6-67 (c) continued The only mechanism of these two involving C-H bond cleavage is the E l , but the C-H cleavage does NOT occur in the slow, rate-determining step. Kinetic isotope effects are observed only when C-H (C---D) bond cleavage occurs in the rate-determining step. Thus, we would expect to observe no change in rate for the deuterium-substituted molecules in the El or SNI mechanisms. (In fact, this technique of measuring isotope effects is one of the most useful tools chemists have for determining what mechanism a reaction follows.) 6-68 Both products are formed through E2 reactions. The difference is whether a D or an H is removed by the base. As explained in Problem 6-67, C-D cleavage can be up to 7 times slower than C-H cleavage, so the product from C-H cleavage should be formed about 7 times as fast. This rate preference is reflected in the 7 : 1 product mixture. ("Ph" is the abbreviation for a benzene ring.) D !f " .:-�H " ,C - C H" " Ph Br - D, Br .. H H " / C = C Ph/ 'H requires C-D bond cleavage; slow; minor product
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Benzene, Transition state, Electric charge, Molecule, R R, C-H bond