Page 140 - Mechanism and Theory in Organic Chemistry

Strengths of Weak Brensted Bases 129
w-. -______ _ --.- --
bases can be studied in water only if they are stronger bases than water and

The phenomenon described above for water also applies to other amphoteric
solvents. It is termed the leveling efect, and may be summarized by the following
statements :

1. No acid strong* than the conjugate acid of a solvent can exist in
appreciable concentration in that solvent.
2. No base stronger than the conjugate base of a solvent can exist in
appreciable concentration in that solvent.
Useful correlaries of these statements are the following:
1. Relative strengths of acids stronger than the conjugate acid of a solvent
cannot be determined in that solvent.
2. Relative strengths of bases stronger than the conjugate base of the solvent
cannot be determined in that solvent.
The acids in which we are interested span a range of roughly 60 pK units,
from the strongest acids (HI, HClO,) to the weakest (methane, cyclohexane),
and since there is no single solvent that is suitable for the entire range, it is
necessary to use several different solvents and to try to make connections among
the results obtained.
Water is taken as the standard solvent for setting up a scale of acidity. It
has the advantage, in addition to convenience, of having a high dielectric con-
stant and being effective in solvating ions. As we noted in Section 2.4 (p. 85),
the result of these properties is that positive and negative ions separate, and
complications that result from association of ions in pairs or in larger aggregates
are avoided.12 For acids too strong to be investigated in water solution, more
acidic media such as acetic acid or mixtures of water with sulfuric or perchloric
acid are commonly used; for very weak acids, solvents such as liquid ammonia,
dimethylsulfoxide, and cyclohexylamine have been employed.
The experimental procedures and the results obtained with some of these
solvents are discussed in Sections 3.2 and 3.3. We note here only that the task of
relating the results of acidity measurements obtained in different solvents is by no
means a simple one, and that it has not proved possible to establish a scale that
provides unambiguously and quantitatively relative acidities of substances
over the whole range of interest to chemists. Thus the relative acidities of two
acids may be different in different solvents, and we may have to be content with
qualitative results if we wish to generalize about acid strengths over wide ranges.
If, however, we are willing to restrict our attention to acids that can all be in-
vestigated in the same solvent, it will be possible to obtain quantitative results.

3.2 STRENGTHS OF WEAK BRONSTED BASES

A variety of organic reactions, including dehydration of alcohols, cleavage of
ethers, many additions to olefins, a number of nucleophilic substitutions, and
various rearrangements, are catalyzed by acids. Since the substrates in these

l2 I. M. Kolthoff and S. Bruckenstein, J. Amer. Chem. Soc., 78, 1 (1956).