Page 95 - Mechanism and Theory in Organic Chemistry

of all the rotational degeneracies of the molecule. For example, in acetone the
molecule as a whole has a twofold axis of symmetry (this is called the external
axis). Each of the two methyl groups has a threefold axis of symmetry (called
internal axes). Thus for acetone u is 2 x 3 x 3 and So is calculated :

\ ,-
-Rho= -Rln18= -5.74
So (acetone) = + 70.09 cal mole-' OK- '
The experimental value is 70.5 cal mole-l OK-l.
If a molecule is optically active, R In n must be added to its entropy estim-
ate, where n is the total number of stereoisomers of equal energy.

Guide to the Use of the Group Tables (Tables 2.6-2.10)
1. AH; and So are the heat of formation and entropy, respectively, of a
group when that group is in a molecule in its standard state of hypothetical ideal
gas at 1 atm pressure and 25°C. All values of AH; are in kilocalories per mole,
and all values of So are in calories per mole per degree (K) . For a simple method
of converting So and AH; to other temperatures, see Benson et al., Chem. Rev., 69
(1969), p. 313.
2. In order to assign values to all groups, some groups have had to be
assigned arbitrary values. Groups in brackets in Tables 2.G2.10 are those
groups. Estimated values obtained from a single compound are in parentheses.

2.4 SOLUTIONS 27
The thermochemical additivity scheme outlined in the previous section is based on
gas-phase data. Since most organic reactions are carried out in solution, it would
be most useful to be able to understand and predict the thermochemical changes
that molecules, ions, and transition states undergo when dissolved in various
solvents. Our knowledge of the structure of liquids and of their interactions on the
molecular level with solutes is still too incomplete to permit more than a very
rough qualitative answer to this problem. We shall proceed by discussing briefly
the influence on solvent properties of the most important parameters characteriz-
ing liquids.
Dielectric Constant
The first property of solvents to be considered is the dielectric constant, E. The
dielectric constant of a substance measures the reduction of the strength of the
electric field surrounding a charged particle immersed in the substance, com-
pared to the field strength around the same particle in a vacuum. The dielectric
constant is a macroscopic property; that is, its definition and measurement

The following discussions of solvent effects will provide further information: (a) T. C. Waddington,
Non-Aqueous Solvents, ThomasNelson, London, 1969; (b) E. M. Kosower, An Introduction to Physical
Organic Chemistry, Wiley, New York, 1968, p. 259; (c) T. C. Waddington, Ed., Non-Aqueous Solvent
System, Academic, London, 1965; (d) E. S. Amis and J. F. Hinton, Solvent Effects on Chemical Phcno-
menu, Academic, New York, 1973; (e) J. F. Coetzee and C. D. Ritchie, Eds., Solute-Solvent Interactions,
Marcel Dekker, New York, 1969; (f) A. J. Parker, Chem. Rev., 69, 1 (1969).