Equilibrium Acidities and Homolytic Bond Dissociation Energies (BDEs) of the Acidic H−N Bonds in Hydrazines and Hydrazides

Abstract

The equilibrium acidities in DMSO for phenylhydrazine, five of its p-substituted derivatives, 1,2-diphenylhydrazine, and 1,1,2-triphenylhydrazine were measured and the BDEs of their acidic N−H bonds were estimated by using the following equation: BDE = 1.37pKHA + 23.06Eox(A-) + 73.3 kcal/mol. The α-N−H bonds in the hydrazides CH3CONHNH2, PhCONHNH2, NH2NHCO2Et, and PhSO2NHNH2 were found to be 2 to 4 pKHA units more acidic than the α-N−H bonds in the corresponding amides, and the BDEs were estimated to be 23−27 kcal/mol weaker. Similarly, the BDE of a N−H bond in hydrazine was estimated to be 26 kcal/mol weaker than that of an N−H bond in NH3. Introduction of a RCO group into hydrazine had little or no effect on the BDE, but introduction of RCO into the β-position of PhCONHNH2 caused about an 8 kcal/mol increase in BDE. An increase in BDE was also observed for introduction of an RCO group into aniline. Here the carbonyl group is effectively destabilizing a nitrogen-centered radical by virtue of its strong electron-withdrawing effect. Incorporation of an open-chain carbohydrazide into a ring structure tends to strengthen the acidity of the N−H bond and weaken its BDEs.