1

Chapter 2

Strain and Stability

2

BDE

CH3-NH2

84.9

CH3-OH

92.3

CH3-I

57.2

CH3-Br

70.9

CH3-Cl

84.6

CH3-F

109.9

HCCH230.7

H2C=CH2

174.1

H3C-CH3

90.4

(CH3)3C-H

93.2

(CH3)2CH-H

95.1

CH3CH2-H

98.2

H3C-H

105.1

HCC-H132

H2C=CH-H

110

CH3CH2-H

98.2

71.182.394.5

H H H

3

Heats of Formation for AlkanesAlkane ∆Hf

o ∆∆Hfo

CH3CH3 -20.24

CH3CH2CH3 -24.84 4.6

CH3(CH2)2CH3 -30.15 5.31

CH3(CH2)3CH3 -35.00 4.85

CH3(CH2)4CH3 -39.96 4.96

CH3(CH2)5CH3 -44.89 4.93

CH3(CH2)6CH3 -49.81 4.92

4

Group Increments

5

Strain Energy in Cycloalkanes

∆Hfo

(calc)∆Hf

o (exp) ∆∆Hfo

-29.6 -29.9 0.3

-24.7 -18.3 6.4

-19.7 +6.7 26.4

-14.8 +12.7 27.5

6

Radical Stability

(CH3)3C-H

93.2

(CH3)2CH-H

95.1

CH3CH2-H

98.2

H3C-H

105.1

HCC-H132

H2C=CH-H

110

CH3CH2-H

98.2

R3C-H R3C• + •H∆Ho

7

Carbocation Stability

NCCH2+

318

FCH2+

290

HOCH2+

243

H2NCH2+

218

(CH3)3C+

231

(CH3)2CH+

246

CH3CH2+

273

+CH3

312

287

H2C=CH+

287

CH3CH2+

273

R3C-H R3C+ + –H

∆Ho

8

Carbanion StabilityR3C-H R3C

– + +HpKa

(CH3)3C-H

71

CH3CH2-H

50

HCC-H24

H2C=CH-H

44

9

Conformational Analysis-Ethane

10

Conformational Analysis-Butane

11

g+g– Pentane Interaction

12

Cycloalkane Conformations (3-5)

13

Cyclohexane Conformations

14

A ValuesGroup A Value Group A Value

H 0 CH3 1.74

D 0.006 C2H5 1.79

F 0.25-0.42 CH(CH3)2 2.21

Cl 0.53-0.64 C(CH3)3 4.7-4.9

Br 0.48-0.67 CF3 2.4-2.5

I 0.47-0.61 Si(CH3)3 2.5

OH 0.60-1.04 C6H5 2.8

OCH3 0.55-0.75 C6H11 2.2

NH2 1.23-1.70 CH=CH2 1.5-1.7

N(CH3)2 1.5-2.1 COCH3 1.0-1.5

15

Cycloalkanes (CnH2n) Strain Energies

n Strain Energy

n Strain Energy

3 27.5 10 12.4

4 26.3 11 11.3

5 6.2 12 4.1

6 0.1 13 5.2

7 6.2 14 1.9

8 9.7 15 1.9

9 12.6 16 2.0

Small Ring

CommonRings

LargeRingsMedium

Rings

MediumRings

16

Transannular Strain in Cyclodecane

17

Polycyclic Systems

18

trans-Cycloalkenes

trans-cycloheptane trans-cyclooctene

19

Bicyclooctenes

20

A values in heterocycles

Group

Me 1.8 2.9 4.0 0.8 1.8 1.0

i-Pr 2.1 4.2 0.7 1.5 0.8

t-Bu >4.5 1.4 >2.7

R

O

R

OO

R

OO

R

SS

R

SS

R

21

Donor-Acceptor Ability

Donorlone pair > bonding pairC:– > N: > O: (p) > O: (sp2) > F:I: > Br: > Cl: > F:C-H > N-H > O-H > F-HC-I > C-Br > C-Cl > C-FC-S > C-C > C-N > C-O > C-FAcceptor

C-F > C-O > C-N > C-CC-I > C-Br > C-Cl > C-FC-S > C-OC-P > C-N