Kinetics and mechanism of hydrolysis of some amine–cyanoborances in acid perchlorate media

Abstract

In acid media ([H⁺]= 0.02–0.2 mol dm^–3), isoproplyamine– and diisopropylamine–cyanoborane are hydrolysed completely to boric acid, hydrogen, hydrogen cyanide and the salt of the corresponding amine. The reaction occurs in steps, viz. (1) rapid loss of CN^– as HCN and one H^– as H₂, (2) rate-determining B–H bond breaking and loss of the second H^– as H₂ and (3) very slow B–N bond breaking. The B–H bond breaking is rate determining also in the acid hydrolysis of BH₃(CN)^– ion. Step (2) is first order both in [substrate] and [H⁺]. Rate constant and activation parameters for step(2) have been determined. Cyanoborane adducts of tertiary amines, Me₃N and Prⁱ₃N, Do not hydrolyse and it appears that the steric bullk of the non-leaving amine group has the dominant effect on the rate of hydrolysis of the adducts.