TYPES OF HETEROFUNCTIONAL COMPOUNDS

Lesson 7

POLY- AND HETEROFUNCTIONAL COMPOUNDS

Organic compounds of biological importance contain very often not one, but several functional groups. These groups can be identical or different. For example, ethylene glycol, HOCH₂CH₂OH, and glycerol, HOCH₂CH(OH)CH₂OH, contain two and three hydroxyl groups, respectively. Dicarboxylic acids, such as oxalic, malonic, succinic, have been discussed in the preceding chapter. These compounds are referred to as polyfunctionalones.

A significant importance in living systems belongs to heterofunctionalcompounds that involve different functional groups in the same molecule.

TYPES OF HETEROFUNCTIONAL COMPOUNDS

Hydroxyl, amino, oxo, and carboxyl groups are encountered most widely in heterofunctional compounds. A combination of different functional groups results in the formation of mixed classes of organic compounds, some of them are given in Table 1 (other combinations are possible, of course).

Table 1.Some types of combining functional groups in heterofunctional compounds

At the first approximation, the chemical behavior of heterofunctional compounds can be represented as a sum of properties of separate monofunctional classes. For instance, pyruvic acid (an oxo acid) can be esterified and transformed into derivatives on its carbonyl group. Salicylic and lactic acids (hydroxy acids) form esters in the reaction with alcohols, as well as their hydroxyl group can be acylated or alkylated. The reaction of salicylic acid with acetic anhydride is used to synthesize aspirin. Esterification of the same acid with methanol results in the formation of methyl salicylate.

In consideration of various combinations of functional groups we will mainly attend to new properties arising in such combinations without resort to familiar reactions of individual functional groups.

When the functional groups are close to each other their interaction is more sharply pronounced. This may be illustrated by comparing acidic and electrophilic properties of some heterofunctional carboxylic acids.

In the aliphatic series, all groups listed in Table 1 are electron-withdrawing substituents, therefore one group has an influence on another. Thus, lactic and pyruvic acids are stronger (pK_a 3.9 and 2.4, respectively) than propionic acid (pK_a 4.9). The hydroxyl group in lactic acid and the oxo group in pyruvic acid decrease an electron density on the carboxylic carbon (the leftmost and middle structures below).

On the other hand, the inductive effect of the carboxyl group results in a similar increase of δ+ on the atom C-2 as shown for pyruvic acid in the rightmost structure above. Both carbonyl carbons in pyruvic acid are stronger electrophilic sites as compared with monofunctional three-carbon analogues, i. e. acetone and propionic acid. Therefore pyruvic acid reacts with nucleophiles more readily by both nucleophilic addition and nucleophilic substitution reactions.

Problem 1.Classify, by writing the structural formulas, the following compounds as polyor heterofunctional ones, if any: (a) chloroform (trichloromethane); (b) glycolaldehyde (hydroxyethanal); (c) ethylene glycol (1,2-ethanediol); (d) phenetol (ethoxybenzene).