20.11: Summary

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Carboxylic acids are among the most useful building blocks for synthesizing other molecules, both in nature and in the laboratory. Thus, an understanding of their properties and reactions is fundamental to understanding biological chemistry. In this chapter, we’ve looked both at acids and at their close relatives, nitriles ($\ce{RC≡N}$).

Carboxylic acids are named systematically by replacing the terminal -e of the corresponding alkane name with -oic acid. Like aldehydes and ketones, the carbonyl carbon atom is sp²-hybridized; like alcohols, carboxylic acids are associated through hydrogen-bonding and therefore have high boiling points.

The distinguishing characteristic of carboxylic acids is their acidity. Although weaker than mineral acids such as HCl, carboxylic acids dissociate much more readily than alcohols because the resultant carboxylate ions are stabilized by resonance between two equivalent forms.

Most carboxylic acids have pK_a values near 5, but the exact pK_a of a given acid depends on structure. Carboxylic acids substituted by electron-withdrawing groups are more acidic (have a lower pK_a) because their carboxylate ions are stabilized. Carboxylic acids substituted by electron-donating groups are less acidic (have a higher pK_a) because their carboxylate ions are destabilized. The extent of dissociation of a carboxylic acid in a buffered solution of a given pH can be calculated with the Henderson–Hasselbalch equation. Inside living cells, where the physiological pH = 7.3, carboxylic acids are entirely dissociated and exist as their carboxylate anions.

Methods of synthesis for carboxylic acids include:

oxidation of alkylbenzenes,
oxidation of primary alcohols or aldehydes,
reaction of Grignard reagents with CO₂ (carboxylation), and
hydrolysis of nitriles.

General reactions of carboxylic acids include:

loss of the acidic proton,
nucleophilic acyl substitution at the carbonyl group,
substitution on the α carbon, and
reduction.

Nitriles are similar in some respects to carboxylic acids and are prepared either by S_N2 reaction of an alkyl halide with cyanide ion or by dehydration of an amide. Nitriles undergo nucleophilic addition to the polar $C≡N$ bond in the same way that carbonyl compounds do. The most important reactions of nitriles are their hydrolysis to carboxylic acids, reduction to primary amines, and reaction with Grignard reagents to yield ketones.

Carboxylic acids and nitriles are easily distinguished spectroscopically. Acids show a characteristic IR absorption at 2500 to 3300 cm^–1 due to the O−H bond and another at 1710 to 1760 cm^–1 due to the $C═O$ bond; nitriles have an absorption at 2250 cm^–1. Acids also show ¹³C NMR absorptions at 165 to 185 δ and ¹H NMR absorptions near 12 δ. Nitriles have a ¹³C NMR absorption in the range 115 to 130 δ.

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