Rate and Equilibrium Constants for Reactions Transferring a Dimethylcarbamoyl Group between Pyridines

We have studied 18 reactions, including four identity reactions, involving transfer of a dimethylcarbamoyl group with N-acylpyridinium bonds to pyridine and its 4-substituted derivatives in acetonitrile solutions at 298 K. The rate constants k _ij varied within the range 0.4 to 10^–6 L/mol·s; the equilibrium constants K _ij varied from 10⁷ to 10^–5. The rate and equilibrium for exchange of carbamoyl groups are described satisfactorily by the Brönsted equation. We have shown that all the reactions occur according to a forced concerted S _N2 mechanism. The structure of the transition state and its position on the reaction coordinate for identity transfer are considered using a More O'Ferrall-Jencks diagram.