Handbook of Solvents - George Wypych - ChemTech - Ventech!

5.5 The phenomenological theory of solvent effects 295
Notice in Table 5.5.5 that all full cancellation systems give substantial negative ΔgA values.
If g is constant, ΔgA=gΔA, and the negative ΔA value leads to a solvophobic driving force
of gΔAγ for complex formation. (The dioxane system in Table 5.5.5 is unassigned because
its K 1 value suggests partial cancellation whereas its G value suggests full cancellation).
Table 5.5.5. Parameter values of the 4-nitroaniline/α-cyclodextrin and methyl orange/
α-cyclodextrin systems 19,20
Cosolvent K1 G a
Cancellation assignment
4-Nitroaniline
Acetonitrile 55 +3 Partial
2-Propanol 46 -3 Partial
Ethanol 29 -9 Partial
Acetone 10 -57 Full
Methanol 3.1 -68 Full
Methyl orange
Acetone 46 -3 Partial
2-Propanol 43 -11 Partial
Acetonitrile 40 -13 Partial
Dioxane 31 -38 (Unassigned)
Ethylene glycol 7.7 -58 Full
DMSO 6.4 -66 Full
Methanol 4.9 -43 Full
a Units are � 2 molecule -1
5.5.3.5 Chemical kinetics
Treatment of the solvent effect on chemical reaction rates by means of the
phenomenological theory is greatly facilitated
by the transition state theory, which
postulates that the initial and transition
states are in (virtual) equilibrium. Thus the
approach developed for complex formation
is applicable also to chemical kinetics.
Again we begin with a thermodynamic cycle,
Figure 5.5.9, where R represents the reactant
(initial state) in a unimolecular
reaction, R � is the transition state, and P is
Figure 5.5.9. Thermodynamic cycle for a unimolecular
reaction. (Reproduced with permission from the Journal
of Pharmaceutical Sciences, reference 21.)
the product. From Figure 5.5.9 we write eq.
[5.5.47], where ΔG � rxn(1) subsequently writ-