Regioselectivity in epoxide ring opening is characteristic of an organic chemistry passage subject on the MCAT, but which is a bit too abstruse to be prior required knowledge for the exam. A shift in conditions to acid catalysis shifts the regioselectivity of epoxide ring opening from less highly substituted carbon to the carbon with more aliphatic substituents. In other words, anionic nucleophiles attack the less hindered carbon, while with acid catalysis, the other carbon is favored for attack. Under acid conditions, the epoxide oxygen is first protonated, and then carbocation character develops at the carbon under attack by the nucleophile in the rate-determining step. More highly substituted carbons can better support positive charge because of the electron releasing character of aliphatic substituents.
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