TRANSITION-STATE STRUCTURAL VARIATIONS IN THE MENSHUTKIN REACTION - ACOMPUTATIONAL STUDY OF STERIC AND ELECTRONIC SUBSTITUENT EFFECTS

Transition structures for the gas phase S(N)2 reactions of substituted pyridines with methyl chloride, bromide, and iodide, respectively, ha ve been localized by the AMPAC program using the AM1 Hamiltonian. The reactions are strongly endothermic (Delta E approximate to 40-60 kcal/ mol) and have activation barriers of ca. 70 kcal/mol, and the calculat ed order of leaving group (X) abilities is I < Br < Cl. The carbon-nit rogen bond is longer for the more reactive leaving group. The structur al parameters are relatively insensitive to substitutions in the pyrid ine ring. Increasing the steric strain in the ortho position (methyl t o tert-butyl or 2,6-dimethyl) leads to looser and earlier transition s tates, whereas substitution with electron-withdrawing groups in the pa ra position has very little effect on the transition structure. Solven t effects were briefly mimicked by means of a dipole placed on the N.. .C...X axis with the positive charge pointing toward X. This results i n a decrease of the activation barrier, a less endothermic reaction, e arlier transition state, and a larger negative charge on X. The conseq uences of steric effects in this reaction are discussed.