Action of Heat on Dicarboxylic Acids

Thermal Behavior of Dicarboxylic Acids

The reactions that occur when dicarboxylic acids are heated depend critically upon the chain length separating (intervening) the two carboxyl groups. Cyclization usually is favored if a strainless five- or six-membered ring can be formed.
Dicarboxylic acids can be divided into three groups on the basis of effect of heat on them.
1. With one or no Intervening Carbon
2. With Two or Three Intervening Carbon
3. With Four or Five Intervening Carbon

1. With one or no Intervening Carbon

Dicarboxylic acids in which the two COOH groups are directly linked to each other or to the same carbon atom, when heated split out a molecule of carbondioxide from one of the COOH groups to form monocarboxylic acids.
Examples-
Oxalic acid and Malonic acid on heating gives formic acid and acetic acid respectively.

2. With Two or Three Intervening Carbon

Dicarboxylic acids in which the two COOH groups are separated by two or three carbon atoms eliminates a molecule of water to form cyclic anhydride when heated.
Examples-
Succinic acid and Glutaric acid on heating gives Succinic anhydride and glutaric anhydride respectively.

3. With Four or Five Intervening Carbon

Dicarboxylic acids in which the two COOH groups are separated by four or five carbon atoms eliminates a molecule of water and carbondioxide to form cyclic ketones when heated alone or distilled with acetic anhydride.
Examples-
Adipic acid and Pimelic acid on heating gives cyclic pentanone and cyclohexanone respectively.