Handbook of Solvents - George Wypych - ChemTech - Ventech!

590 Javier Catalán
dipolar). This transition undergoes
a strong hypsochromic shift
as solvent polarity increases, so
much so that it occasionally overlaps
with strong π→ π*bands
and
results in imprecise localization of
the maximum of the charge-transfer
band.
The Z values for the 20 solvents
originally examined by
Kosower 53 spanned the range from
64.2 for dichloromethane to 94.6
for water. The scale was subsequently
expanded to an overall 61
solvents by Marcus 56 and the original range extended to 55.3 kcal mol -1 (for
2-methyltetrahydrofuran). Further expansion of this scale was precluded by the
fact that high-polar solvents shift the charge-transfer band at the shortest wavelength
to such an extent that it appears above the strong first π→ π*transition
of
the compound, thus hindering measurement; in addition, the probe (4) is scarcely
soluble in non-polar solvents. One should also bear in mind that many solvents
require using a high concentration of the probe in order to obtain a measurable
charge-transfer band; as a result, the position of the band often depends on the probe concentration.
One other fact to be considered is that the interest initially aroused by this scale promoted
attempts at overcoming the above-mentioned measurement problems by using correlations
with other solvent-sensitive processes; as a result, many of the Z values currently in
use are not actually measured values but extrapolated values derived from previously established
ratios.
10.3.3.6 The χR scale of Brooker
In 1951, Brooker 57 suggested for the first time that
solvatochromic dyes could be used to obtain measures of solvent
polarity. This author 58 constructed the χR scale on the basis
of the solvatochromism of the merocyanine dye (5), the
electronic transition of which gives rise to a charge-transfer
from the amine nitrogen to a carboxamide group at the other
end of the molecule. Hence, the excited status is more dipolar
than the ground state, and the resulting band is shifted
bathochromically as solvent polarity increases. χR values reflect the position of the maximum
of the first band for the chromophore in kcal mol -1 .
The original scale encompassed 58 solvents spanning χR values from 33.6 kcal mol -1
for m-cresol to 50.9 kcal mol -1 for n-heptane.
10.3.3.7 The Φ scale of Dubois and Bienvenüe
Dubois and Bienvenüe 59 Scheme IV.
developed the Φ polarity solvent scale on the basis of the position
of the n →π*transition for eight selected aliphatic ketones that were studied in 23 solvents,
using n-hexane as reference and the following equation for calculation: