Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.1 Solvent effects on chemical reactivity 739
ity scales being the Gutmann DN, the Kamlet and Taft β, and the Koppel and Palm B. The
other class is said to reflect anion (or negative dipole’s end) solvation. This latter class includes
the famous scales π*, α, E T(30), Z, and last but not least, the acceptor number AN.
Summed up:
Cation (or positive dipole’s end) solvation
• Gutmann DN
• Kamlet and Taft β
• Koppel and Palm B (B*)
Anion (and negative dipole’s end) solvation
• Gutmann AN
• Dimroth and Reichardt E T(30)
• Kosower Z
• Kamlet and Taft α, π*
These two sets of scales agree in their general trend, but are often at variance when values
for any two particular solvents are taken. Some intercorrelations have been presented by
Taft et al., e.g., the parameters E T, AN and Z can be written as linear functions of both α and
π*. 8 Originally, the values of E T and π* were conceived as microscopic polarity scales reflecting
the “local” polarity of the solvent in the neighborhood of solutes (“effective” dielectric
constant in contrast to the macroscopic one). In the framework of the
donor-acceptor concept, however, they obtained an alternative meaning, based on the interrelationships
found between various scales. Along these lines, the common solvents may be
separated into six classes as follows.
1 nonpolar aliphatic solvents
2 protics or protogenetic solvents (at least one hydrogen atom is bonded
to oxygen)
3 aromatic solvents
4 (poly)halogenated solvents
5 (perhaps) amines
6 select (or “normal” according to Abraham) solvents defined as non-protonic,
non-chlorinated, aliphatic solvents with a single dominant bond dipole.
A case study is the plot of AN
versus E T shown in Figure 13.1.2.
While there is a quite good correspondence
for the select solvents
(and likely for the nonpolar
aliphatic solvents), the other
classes are considerably off-line. 9
Figure 13.1.2. Relationship between the E T(30) values and the acceptor
number [from ref. 21]. Triangles: protic solvents, squares: aromatic
and chlorinated solvents.
This behavior may be interpreted
in terms of the operation of different
solvation mechanisms such as
electronic polarizability, dipole
density, and/or hydrogen-bonding
(HB) ability. For instance, the
main physical difference between
π* and E T(30), in the absence of