Handbook of Solvents - George Wypych - ChemTech - Ventech!

584 Javier Catalán
by ethanol, established in 1862 by Berthelot and Saint-Gilles, 2 and on the rate of
quaternization of tertiary amines by alkyl halides, discovered in 1890 by Menschutkin. 3 In
his study, Menschutkin found that even so-called “inert solvents” had strong effects on the
reaction rate and that the rate increased by a factor about 700 from hexane to acetophenone.
Subsequent kinetic studies have revealed even higher sensitivity of the reaction rate to the
solvent. Thus, the solvolysis rate of tert-butyl chloride increases 340 000 times from pure
ethanol to a 50:50 v/v mixture of this alcohol and water, 4,5 and by a factor of 2.88×10 14 from
pentane to water. 6 Also, the decarboxylation rate of 6-nitrobenzisoxazol 3-carboxylate increases
by a factor of 9.5×10 7 from water to HMPT. 7
10.3.1 THE SOLVENT EFFECT AND ITS DISSECTION INTO GENERAL AND
SPECIFIC CONTRIBUTIONS
Rationalizing the behavior of a solvent in a global manner, i.e., in terms of a single empirical
parameter derived from a single environmental probe, appears to be inappropriate because
the magnitude of such a parameter would be so sensitive to the nature of the probe that the
parameter would lack predictive ability. One must therefore avoid any descriptions based
on a single term encompassing every potential interaction of the solvent, often concealed
under a global concept called “solvent polarity”. One immediate way of dissecting the solvent
effect is by splitting it into general (non-specific) interactions and specific interactions.
In relation to general interactions, the solvent is assumed to be a dielectric continuum.
The earliest models for this type of interaction were developed by Kirkwood 8 and Onsager, 9
and were later modified with corrections for the effect of electrostatic saturation. 10,11 The intrinsic
difficulty of these models in accurately determining the dimensions of the cybotactic
region (viz. the solvent region where solvent molecules are directly perturbed by the presence
of solute molecule) that surrounds each solute molecule in the bulk solvent, have usually
raised a need for empirical approximations to the determination of a parameter
encompassing solvent polarity and polarizability. An alternative approach to the general effect
was recently reported that was derived from liquid-state theories. One case in point is
the recent paper by Matyushov et al., 12 who performed a theoretical thermodynamic analysis
of the solvent-induced shifts in the UV-Vis spectra for chromophores; specifically, they
studied p-nitroanisole and the pyridinium-N-phenoxide betaine dye using molecular theories
based on long-range solute-solvent interactions due to inductive, dispersive and dipole-dipole
forces.
A number of general empirical solvent scales have been reported; 1,13-15 according to
Drago, 14 their marked mutual divergences are good proof that they do not reflect general effects
alone but also specific effects of variable nature depending on the particular probe
used to construct each scale. In any case, there have been two attempts at establishing a pure
solvent general scale over the last decade. Thus, in 1992, Drago 14 developed the “unified
solvent polarity scale”, also called the “S� scale” by using a least-squares minimization
program 16 to fit a series of physico-chemical properties (χ) for systems where specific interactions
with the solvents were excluded to the equation Δχ =PS�+ W. In 1995, our group reported
the solvent polarity-polarizability (SPP) scale, based on UV-Vis measurements of
the 2-N,N-dimethyl-7-nitrofluorene/2-fluoro-7-nitrofluorene probe/homomorph pair. 15
According to Drago, 17 specific interactions can be described as localized donor-acceptor
interactions involving specific orbitals in terms of two parameters, viz. E for electrostatic
interactions and C for covalent interactions; according to Kamlet and Taft, 18a such
interactions can be described in terms of hydrogen-bonding acid-base interactions. In fact,