Handbook of Solvents - George Wypych - ChemTech - Ventech!

604 Javier Catalán
hexafluoro-2-propanol, acetic acid and trifluoroacetic acid, with λ max values of 550, 536,
524, 517, 534 and 507 nm, respectively. In addition, the spectral envelope undergoes no significant
change in the solvents studied, so a potential protonation of the probe -even a partial
one- can be discarded.
The shift in the n →π*band is largely caused by solvent acidity; however, an n →π*
transition is also obviously sensitive to the polarity of the medium, the contribution of
which must be subtracted if acidity is to be accurately determined. To this end, our group
used the solvatochromic method of Kamlet and Taft 18a to plot the frequency of the absorption
maximum for the n →π*transition against solvent polarity (the SPP value). As expected,
non-acidic solvents exhibited a linear dependence [eq. (10.3.13)], whereas acidic
solvents departed from this behavior -the more acidic the greater the divergence. This departure
from the linear behavior described by eq. [10.3.13] is quantified by Δv DETZ,
v DETZ = 1.015SPP + 17.51 [10.3.13]
with n = 13, r = 0.983 and sd = 23 cm -1 .
The SA value of an acid solvent as determined using the DETZ probe is calculated using
the expression established from the information provided by a series of solvents of increased
acidity measurable by the TBSB/DTBSB probe/homomorph couple (viz.
1,2-butanediol, 1,3,-butanediol, ethanol, methanol and hexafluoro-2-propanol); this gives
rise to eq. [10.3.14], which, in principle, should only be used to determine SA for solvents
with Δv DETZ values above 100 cm -1 :
SA = 0.833Δv DETZ + 0.339 [10.3.14]
withn=6andr=0.987.
Table 10.3.1 gives the SA values for a wide range of highly acidic solvents. Data were
largely obtained from measurements made in our laboratory; 70 some, however, are reported
here for the first time.
A brief analysis of these SA data allow one to draw several interesting conclusions
from structural effects on solvent acidity. Thus:
• Substitution into alcoholic compounds allows the entire range of the solvent acidity
scale to be spanned. Thus, 2-octanol can be considered to be scarcely HBD
(SA=0.088) and ethylene glycol to be highly HBD (SA=0.717). Perfluoroalkanols
are even more highly HBD.
• The acidity of 1-alkanols and carboxylic acids decreases significantly with
increasing chain length but levels off beyond nonanol and octanoic acid,
respectively.
• Cyclization decreases solvent acidity. Thus, SA decreases from 0.318 to 0.257
between 1-pentanol and cyclopentanol, and from 0.298 to 0.136 between 1-octanol
and cyclooctanol.
• Aromatic rings make aniline (SA=0.131) and pyrrole (SA=0.386) more HBD than
their saturated homologs: cyclohexylamine and pyrrolidine, respectively, both of
which are non-HBD (SA=0).
• The acidity of alkanols decreases dramatically with increasing alkylation at the
atom that bears the hydroxyl group [e.g., from 1-butanol (SA=0.340) to 2-butanol
(SA=0.221) to tert-butyl alcohol (SA=0.146)].