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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