Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 123

acyl derivatives in Section 7.8 . In this, hydroxide substitutes the triiodocarbanion
to give the carboxylic acid. After proton exchange, the solid iodoform is deposited.
7.8 CARBOXYLIC ACIDS AND ACYL DERIVATIVES
Chapter 2 gave the structural features of carboxylic acids and acyl derivatives.
These compounds can be seen as functional classes with one heteroatom
bond which is modified by a carbonyl group. In Chapter 6, we saw how this
explains the acidity of carboxylic acids in which the alcohol and carbonyl
parts acted together. The acidic nature of carboxylic acids was limited to the
O-H bond.
In Section 7.7 , we saw how the reactions of the carbonyl group were mostly
started by nucleophilic attack to give addition products. Nucleophilic attack is
also the major reaction of acyl derivatives. However, instead of overall addition,
the reaction leads to substitution products as shown in Figure 7.37 .
7.8.1 Nucleophilic Acyl Substitution
The change from addition to substitution is because of the potential leaving group
Y in Figure 7.37 . In aldehydes and ketones, there is no simple leaving group. Only
hydride or carbanions are possible. Because these are both very strong nucleo-
philes, they are very poor leaving groups. Acyl derivatives such as acyl halides,
acid anhydrides, esters, carboxylic acids, and amides have better leaving groups.
These include halide, carboxylate, alkoxide, hydroxide, and amine anion.
The overall substitution reactions give the same result as the one-step S N 2 reac-
tions in Section 7.5.2 . However, they occur by a different mechanism. Figure 7.37
shows nucleophilic acyl substitution as a two-step process which goes through
an intermediate oxyanion.
FIGURE 7.37
Nucleophilic addition versus acyl substitution.
The relative reactivity of acyl derivatives in substitution is controlled by the
polarization in each derivative. The higher the polarity of the C-Y bond, the