Page 84 - Mechanism and Theory in Organic Chemistry

Thermochemistry 73

4 C-(H),(C) = -40.32
2 C-(H)(C), = - 3.80
2 gauche corrections = + 1.60
AH; (2,3-dimethylbutane) = -42.52 kcal mole-I
The experimental value is - 42.49 kcal mole- l.

Alkenes
There are two types of corrections that are sometimes necessary in calculating
AH/" for alkenes. For a compound that contains a cis double bond, a correction
factor of + 1 .OO must be added. (If one or both of the cis substituents is t-butyl,
the correction factor is larger: see Table 2.6, footnote a.) For example, the AH:
(cis-2-butene) is calculated as follows :
2 C-(H3)(C) = - 20.16
2 Cd-(H)(C) = + 17.18
1 cis correction = + 1.00
AH; (cis-2-butene) = - 1.98 kcal mole-I
The experimental value is - 1.67 kcal mole-l. Note that there is no AH," for
C-(H),(C,) in Table 2.6; a methyl bonded to an sp2 carbon has the same AH;
group value as a methyl bonded to an sp3 carbon. This assumption was made in
the original determination of AH; group values.
If one side of the double bond is substituted as in 20 or 21, in which R
stands for an alkyl group, an alkene gauche correction of 0.50 kcal mole-I must
be added. Thus the calculated AH/" for 2,3-dimethylbut-1-ene (22) is as follows:
3 C-(H),(C) = -30.24
1 Cd-(H), = +6.26
1 Cd-(C), = + 10.34
1 C-(Cd) (C,) (H) = - 1.48
1 alkene gauche correct~on = + 0.50
AH; (2,3-dimethylbut-1-ene) = - 14.62 kcal mole-'
The experimental value is - 15.85 kcal mole- l.

R R CH3
I I I
R-C-C= R-C-C = CH3-C-C=CH2
A 8 R R H CH,
I I
I
I

Aromatics
In alkylated benzenes a correction factor of 0.57 must be applied if two substi-
tuents-are ortho to each other. For example, AH; of 1,2:dimethylbenzene is
calculated as follows :
4 CB-(H) = + 13.20
2 CB-(C) = + 1 1.02
2 C(H),(C) = -20.16
1 ortho correction = + 0.57
AH; (1,2-dimethylbenzene) = + 4.63 kcal mole-l