A THEORETICAL-STUDY OF ALKYL SUBSTITUENT EFFECTS IN INTRAMOLECULAR SN2 REACTIONS .3. PERMISSIBLE DEVIATIONS FROM THE IDEAL TRANSITION GEOMETRY

In Parts I and II, transition structures for several (Cl-+RCl), (amine +RCl) and (R-+RCl) S(N)2 reactions were determined. In this paper ''pe rmissible'' deviations (costing less than 1 kcal/mol) from these struc tures are studied. For this purpose, the potential surface near the sa ddle-point was explored by freezing the substrate in its TS geometry a nd moving the nucleophile. It was found that: Moving along the Nu-C in cipient bond is more difficult than moving in a perpendicular directio n. The maximal value for axial displacement is approximately 0.30 angs trom and is double this for lateral displacements. Moving the nucleoph ile towards the reaction center is more difficult than moving away fro m it. For the same cost in energy, the ratio of the two displacements is approximately 2:3. If the nucleophile lies on the pseudo-ternary ax is in the transition state, displacements in the vertical and lateral directions are equally facile. The deviation of the direction of attac k is then less-than-or-equal-to 8-degrees. If the nucleophile lies out side this axis, ''permissible'' vertical deviations are about 2-degree s smaller than lateral deviations. The connections between these findi ngs and the critical distance of approach and the activation entropy i n intramolecular substitutions are discussed. In particular, it is sug gested that, paradoxically, good nucleophiles and leaving groups may s ometimes render the intramolecular S(N)2 more difficult, or even impos sible.