Mechanisms and Selectivities of Alcohol Dehydration

Catalytic dehydration can proceed according to the following types of mechanisms, where A and B stand for acidic and basic centers of catalysts, respectively.  

ElcB mechanism The first step of dehydration is the formation of a ctu banion, 

E2 mechanism The elimination of a proton and a hydroxyl group from alcohols are concerted without formation of ionic intermediates. Alumina is a typical E2 oxide. 

The three mechanisms can be distinguished in various ways, though, unlike the case of liquid phase reactions, the kinetic method cannot be used. The product distribution is one of the most significant clues. With the El mechanism, isomerization takes place in the carbenium ion stage. Thus, the formation of 2-butene from 1-butanol is indicative of the El mechanism. High selectivity for 1-butene (Hofmann orientation) from butan-2-ol is indicative of lcB, whereas 1 and 2 give mainly 2-butene (Sayt-zeff orientation). 

The dehydration of isobutyl alcohol over Si02—AI2O3 yields a mixture of butenes in which the fraction of n-butene is 33%. Since the rate of skeletal isomerization of isobutene to n-butenes is significandy lower than the rate of formation of a-butenes in dehydration, n-butenes must be primary products. This indicates that the reaction proceeds via the 1 mechanism. Formation of n-butenes is associated with the formation of the least stable isopropyl carbenium ions, which are readily rearranged by hydride or methyl transfer to form more stable tertiary or secondary carbenium ions. 

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