Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.1 Solvent effects on chemical reactivity 763
trast to boiling point conditions, not unimportant contributions of S att even for some aprotic
liquids (see below).
On the other hand, the g K factor is, loosely speaking, a measure of the deviation of the
relative dielectric constant of the solvent with the same dipole moment and polarizability
would have if its dipoles were not correlated by its structure. However, the g K factor is only
the average cosine of the angles between the dipole moments of neighboring molecules.
There may thus be orientational order in the vicinity of a molecule despite a g K of unity if
there are equal head-to-tail and antiparallel alignments. Furthermore, the g K factor is not related
to positional order.
The better starting point for assessing order would be experimental room temperature
entropies of vaporization upon applying the same method as described above for the boiling
point conditions. (Note however, that the packing density, and hence the molecular HS diameter,
varies with temperature. Therefore, in the paper 122 the packing densities have been
calculated for near boiling point conditions.) Thus we choose the simplest fluid as the reference
system. This is a liquid composed of spherical, nonpolar molecules, approximated to a
HS gas moving in a uniform background or mean field potential provided by the attractive
forces. 43 Since the mean field potential affects neither structure nor entropy, the excess entropy
S ex
( ) ()
S / R = ln V / V + f η −Δ
H / RT
[13.1.23]
ex g o v
may be viewed as an index of orientational and positional order in liquids. It represents the
entropy of attractions plus the contributions arising from molecular nonsphericity. This latter
effect can be estimated by comparing the entropy deficits for spherical and hard convex
body repulsions in the reference system. Computations available for three n-alkanes suggest
that only ≈ 20% of S ex are due to nonsphericity effects.
Table 13.1.6. Some liquid properties concerning structure. Data are from ref. 55 and
128 (g K)
Solvent ΔvH/RT ΔvSo/R -Sex/R gK
c-C6 18.21 17.89 0.32
THF 12.83 12.48 0.35
CCl4 13.08 12.72 0.36
n-C5 10.65 10.21 0.44
CHCl3 12.62 12.16 0.46
CH2Cl2 11.62 11.14 0.48
Ph-H 13.65 13.08 0.57
Ph-Me 15.32 14.62 0.70
n-C6 12.70 11.89 0.81
Et2O 10.96 10.13 0.83
c-hexanone 18.20 17.30 0.90