Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 603
teresting spectroscopic behavior. Thus, in a non-HBD solvent (i.e., one that cannot interact
directly with the oxygen lone pair), the band is structured; on the other hand, the band loses
its structure upon interaction with an HBD solvent. 75
The SA acidity scale was established by comparing the solvatochromism of the probes
TBSB and DTBSB using the method of Kamlet and Taft. 18a In this method, the
solvatochromism of DTBSB in a solvent is used as the reference for zero acidity. Consequently,
non-acidic solvents obey the equation
v TBSB = 1.409v DTBSB - 6288.7 [10.3.11]
with n = 50, r = 0.961 and sd = 43.34 cm -1 .
Based on eq. [10.3.11] for non-acidic solvents, the SA value for a given solvent can be
obtained from the following expression, where a value of 0.4 is assigned to ethanol, the acid
solvatochromic behavior of which is exhibited at 1299.8 cm -1 :
SA =[[v TBSB - (1.409v DTBSB - 6288.7)] / 1299.8]0.4 [10.3.12]
Table 10.3.1 gives the SA values for a wide variety of C-H, N-H and O-H acid solvents.
Data were all obtained from measurements made in our laboratory. 74 The
homomorph was insoluble in some solvents, which therefore could not be measured, so they
where assumed not to interact specifically and assigned a zero SA value if the probe exhibited
a structured spectrum in them.
Evaluating the acidity of weakly acidic solvents entails using a probe basic enough to
afford measurement of such an acidity; as a result, the probe is usually protonated by
strongly acidic solvents and useless for the intended purpose. Stilbazolium betaines are subject
to this constraint as they have pK values of about 10 -(11) and DTBSB have a pKa of
8.57 76 and ca. 10, 77 respectively, which makes them unsuitable for the evaluation of solvents
more acidic than methanol. This forced us to find a suitable probe with a view to expanding
the SA scale to more acidic solvents. This problem is not exclusive to our scale; in fact, it affects
all acidity scales, which usually provide little information about strongly acidic solvents.
The problem was solved thanks to the exceptional behavior of
3,6-diethyltetrazine 70 (DETZ, 14). This compound possesses two lone electron
pairs located on opposite sites of its hexagonal ring; the strong interaction between
the two pairs results in one antibonding n orbital in the compound lying at
an anomalously high energy level; this, as shown below, has special spectroscopic
implications. The π-electron system of DETZ is similar to that of benzene.
Thus, the first π→ π*transition
in the two systems appears at λmax = 252 nm in
DETZ and at 260 nm in benzene. On the other hand, the π→ π*
transition in
DETZ is strongly shifted to the visible region (λmax ≅ 550 nm), which is the origin
of its deep color. This unique feature makes this compound a firm candidate for
use as an environmental probe as the wide energy gap between the two transitions
excludes potential overlap. Problems such as protonation of the DETZ
probe by acid solvents can be ruled out since, according to Mason, 78 this type of substance
exhibits pK values below zero.
The sensitivity of the n →π*transition in DETZ to the solvent is clearly reflected in
its UV/Vis spectra in methylcyclohexane, methanol, trifluoromethanol,