The Stability of Carbonium Ions
Organic chemists are taught that carbonium ion stability is predominantly due to
electronic factors such as whether they are primary, secondary, tertiary, allylic,
benzylic etc. What is mentioned much less is that their relative stability is also very
sensitive to their geometries, and in particular the angles of the three substituents
at the carbon. We will use a technique known as
Molecular Mechanics to predict the relative stabilities of the species shown below.
The only surprising result is that the MM method
predicts that the tertiary carbonium ion is actually LESS stable than the
preceeding primary ion, which is surprising to say the least. Clearly, electronic factors
must be playing an important role as well. Suffice to say that even without ANY
consideration of electronic factors, the MM2 method does not do too badly in its
prediction of relative stabilities of this set of carbonium ions.
This re-inforces the conclusion that bond angles
in carbonium ions are just as important as substitution! Take a look for example at the
angle contribution to the MM energies, which varies most. Also included are the
semii-empirial (PM3) energies, which take into account lots of other factors, including
effects such as hyper-conjugation, and p conjugation.
In this particular system, the differences in these effects are only secondary in
controlling the stability.
