Page 262 - Mechanism and Theory in Organic Chemistry

Unimolecular Electrophilic Substitutions-Carbanions 251

have the nucleophilic, and details of mechanism are not as well defined. Never-
theless, there are a number of transformations that can be profitably considered in
terms of the S,1 process.llg
In Section 4.5 we discussed reactions in which electrophilic substitution of a
metal ion takes place by a bimolecular pathway. The unimolecular substitution
is less common, although there are some examples in cases where the carbanion
is well stabilized.120 For our purposes here the most important S,1 reactions are
those in which the leaving group is a proton or a neutral carbon molecule.

Proton Leaving Group
Cleavage of a carbon-hydrogen bond to yield a carbanion and proton is a
Brransted acid-base reaction (Equations 5.24 and 5.25). The mechanism is not

strictly speaking a unimolecular one, because there are two molecules taking part
in the ionization step. It is, however, analogous to an acid-catalyzed nucleophilic
substitution, in which a Lewis acid helps to pull off the leaving group. Those
reactions are ordinarily included in the S,1 category. We may therefore consider
Reactions 5.24-5.25 in the present context.
The most elementary example of the electrophilic substitution with hydro-
gen leaving group is the exchange of one proton for another, a process that can be
studied by isotopic labeling. We have considered in Section 3.3 the equilibrium
aspects of C-H acidity; data were given there that allow a rough assessment of
relative stabilities of various carbanion structures (Table 3.1, p. 146). The
parallel between rates of proton removal and anion stability as measured by the
acid dissociation constant was also considered there. In general, the more highly
stabilized the anion the more rapidly a given base will produce it by proton
removal.
The stereochemical outcome of a substitution by way of a carbanion depends
on the geometrical preferences of the anion and on its degree of association with
other species present in the medium. Elementary consideration of carbanion
structure leads to the conclusion that :CH,- and simple alkyl homologs should be
pyramidal. The isoelectronic ammonia and amines undergo fast inversion; the
same might be expected for carbanions 40 .+ 41.121 If this change takes place
rapidly, an anion generated from a chiral precursor would lose its configuration

For a review of electrophilic substitution and the carbanion field, see D. J. Cram, Fundamentals
ofcarbanion Chemistry, Academic Press, New York, 1965.
lao See, for example: (a) B. J. Gregory and C. K. Ingold, J. Chem. Soc. B, 276 (1969); (b) D. Dodd
and M. D. Johnson, J. Chem. Soc. B, 1071 (1969); (c) D. Dodd, C. K. Ingold, and M. D. Johnson,
J. Chem. Soc. B, 1076 (1969).
lal A. Rauk, L. C. Allen, and K. Mislow, Angew. Chem. Int. Ed., 9, 400 (1970) review theoretical
and experimental aspects of pyramidal inversion.