Kinetics and Mechanisms of Dehydration of Secondary Alcohols under Hydrothermal Conditions

Alcohol dehydration by elimination of water is central to a series of functional group interconversions that have been proposed as a reaction pathway that connects hydrocarbons and carboxylic acids under geochemically relevant hydrothermal conditions such as in sedimentary basins. Hydrothermal dehydration of alcohols is an example of an organic reaction that is quite different from the corresponding chemistry under ambient laboratory conditions. In hydrothermal dehydration, water acts as the solvent and provides the catalyst, and no additional reagents are required. This stands in contrast to the same reaction at ambient conditions, where concentrated strong acids are required. Hydrothermal dehydration is thus of potential interest in the context of green chemistry. We investigated the mechanism of hydrothermal alcohol dehydration for a series of secondary alcohols using studies of kinetics and stereoelectronic effects to establish reaction mechanisms. The E1 elimination mechanism dominates over the corresponding E2 mechanism, with the E2 mechanism being competitive with E1 only for the most favorable stereoelectronically restricted alcohols included in the present study. These results are relevant to understanding the kinetics and product distributions of alcohol dehydration reactions in natural geologic systems and can guide the development of organic chemical reactions that mimic geologic organic reactions under laboratory green chemistry conditions.