Equilibrium acidities and homolytic bond dissociation energies (BDEs) of the acidic H-N bonds in hydrazines and hydrazides

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 = 1,37pK(HA) + 23.06E(OX)(A(-)) + 73.3 kcal/mol. The alpha-N-H bonds in the hydrazides CH3CONHNH2, PhCONHNH2, NH2NHCO2Et, and PhSO2NHNH2 were found to be 2 to 4 pK(HA) units more acidic than the alpha-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 beta-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.