Page 224 - Mechanism and Theory in Organic Chemistry

Chapter 5

UNIMOLECULAR

SUBSTITUTIONS

AND RELATED

REACTIONS

In this chapter we take up unimolecular and borderline substitutions and discuss
carbocations, carbanions, and carbenes, three of the important reactive organic
intermediates.
5.1 LIMITING UNIMOLECULAR NUCLEOPHILIC REACTIONS.
KINETICS AND STEREOCHEMISTRY l
Diphenylchloromethane (benzhydryl chloride, I), when dissolved in solvents
such as aqueous ethanol, aqueous acetone, acetic acid, or formic acid, undergoes
substitution of chloride by a nucleophilic group derived from the solvent.

1
Equation 5.1, in which SOH stands for a molecule of some hydroxylic solvent,
illustrates the process. This type of reaction, where the solvent takes the role of the
+ SOH -
+\c, C1 +\(yes + HCl (5.1)
+/ "H 4/ 'H
nucleophile (Lewis base) in the substitution, is called solvolysis.

' Unimolecular substitutions are discussed in detail in the following sources: (a) C. A. Bunton,
Nucbophilic Substitution at a Saturated Carbon Atom, Elsevier, Amsterdam, 1963; (b) C. K. Ingold,
Structure and Mechanism in Organic Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1969;
(c) A. Streitwieser, Jr., Soluolytic Displacement Reactions, McGraw-Hill, New York, 1962; (d) E. R.
Thornton, Soluolysis Mechanisms, Ronald Press, New York, 1964.