Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 601
polarizabilities (20.38 and 22.78 α/J -1 C 2 m 2 for NI and MNI, respectively). Both also have
the same sensitivity to solvent acidity as their surface electrostatic potential minima are virtually
coincident [V S,min(NI) = -47.49 kcal mol -1 ,V S,min(MNI) = -47.58 kcal mol -1 ]; according
to Politzer et al., 71 this means that they exhibit the same hydrogen-bonding basicity. In
addition, the electrostatic potential surface in both molecules suggests that sole
electrophilic sites are those on the oxygen atoms of the nitro group.
Obviously, the difference in solvatochromism between NI and MNI [Δv(solvent) = v NI
-v MNI] will cancel many spurious effects accompanying the basicity effect of the solvent. In
addition, NI and MNI possess several advantageous spectral features; thus, they exhibit a
sharp first absorption band that overlaps with no higher-energy band in any solvent, and
both have the same spectral envelope in such a band (see Figure 1 in ref. 72), which facilitates
comparison between the two spectra and the precise establishment of the basicity parameter,
SB. The basicity of a solvent (SB) on a scale encompassing values between zero
for the gas phase -the absence of solvent- and unity for tetramethylguanidine (TMG) can be
directly obtained from the following equation:
SB(solvent) = [Δv(solvent) - Δv(gas)] / [Δv(TMG) - Δv(gas)] [10.3.10]
Table 10.3.1 lists the SB values for a wide range of solvents. All data were obtained
from measurements made in our laboratory; most have been reported elsewhere 70,72 but
some are published here for the first time.
A brief analysis of these SB data allows one to draw several interesting conclusions as
regards structural effects on solvent basicity. Thus:
• Appropriate substitution in compound families such as amines and alcohols allows
the entire range of the solvent basicity scale to be spanned with substances from
such families. Thus, perfluorotriethylamine can be considered non-basic
(SB=0.082), whereas N,N-dimethylcyclohexylamine is at the top of the scale
(SB=0.998). Similarly, hexafluoro-2-propanol is non-basic (SB=0.014), whereas
2-octanol is very near the top (SB=0.963).
• The basicity of n-alkanols increases significantly with increase in chain length and
levels off beyond octanol.
• Cyclization hardly influences solvent basicity. Thus, there is little difference in
basicity between n-pentane (SB=0.073) and cyclopentane (SB=0.063) or between
n-pentanol (SB=0.869) and cyclopentanol (SB=0.836).
• Aromatization decreases solvent basicity by a factor of 3.5-5.5, as illustrated by the
following couples: pyrrolidine/pyrrole (0.99/0.18), N-methylpyrrolidine/
N-ethylpyrrole (0.92/0.22), tetrahydrofuran/furan (0.59/0.11), 2-methyltetrahydrofuran/2-methylfuran
(0.56/0.16), cyclohexylamine/aniline (0.96/0.26),
N-methylcyclohexylamine/N-methylaniline (0.92/0.21), N,N-dimethylcyclohexylamine/N,N-dimethylaniline
(0.99/0.30) and a structurally less similar couple such
as piperidine/pyridine (0.93/0.58).
The potential family-dependence of our SB scale was examined and discarded elsewhere.
73
10.3.6 SOLVENT ACIDITY: THE SA SCALE 74
As stated above, a probe for solvent acidity should possess a number of features to be usable
in UV-Vis spectroscopy. One is that it should be basic enough in its electronic ground state