Page 388 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

The concept of acidity was discussed earlier in relation to aqueous solutions and 369
the acidity of carboxylic acids and alcohols (see p. 321–322). The acidity of such
compounds can measured and expressed as the pK . TOPIC 3.1
a
Acidity of Hydrocarbons
−
+
S H X
pK =−logK where K =
a
HX S
Determination of the acidity of hydrocarbons is more difficult. As most are
very weak acids, very strong bases are required to cause deprotonation. Water and
alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for
generation of hydrocarbon anions. A strong base deprotonates the solvent rather than
the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, THF, and
dimethoxyethane are used, but for equilibrium measurements solvents that promote
dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such
as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and cyclohexylamine are
used in the preparation of moderately basic carbanions. The high polarity and cation-
solvating ability of DMSO and DMF facilitate dissociation of ion pairs so that the
equilibrium data obtained refer to the free ions, rather than to ion aggregates.
The basicity of a base-solvent system can be specified by a basicity constant H .
_
The H is a measure of solution basicity, analogous to the acidity function H (see
_ 0
Section 3.7.1.3). The value of H approximates the pH of strongly basic solutions. The
_
larger the value of H , the greater the proton-abstracting ability of the medium. Use
_
of a series of overlapping indicators permits assignment of H values to base-solvent
_
systems, and allows pK’s to be determined over a range of 0–30 pK units. Table 3.35
presents H values for some solvent-base systems.
_
The acidity of a hydrocarbon can be determined in an analogous way. 163 If
the electronic spectra of the neutral and anionic forms are sufficiently different, the
concentrations of each can be determined directly, and the position of the equilibrium
constant is related to pK by the equation
R−H
pK R−H = H +log
−
R
−
A measurement of the ratio RH R at a known H yields the pK. If the electronic
−
_
spectrum of the hydrocarbon and its anion are not sufficiently different, an indicator is
Table 3.35. Values of H for Some Solvent-
_
Base Systems
Solvent H _ a
5M KOH 15 5
10M KOH 17 0
1M KOH 18 5
0 01M NaOMe in 1:1 DMSO-MeOH 15 0
0 01M NaOMe in 10:1 DMSO-MeOH 18 0
0 01M NaOEt in 20:1 DMSO-MeOH 21 0

a. From J. R. Jones, The Ionization of Carbon Acids, Academic
Press, New York, 1973, Chap. 6.
163
D. Dolman and R. Stewart, Can. J. Chem., 45, 911 (1967); E. C. Steiner and J. M. Gilbert, J. Am.
Chem. Soc., 87, 382 (1965); K. Bowden and R. Stewart, Tetrahedron, 21, 261 (1965).