Page 174 - Mechanism and Theory in Organic Chemistry

Lewis Acids and Bases 163

order 4,N > 4,NH, 4NH2, and warn us that the solution order is probably again
caused by solvation and not by internal electron distribution properties of the
bases.lO'
The carbonyl bases constitute another important class of weak bases that
present interesting possibilities for investigation of structural effects. In solution,
experiments with these compounds are subject to severe difficulties. The result
is a serious lack of agreement among different investigators about pKa. Arnett
and co-workers point out that pKa values reported for acetophenone cover a
range of over four units ( - 3.65 to - 7.99), while those for acetone span seven
units ( - 0.2 to - 7.2).lo2 In view of these uncertainties, it is impossible to say
whether aldehydes, ketones, or carboxylic acids are the most basic in solution.
Gas-phase data are available for some of these substances.
Table 3.18 summarizes gas-phase basicities for a number of compounds. It
is possible to measure relative gas-phase basicities quantitatively, and the table
includes proton affinity, which is the negative of the enthalpy change for reaction
3.60.'03
B, + Hg+ - BHg+ . AH = - (proton affinity) (3.60)
Table 3.18 contains data for three carbon bases: propene, ethylene, and
methane. The position of methane indicates that the saturated hydrocarbons,
among the weakest of the Bransted acids, are also among the weakest bases. The
unsaturated hydrocarbons have electrons in higher energy orbitals and accept a
proton more easily. There is information available, primarily from acidity
function techniques, about solution basicities of a number of unsaturated hydro-
carbons.lo4

3.5 LEWIS ACIDS AND BASESo5

In 1923 G. N. Lewis proposed a definition of acids and bases somewhat different
from that of Bransted :Io6
An acid is an electron-pair acceptor.
A base is an electron-pair donor.
Lewis acids are thus electron-deficient molecules or ions such as BF, or carbo-
cations whereaiLEwis bases are mole~ules or ions containing available ektx- ons,
---
such as ambs, ethers, alkoxide ions, and SO forth.,A Lewis acid-base reaction is
the combination of an acid and a base to form a complex, or adduct. The
stabilities of these adducts depend on the structures of the constituent acid and
base and vary over a wide range. Some examples of Lewis acid-base reactions are
given in Table 3.19. Lewis acid-base reactions abound in organic chemistry:

lo' I. Dzidic, J. Amer. Chem. Soc., 94, 8333 (1972).
Io2 E. M. Arnett, R. P. Quirk, and J. W. Larsen, J. Amer. Chem. Soc., 92, 3977 (1970).
lo3 See note 94 (b), p. 159.
lo4 For further details see note 84 (a), p. 151.
Ins A general discussion of Lewis acids and bases is given by R. J. Gillespie in Friedel-Crafts and
Relaled Reactions, Vol. 1, G. A. Olah, Ed., Wiley-Interscience, New York, 1963, p. 169.
log G. N. Lewis, Valence and the Slructure of Atoms and Molecules, American Chemical Society Mono-
graph, The Chemical Catalog Co., New York, 1923. Lewis also gave a definition equivalent to that
of Bronsted at this time, but he considered the electron-pair definition to bk more general.