# 18: Carboxylic Acids and Their Derivatives


Almost all of the basic types of reactions now have been covered: addition, elimination, substitution, and rearrangement by polar, radical, and concerted mechanisms. Indeed, if you have been looking for similarities, you will have seen that most of the reactions discussed in the preceding three chapters are variations on basic types we have discussed earlier. Furthermore, most of the basic structural effects that determine chemical reactivity also have been covered in previous chapters: bond energies, steric hindrance, electronegativity, electron delocalization, hydrogen bonding, solvation, and conformational influences.

You might well ask what is left. The answer is, a great deal - but now we will be concerned mostly with putting concepts together, moving from the simple to the complex. For example, in this chapter we will be trying to understand the ways that carboxylic acids, which possess the $$\ce{-COOH}$$ functional group, are similar to and different from alcohols, which have the $$\ce{-OH}$$ group, and aldehydes and ketones, which have $$\ce{C=O}$$ bonds.

• 18.1: Prelude to Carboxylic Acids and Their Derivatives
Almost all of the basic types of reactions now have been covered: addition, elimination, substitution, and rearrangement by polar, radical, and concerted mechanisms. Indeed, if you have been looking for similarities, you will have seen that most of the reactions discussed in the preceding three chapters are variations on basic types we have discussed earlier.
• 18.2: Physical Properties of Carboxylic Acids
Carboxylic acids show a high degree of association through hydrogen bonding. Carboxylic acids have substantially higher melting points and boiling points of acids relative to alcohols, aldehydes, ketones, and chlorides can be attributed to the strength and degree of hydrogen bonding. Hydrogen bonding also is responsible for the high water solubility of the simple carboxylic acids with less than five carbons; water molecules can solvate the carbonyl group through hydrogen bonds.
• 18.3: Some Chemical Properties of Carboxylic Acids
Most of the reactions of carboxylic acids belong to one of four principal classes, depending on the point in the molecule where the reaction occurs. (1) Reactions involving the -O-H bond, (2) Reactions at the carbonyl bond, (3) Decarboxylation, and (4) Substitution on the R group.
• 18.4: Reactions at the Carbonyl Carbon of Carboxylic Acids
Many important reactions of carboxylic acids involve attack on the carbon of the carbonyl group by nucleophilic species. These reactions frequently are catalyzed by acids, because addition of a proton or formation of a hydrogen bond to the carbonyl oxygen makes the carbonyl carbon more vulnerable to nucleophilic attack.
• 18.5: Decarboxylation of Carboxylic Acids
The decarboxylation of RCO2H to give RH and CO2 can be calculated from bond energies and the stabilization energy of the carboxyl group to have ΔH°=−7 kcal/mol. This does not mean that the reaction goes easily. Special structural features are required. The simple aliphatic carboxylic acids do not lose carbon dioxide on heating, but when there are strongly electron-attracting groups attached to the αα carbon, decarboxylation often proceeds readily at 100 - 150°.
• 18.6: Reactions at the $$\alpha$$ Carbons of Carboxylic Acids
The halogen of an α-haloalkanoic acid is replaced readily by nucleophilic reagents. Thus a variety of αα -substituted carboxylic aids may be prepared by reactions that are analogous to SN2 substitution of alkyl halides. However, The SN1 reactivity of α-haloalkanoic acids is particularly low.
• 18.7: Functional Derivatives of Carboxylic Acids
A functional derivative of a carboxylic acid is a substance formed by replacement of the hydroxyl group of the acid by some other group, X , such that it can be hydrolyzed back to the acid.
• 18.8: Reactions at the Carbonyl Carbon of Acid Derivatives
Hydrolysis of most acid derivatives to the parent acids is acid- or base-catalyzed. Carboxylic acid derivatives also react with organomagnesium and organolithium compound
• 18.9: Reactions at the $$\alpha$$ Carbons of Carboxylic Acid Derivatives
Many important synthetic reactions in which C−CC−C bonds are formed involve esters and are brought about by basic reagents. This is possible because the αα hydrogens of an ester are weakly acidic, and a strong base, such as sodium ethoxide, can produce a significant concentration of the ester anion at equilibrium. The acidity of αα hydrogens is attributed partly to the electron-attracting inductive effects of the ester oxygens, and partly to resonance stabilization of the resulting anion.
• 18.10: Reactions of Unsaturated Carboxylic Acids and Their Derivatives
Unsaturated carboxylic acids of the type RCH=CH(CH2)nCOOH usually exhibit the properties characteristic of isolated double bonds and isolated carboxyl groups when n is large and the functional groups are far apart. As expected, exceptional behavior is found most commonly when the groups are sufficiently close together to interact strongly. We shall emphasize those properties that are exceptional.
• 18.11: Dicarboxylic Acids
Acids in which there are two carboxyl groups separated by a chain of more than five carbon atoms (n>5) for the most part have unexceptional properties, and the carboxyl groups behave more or less independently of one another. However, when the carboxyl groups are closer together the possibilities for interaction increase; we shall be interested primarily in such acids.
• 18.12: Methods of Preparation of Carboxylic Acids and Their Derivatives
• 18.E: Carboxylic Acids and Their Derivatives (Exercises)
These are the homework exercises to accompany Chapter 18 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).