Alcohol Dehydration: Alkene Product

The elimination of water upon protonation of 2,3-dimethyl-2-butanol generates the 2,3-dimethyl-2-butanium ion, a tertiary cation. This cation can loose a beta-hydrogen as a proton to form an alkene. There are beta-hydrogen atoms available on two methyl groups as well as on the isopropyl group. The H atom of the isopropyl group is eliminated preferentially as it leads to the more stable alkene, the 2,3-dimethyl-2-butene. The proton transfer to water is endothermic by 26.7 kcal/mol and overall the reaction is endothermic by 22.4 kcal/mol. This alkene is formed in about 65% yield; the less stable alkene 2,3-dimethyl-1-butene is formed as a side product in about 35% yield.


Figure. The MP2(full)/6-31G* optimized structure of 2,3-dimethyl-2-butene.