The effect of silyl substitution at carbon on the SN2 reactivity is even more dramatic. The calculated barriers for the sN2 hydride exchange reactions2'5a, 5b, and 5c are 47.4 (44.3), 50.1 (47.4), and 35.9 (35.3) kcalamol-', respectively, at 6-31G*//3-21G (the values in parentheses are at 3-21G//3-21G).