(BQ) Part 2 book "Advanced organic chemistry (Part A: Structure and mechanisms)" has contents: Carbanions and other carbon nucleophiles, addition; condensation and substitution reactions of carbonyl compounds; aromaticity; aromatic substitution; concerted pericyclic reactions; free radical reactions; photochemistry. | 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 . 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 we study carbanions that are stabilized by functional groups, with emphasis on carbonyl compounds. In Section the neutral nucleophilic enols and enamines are considered. Finally in Section we look at some examples of carbanions as nucleophiles in SN 2 reactions. . 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 hydrocarbons is more difficult. As most are quite weak acids, very strong bases are required 579 580 CHAPTER 6 Carbanions and Other Carbon Nucleophiles to effect deprotonation.
