ENADE Química Practice Questions - ENADE Química - Exame Nacional De Desempenho Dos Estudantes Exam

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When 2-bromo-2-methylbutane is treated with potassium tert-butoxide (t-BuOK) in tert-butanol under reflux, the major elimination product formed is 2-methyl-1-butene rather than 2-methyl-2-butene. Which mechanistic rationale explains this regiochemical preference?

Steric hindrance of the bulky tert-butoxide base restricts deprotonation to the more accessible methyl protons, yielding the less substituted Hofmann alkene. The bulky base destabilizes the anti-periplanar transition state required for the Hofmann product, forcing formation of the thermodynamic alkene. The reaction proceeds via a free carbocation E1 mechanism where hyperconjugation stabilizes the primary alkyl radical. Potassium tert-butoxide undergoes unimolecular conjugate addition, favoring the Zaitsev thermodynamic product at elevated temperatures.

Correct Answer:

Steric hindrance of the bulky tert-butoxide base restricts deprotonation to the more accessible methyl protons, yielding the less substituted Hofmann alkene.

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