TRANSITION STRUCTURES OF THE FRIEDEL-CRAFTS REACTION IN SOLUTION

We have studied the Friedel-Crafts reaction between HF and CH4 both uncatalysed and catalysed by BF3. Stationary structures have been located and characterized at the HF/6-31 + G level of theory both in the gas phase and in solution. For the uncatalysed process, by using the continuum model, we have located a transition structure in solution which is different from that in the gas phase. This structure is also different from that obtained including the solvation effects in the reaction path constructed in the gas phase. Discrete calculations including two solvent molecules agree with the results obtained with the continuum model. Two different mechanisms have been investigated for the catalysed process, an acid catalysis and a bifunctional one. Solvent effects also produce important changes in the transition structures of the catalysed processes from both the nuclear and the electronic points of view. The relative stability of the transition states of the acid and bifunctional catalysis is changed, showing that the solvent can modify the mechanism in solution, compared with the gas phase, for the catalysed process.