The intermolecular isotope effects of
methyl-labelled and alcoholic carbon-labelled pinacol converted to pinacone
have been studied over a temperature range of 60-113.5 �C. Tests were made to
establish the validity of the results by using two starting materials for
synthesizing methyl-labelled pinacol, and several different methods of
analysing the products. The isotope effects were determined by measurements
made both on (i) pinacol and (ii) pinacone.
The results above 80 �C can be interpreted
in terms of the synartetic ion mechanism provided both the carbonium ion and
the transition state are largely unhydrated, and both the formation of the
carbonium ion and the methyl migration are slow steps.
Below 60 �C an alternative mechanism is
operative. Unless unsuspected exchange reactions are present, the results
suggest that the transition state is not so greatly distorted from a simple
ethane-type configuration as in the high temperature reaction. Results at
intermediate temperatures indicate that the alternative route proceeds via a
stable intermediate.