The compounds discussed in this chapter are all considered to be derived from carboxylic acids, and include acid halides, acid anhydrides, esters and amides (thioesters and acyl phosphates are also briefly mentioned). As you proceed through the chapter, you should be looking for similarities in behaviour among the various classes of compounds. These similarities can be readily understood once you appreciate the fact that, in most of their reactions, carboxylic acid derivatives react via the nucleophilic acyl substitution mechanism. In this chapter, we describe the nomenclature of the various types of carboxylic acid derivatives, and explain the relative reactivity of these compounds in terms of resonance contributions to the ground state of each type of compound. We describe the reactions of carboxylic acids, acid halides, acid anhydrides, esters, amides, polyamides and polyesters in detail, and discuss the biological importance of thiol esters briefly. The chapter concludes with a look at how infrared spectroscopy and NMR spectroscopy can be used in the identification of unknown carboxylic acid derivatives. Chapter Objectives 21.0 Introduction 21.1 Naming Carboxylic Acid Derivatives 21.2 Nucleophilic Acyl Substitution Reactions 21.3 Nucleophilic Acyl Substitution Reactions of Carboxylic Acids 21.4 Chemistry of Acid Halides 21.5 Chemistry of Acid Anhydrides 21.6 Chemistry of Esters 21.7 Chemistry of Amides 21.8 Chemistry of Thioesters and Acyl Phosphates: Biological Carboxylic Acid Derivatives 21.9 Polyamides and Polyesters: Step-Growth Polymers 21.10 Spectroscopy of Carboxylic Acid Derivatives 21.S: Carboxylic Acid Derivatives (Summary)