Dehydration of Methylcyclohexanols Part IV

I want to pursue point #5 further by first grappling with the current literature concerning the “Evelyn Effect.” The JCE article by David Todd, “The Dehydration of 2-Methylcyclohexanol Revisited: The Evelyn Effect” observes a kinetic effect that can be explained by proposing that in a mixture of cis/trans 2-Methylcyclohexanol the cis isomer reacts much faster than the trans isomer to give predominately 1-methylcyclohexene. The formation of 1-methylcyclohexene from cis-2-methylcyclohexanol would involve an “E2-like” anti-elimination of proton and the protonated alcohol. The dehydration of the trans isomer would go through a E1 mechanism that requires the formation of a carbocation before elimination of a proton. A follow-up study by Cawley and Linder: “The Acid Catalyzed Dehydration of an Isomeric 2-Methylcyclohexanol Mixture” involves a detailed kinetic study. Students began with a 36.6/63.4 cis/trans mixture of 2-methylcyclohexanol with a cyclohexanol impurity (% impurity was not reported). They performed thy typical reaction+distillation and collected fractions at 4, 8, 16, 24, and 28 minutes. They also collected a 0.1 mL volume of the sample of the reaction mixture at each of these time intervals. These fractions were analyzed by 1H NMR and GC for composition. The cis/trans rate constants for the dehydration of reaction were determined to be 8.4/1.0 – much less than 30/1 ratio reported in 1931 by Vavon and Barbier. An intriguing study! It would be very interesting to have the raw (student) data on this one. Very little is said about the product ratios in the distillate fractions, they just report that they obtained 2.1% methylene cyclohexene and not the 4% previously reported.

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