Page 597 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

6

Carbanions and Other Carbon

Nucleophiles

Introduction

This chapter is concerned with carbanions, which are the conjugate bases (in the
Brønsted sense) formed by deprotonation at carbon atoms. Carbanions are very
important in synthesis because they are good nucleophiles and formation of new
carbon-carbon bonds often requires a nucleophilic carbon species. Carbanions vary
widely in stability, depending on the hybridization of the carbon atom and the ability
of substituent groups to stabilize the negative charge. In the absence of a stabilizing
substituent, removal of a proton from a C–H bond is difficult. There has therefore
been much effort devoted to study of the methods of generating carbanions and
understanding substituent effects on stability and reactivity. Fundamental aspects of
carbanion structure and stability were introduced in Section 3.4.2. In this chapter we
first consider the measurement of hydrocarbon acidity. We then look briefly at the
structure of organolithium compounds, which are important examples of carbanionic
character in organometallic compounds. In Section 6.3 we study carbanions that are
stabilized by functional groups, with emphasis on carbonyl compounds. In Section 6.4
the neutral nucleophilic enols and enamines are considered. Finally in Section 6.5 we
look at some examples of carbanions as nucleophiles in S 2 reactions.
N

6.1. Acidity of Hydrocarbons

In the discussion of the relative acidity of carboxylic acids in Chapter 1 (p. 53–54),
the thermodynamic acidity, expressed as the acid dissociation constant in aqueous
solution, was taken as the measure of acidity. Determining the dissociation constants
of carboxylic acids in aqueous solution by measuring the titration curve with a
pH-sensitive electrode is straightforward, but determination of the acidity of hydro-
carbons is more difficult. As most are quite weak acids, very strong bases are required
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