N−H and α(C−H) Bond Dissociation Enthalpies of Aliphatic Amines

Abstract

Bond dissociation enthalpies (BDEs) of a large series of aliphatic amines (21) were measured by means of photoacoustic calorimetry. Despite the different structures studied in the primary, secondary, and tertiary amine series, the α(C−H) BDEs were found to be very similar for unconstrained amines with values very close to 91 kcal/mol. αC- and N-alkylation or introduction of an hydroxy group only slightly affect the BDEs, a fact in perfect agreement with calculations performed at different CBS levels. This demonstrates the predominance of the two-orbital−three-electron interaction involving the N and αC^• orbitals. On the other hand, the N−H BDE decreases when going from primary to secondary amines. This result is interpreted in term of a hyperconjugation in σC−C bonds, which leads to a stabilization of the aminyl radical. For cyclized amines, the BDEs depend on the relative geometry of the singly occupied αC^• orbital with respect to that of the N atom, disfavoring the two-orbital−three-electron interaction. However, such structures can exhibit through-bond interaction. For a crowded structure such as triisopropylamine, for which the αC^• orbital is not coplanar with the nitrogen one, the relaxation of a strain energy allows the BDE to be comparable to flexible structures.