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, E
s) 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, E
s, 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 sp
n
orbital of C and the
nonbonded
doubly occupied sp
n
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 sp
n
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.
