Handbook of Solvents - George Wypych - ChemTech - Ventech!

600 Javier Catalán
Solvents SPP SB SA
sulfolane 1.003 0.365 0.052
m-cresol 1.000 0.192 0.697
1,1,1,3,3,3-hexafluoro-2-propanol 1.014 0.014 1.00
ααα-trifluoro-m-cresol 1.085 0.051 0.763
a Assumed value because it is considered non acid solvent.
a Assumed value because the first band of the TBSB spectra exhibits vibronic structure.
A brief analysis of these SPP data allows one to draw several interesting conclusions
from structural effects on solvent polarity, namely:
• Cyclohexane is used as the non-polar reference in many scales. In the SPP scale, the
polarity gap between cyclohexane and the gas phase (0.557 SPP units) is as wide as
that between cyclohexane and the highest polarity (0.443 SPP units).
• Alkanes span a wide range of SPP values (e.g., 0.214 for perfluoro-n-hexane, 0.479
for 2-methylbutane and 0.601 for decalin).
• Unsaturation increases polarity in alcohols. Thus, the SPP values for n-propanol,
allyl alcohol and propargyl alcohol are 0.847, 0.875 and 0.915, respectively.
10.3.5 SOLVENT BASICITY: THE SB SCALE 72
As stated above, for a probe of solvent basicity to be usable in UV-Vis spectroscopy, it
should meet a series of requirements. One is that it should be acidic enough in its electronic
ground state to allow characterization of the basicity of its environment. In addition, its
acidity should increase upon electronic excitation such that its electronic transitions will be
sensitive to the basicity of the medium. This behavior will result in a bathochromic shift in
the absorption band the magnitude of which will increase with increasing basicity of the environment.
The probe should also be free of potential conformational changes that might influence
the electronic transition to be evaluated. Finally, its molecular structure should be
readily converted into a homomorph lacking the acid site without any side effects that might
affect the resulting solvent basicity.
5-Nitroindoline (NI, 9) possesses the above-described electronic and structural features.
It is an N-H acid with a single acid site borne by a donor group whose free rotation is
hindered by an ethylene bridge on the ring. However, if charge transfer endows the compound
with appropriate acid properties that increase with electronic excitation, the basicity
of the acceptor (nitro) group will also increase and the polarity of the compound will be altered
as a result. Both effects will influence the electronic transition of the probe that is to be
used to evaluate basicity.
Replacement of the acid proton (N-H) by a methyl group in this molecular structure
has the same side effects; as a result, the compound will be similarly sensitive to the polarity
and acidity of the medium, but not to its basicity owing to the absence of an acid site. Consequently,
1-methyl-5-nitroindoline (MNI, 10) possesses the required properties for use as a
homomorph of 5-nitroindoline in order to construct our solvent basicity scale (SB). The
suitability of this probe/homomorph couple is consistent with theoretical MP2/6-31G**
data. Thus, both the probe and its homomorph exhibit the same sensitivity to solvent polarity/polarizability
because of their similar dipole moments (µ NI = 7.13, µ MNI = 7.31 D) and