieldK. Ziegler (1942)

Et2O30 min.

h

Ionic Preparation and Destruction of NBS

pKa 9.5

N

O

O

HNaOH

0°C N-

O

O

N

O

O

BrBr2

+ NaBr

N

OH

O

Br+

N

OH

O

undo with HBr

N

OH

O

Br

+

Br-

“enol” to “ketone”

Br-

+ Br2

BrBr

How to controlAddn. vs. Subst.?

Rate [Br2]2

Br2 helps Br- leave from “Br+” in nonpolar solvent

(like protonation of OH)

Whenever a Br2 molecule is consumed, one new Br2 molecule is created.

Allylic Reactivity - Radical

Automatically maintains minimal [Br2].

+ Br2 + Br-Br-Br-

Br+

CH2Cl2

25°C Dark

H

H

or H

Br

Br

H

Br2 + Br

N

O

O

Br N

O

O

H

Addition

Substitution

2

HBr•

initiator(h, peroxide, etc.)

Keep dark

+ HBrslow

(selective)

or minimize [Br2] (tedious to impossible?)

Cl• also attacks this CH2 group

Conjugation worth

~7 kcal

Conjugation worth

~8 kcal

Hcombustion

768.9 ±0.3

761.6 ±0.2

17.7

25.4

Hformation

Diene Stabilization:

Conjugation worth ~ 4 kcal/mole

Hhydrogenation

(kcal/mole)

-30.2

-29.8

-30.0

-56.5

-60.4

-60.0

End of Lecture 54February 21, 2011

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