Elimination reactions where a quaternary ammonium ion is the leaving group obey the Hofmann rule, yielding the less substituted alkene. This is the opposite of the Zaitsev rule. The reason is that with base-promoted elimination of an alkyl halide, the transition state has substantial double bond character, but in this case, with the presence of a strongly electron withdrawing, positively charged, nitrogen, the activated complex has character substantially determined by the carbon-hydrogen bond cleavage. The β-hydrogen is extracted by the base from the least hindered carbon.

While the Hofmann rule is not required prior knowledge for the MCAT, it would be an ideal passage topic on the test.












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