Handbook of Solvents - George Wypych - ChemTech - Ventech!

740 Roland Schmid
HB interactions, is claimed to lie in different responses to solvent polarizability effects.
Likewise, in the relationship between the π* scale and the reaction field functions of the refractive
index (whose square is called the optical dielectric constant e ∞) and the dielectric
constant, the aromatic and the halogenated solvents were found to constitute special cases. 10
This feature is also reflected by the polarizability correction term in eq. [13.1.2] below. For
the select solvents, the various “polarity” scales are more or less equivalent. A recent account
of the various scales has been given by Marcus, 11 and in particular of π* by Laurence
et al., 12 and of E T by Reichardt. 13
However, solvation is not the only mode of action taken by the solvent on chemical reactivity.
Since chemical reactions typically are accompanied by changes in volume, even
reactions with no alteration of charge distribution are sensitive to the solvent. The solvent
dependence of a reaction where both reactants and products are neutral species (“neutral”
pathway) is often treated in terms of either of two solvent properties. The one is the cohesive
energy density ε c or cohesive pressure measuring the total molecular cohesion per unit volume,
( )
ε c v
= Δ H −RT
/ V
[13.1.1]
where:
ΔHv molar enthalpy of vaporization
V molar liquid volume
The square root of εc is termed the Hildebrand solubility parameter δH, which is the solvent
property that measures the work necessary to separate the solvent molecules (disrupt
and reorganize solvent/solvent interactions) to create a suitably sized cavity for the solute.
The other quantity in use is the internal pressure Pi which is a measure of the change in internal
energy U of the solvent during a small isothermal expansion, Pi =(∂U/∂V) T. Interesting,
and long-known, is the fact that for the highly dipolar and particular for the protic solvents,
values of ε c are far in excess of Pi. 14 This is interpreted to mean that a small expansion does
not disrupt all of the intermolecular interactions associated with the liquid state. It has been
suggested that Pi does not detect hydrogen bonding but only weaker interactions.
At first, solvent effects on reactivity were studied in terms of some particular solvent
parameter. Later on, more sophisticated methods via multiparameter equations were applied
such as 15
( )
XYZ = XYZ0 + s π* + dδ + aα + bβ+ hδH [13.1.2]
where XYZ0, s, a, b, and h are solvent-independent coefficients characteristic of the process
and indicative of its sensitivity to the accompanying solvent properties. Further, δ is a
polarizability correction term equal to 0.0 for nonchlorinated aliphatic solvents, 0.5 for
polychlorinated aliphatics, and 1.0 for aromatic solvents. The other parameters have been
given above, viz. π*, α,β, and δH are indices of solvent dipolarity/polarizability, Lewis acidity,
Lewis basicity, and cavity formation energy, respectively. For the latter, instead of δH, 2 16
should be preferred as suggested from regular solution theory.
δ H
Let us just mention two applications of the linear solvation energy relationship
(LSER). The one concerns the solvolysis of tertiary butyl-halides 17
log k(Bu t 2
Cl) = -14.60 + 5.10π* + 4.17α + 0.73β + 0.0048δH