Handbook of Solvents - George Wypych - ChemTech - Ventech!

5.5 The phenomenological theory of solvent effects 297
For this test, the reaction possesses these very desirable features: (1) the kinetics are
first order, and the rate-determining step is unimolecular; (2) the reaction rate is independent
of pH over the approximate range 4-13; (3) the rate-determining step of the process is
not a solvolysis, so the concentration of water does not appear in the rate equation; and (4)
the reaction is known to display a sensitivity to solvent composition. Figures [5.5.10] and
[5.5.11] show curve-fits, and Tables 5.5.6 and 5.5.7 give the parameter values obtained in
the curve-fitting regression analysis. Observe that ΔgA � is positive. This means that the
transition state occupies a larger volume than does the reactant. This conclusion has been
independently confirmed by studying the pressure dependence of the kinetics. 21,22
Table 5.5.6. Model parameters for solvent effects on the decomposition of
N-chloroalanine 21
Cosolvent K1 K2 ΔgA � , � 2 molecule -1
Methanol 2.8 3.1 16.8
Ethanol 5.7 2.8 23.8
1-Propanol 13.0 3.2 22.0
2-Propanol 5.9 11 20.6
Ethylene glycol 4.4 1.7 34.8
Propylene glycol 5.2 6.0 27.0
Acetonitrile 7.0 2.0 42
Dioxane 8.9 4.3 55
Table 5.5.7. Model parameters for solvent effects on the decomposition of
N-chloroleucine 21
Cosolvent K1 K2 ΔgA � , � 2 molecule -1
Methanol 2.5 4.0 19.3
2-Propanol 2.7 40 24.2
Ethylene glycol 4.4 3.2 38
Acetonitrile 7.5 3.9 43
The success shown by this kinetic study of a unimolecular reaction unaccompanied by
complications arising from solvent effects on pH or water concentration (as a reactant)
means that one can be confident in applying the theory to more complicated systems. Of
course, an analysis must be carried out for such systems, deriving the appropriate functions
and making chemically reasonable approximations. One of the goals is to achieve a practical
level of predictive ability, as for example we have reached in dealing with solvent effects
on solubility.