C−H Bond Dissociation Energies of Alkyl Amines:  Radical Structures and Stabilization Energies

A previous experimental study of the αC−H bond dissociation energies (BDEs) of amines indicated a significant decrease in BDE (or increase in radical stabilization energy, Es) in the series primary, secondary, and tertiary. However, this was not supported by theoretical investigations. The αC−H BDEs of trimethylamine ((CH3)3NH), triethylamine ((C2H5)3NH), and tri-n-butylamine ((C4H9)3NH) and of the cyclic secondary amines piperidine, piperazine, morpholine, and pyrrolidine were therefore determined by photoacoustic calorimetry in benzene solvent. Ab initio procedures, which incorporated isodesmic reactions to minimize residual correlation errors, were used to obtain the BDEs of several of these for direct comparisons. Also the BDEs of methylamine (CH3NH2), ethylamine (C2H5NH2), isopropylamine ((CH3)2CHNH2)), and dimethylamine ((CH3)2NH) were calculated as a check on the earlier results. The experimental BDEs in kJ mol-1 at 298 K (±10 kJ mol-1), estimated from the photoacoustic calorimetric measurements, were as follows: trimethylamine 372, triethylamine 381, tri-n-butylamine 381, piperidine 385, piperazine 385, morpholine 389, and pyrrolidine 377. The ab initio results were in excellent agreement with these values. From earlier work and the present calculations the α-to-N C−H BDE of methylamine was estimated to be 388 ± <10 kJ mol-1, corresponding to a radical stabilization energy, Es, of ∼51 kJ mol-1. Contrary to the previous experimental finding, both theory and experiment showed that the increase in Es on alkylation either at N or C is expected to be less than 4 kJ mol-1. Values of for the α-C radicals of the smaller aliphatic amines, except that of methylamine, must therefore be revised. The three-electron two-orbital π-like interaction, which causes the αC radical stabilization, is maximized when the singly occupied spn orbital of C and the nonbonded doubly occupied spn orbital of N are anticoplanar to each other. Alkylamines preferably adopt a conformation in which at least one αC−H bond is anticoplanar to the lone pair on nitrogen, and the most stable carbon centered α-to-N free radical is that derived by abstraction of this H atom. In the five-membered pyrrolidine ring the radical adopts an envelope conformation with the C5 carbon atom at the vertex. This accommodates the favorable alignment of the spn orbitals of C• and N but has no C−H eclipsing interactions like those which occur in the parent. Thus, in effect, there is a reduction of strain on formation of the radical, and the BDE is lowered by ∼8 kJ mol-1 below that of typical secondary amines.