Exploring the Preference for Tertiary Substrates in E2 Reactions: Beyond Transition State Stability

The fundamental bimolecular elimination (E2) reactions are often taught with the assumption that more substituted alkenes are favored due to the stability of the alkene-like transition state. However, this quantum-chemical study shows that in a model E2 reaction involving 2-bromo-2-methylpropane and 1-bromo-2-methylpropane, both yielding 2-methylpropene, the tertiary substrate does not kinetically favor the reaction. The faster reaction with 2-bromo-2-methylpropane is instead attributed to a probability factor, given the greater number of antiperiplanar β-hydrogens. When this factor is absent, as in the case of 2-bromobutane, the Saytzeff product dominates, consistent with the stability of more substituted alkene-like transition states. Therefore, textbooks and curricula should include this probabilistic effect as a key factor explaining the preference for the E2 reaction in the substrate order: primary < secondary < tertiary.