KINETIC STUDIES OF INTRAMOLECULAR HYDROGEN BONDING IN METHYL AND ETHYL SALICYLATES AND SALICYLALDEHYDE BY MEANS OF ULTRASONIC ABSORPTION MEASUREMENT

The Ultrasonic relaxational absorptions in methyl and ethyl salicylates and salicylaldehyde have been studied in the frequency range 2.5 to 95 Mc/sec at the various temperatures from 20 to 40°. The ultrasonic absorption measurements in derivatives of phenol and benzoic acid lead to the conclusion that the ultrasonic relaxations in methyl and ethyl salicylates and salicylaldehyde are associated with intramolecular hydrogen bonding between the phenolic hydroxyl group and an oxygen of adjacent carboxyl or carbonyl group. The kinetic values have been calculated for the reaction of intramolecular hydrogen bonding. The forward and backward rate constants at 30° are 1.2 × 106 and 3.1 × 107 sec-1 for methyl salicylate, 4.7 × 105 and 2.2 × 107 sec-1 for ethyl salicylate, and 4.0 × 105 and 2.6 × 107 sec-1 for salicylaldehyde, respectively. The heats of the hydrogen bonding at constant volume are 2.5, 2.5, and 3.5 kcal/mol for methyl and ethyl salicylates and salicylaldehyde, respectively.