The catalytic effect of water in basic hydrolysis of methyl acetate: a theoretical study

Base catalyzed hydrolysis of methylacetate has been investigated at the ab initio MP2/6-31+G*//HF/6-31+G* level of theory. In the gas phase the B(AC)2 -mechanism is favoured over the S(N)2-mechanism by 22 kJ mol-l in the compu ted free energy of activation. Two water molecules were added to the tetrah edral intermediate and to the third transition state of the B(AC)2-mechanis m to simulate specific solvation of the reacting species, One of the water molecules was found to take part in a water-assisted six-membered cyclic tr ansition state. The polarized continuum solvent model (PCM) was applied to all the optimized geometries to simulate the aqueous bulk medium. The compu ted free energy of activation of the water-assisted second transition state is 14 kJ mol-l lower than that of the corresponding non-assisted transitio n state. The B(AC)2-mechanism in aqueous solution is favoured over the SN2- mechanism by 56 kT mol-l in free energy of activation. The computed free en ergies of activation of the B(AC)2-mechanism in aqueous solution are in goo d agreement with experimental data. (C) 1999 Elsevier Science BV. All right s reserved.