Page 175 - Mechanism and Theory in Organic Chemistry

Table 3.19 EXAMPLES OF LEWIS ACID-BASE REACTIONS

- +/C2H5
BF, + C~H,-O-C~H~ F,B-O\
CZH5

R
'c-0 + CN-
/
R

the electrophiles (for example NO, + , carbocations, Ag + , B2H6, carbonyl
carbon) and nucleophiles (OH-, C1-, amines, carbanions, and mwy others),
which are of such great importance in organic reactions, are Lewis acids and
bases, respectively. The theory of these interactions, still in the process of byg
developed, may well prove to be of very general application. \
The relationship between the Bransted definition and the Lewis definition
is a subtle one, which has caused some confusion and controversy over the years.
There is no particular problem as far as bases are concerned, for it is clear that the
two definitions refer to the same substances. Molecules with available electrons
are capable of accepting a proton and also of coordinating with other electron-
deficient centers, and so fit both definitions. It is with acids that the difficulties
arise. The proton is itself a Lewis acid, and the prototype acid-base reactions of
Equation 3.1 and 3.2 are clearly acid-base reactions in the Lewis sense, with H+
the acid, A- or B the base, and HA and BH + the acid-base complexes. For these
reasons Lewislo7 and otherslo8 have considered the Brransted acid-base reactions
to be special cases of the more general category covered by the Lewis definition.
Adherents of the Brransted theory, however, maintain that since Brransted acid-
base reactions do not involve the bare proton, and since Brransted acids are not,
in general, Lewis acids, it is better to regard the two definitions as distinct.log
The problem is that a substance HA undergoing a reaction in which it
behaves as a Brransted acid is not behaving as an acid in the Lewis sense; it is,
however, behaving as a Lewis acid-base adduct. The issue is further clouded by
the fact that HA can enter into reactions in which it does behave as a Lewis
acid, as for example in the formation of a hydrogen bond (Equation 3.61) ; in
thiB case it is not, however, behaving as a Brransted acid, since the proton remains

lo7 See note 106.
lo8 (a) W. F. Luder and S. Zuffanti, The Electronic Theory of Acids and Bases, Wiley, New York, 1946;
W. F. Luder, Chem. Rev., 27, 547 (1940); (b) D. P. N. Satchell and R. S. Satchell, Quart. Rev. (Lon-
don), 25, 171 (1971).
log (a) R. P. Bell, The Proton in Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1973, p. 7;
(b) I. M. Kolthoff, J. Phys. Chem., 48, 51 (1944).