Solving the Mathematics of a Kinetic and Regiochemical Problem: The Dehydration Reaction of a cis-trans Mixture of 2-Methylcycloalkanols

Abstract

A take-home exam is presented. It is based on an experiment that studied the dehydration of a cis-trans mixture of 2-methylcyclohexanols. The "givens" are the idealized kinetic and regiochemical data. The focus is strictly on the mathematics, not the experimental details. The goal is to calculate six specific rate constants using a little ingenuity and some fundamental mathematics, to show that the Saytzeff elimination rule is followed for the dehydration of each of the two reactant isomeric alcohols.

An Improved Mechanistic Interpretation of the Acid-Catalyzed Dehydration of Methylcyclohexanols

“…The previously published results − and those provided in this work for the acid-catalyzed dehydration of methylcyclohexanols can be rationalized by the E1 reaction pathways shown in Scheme . The regioisomeric methylcyclohexanols 1 , 4 , 10 , and 11 are protonated in the acidic conditions to form a water leaving group.…”

“…The simplistic interpretation of two experimental observations has led to confusion over the mechanistic details of the acid-catalyzed dehydration of 1 . Propagation of the so-called Evelyn effect and the observation of the enhanced rate of dehydration of 1- cis compared to 1- trans , have both resulted in the invocation of an E2 mechanism. The Evelyn effect was based on the observation that the initial and final distillates from the dehydration of 1 contain different ratios of alkenes under certain reaction conditions.…”

“…If the carbocations are accessible in the reaction, all of these cycloalkenes will be commonly observed in the dehydration of methylcyclohexanols . It is not surprising that all of the reported dehydrations of 1-methylcyclohexanol ( 4 ) or 2-methylcyclohexanol ( 1 ) have generated methylcyclohexene ( 2 ) as the major product − as it is the most stable alkene isomer produced from the most stable carbocation. The relative abundances of the other products will largely depend on the starting alcohol(s) and the complicated kinetics of the dehydration.…”

“…An E1 pathway was proposed to rationalize the products generated by carbocation rearrangement. Shortly thereafter, using NMR observations of the reaction at five time points over ∼30 min, Cawley and Lindner 6,7 determined a smaller difference in the experimental rates of dehydration of 1-cis/1-trans: (8.4:1 rather than the previous value of 30:1). 1 They disputed the previous claim 5 that the E1 and E2 pathways were both active and asserted an ill-defined E2-like reaction that followed first-order (E1) kinetics.…”

“…Despite these simple reactions being a common fixture in organic laboratory curricula, featured in laboratory textbooks, , and having inspired multiple educational implementations, − a coherent conceptual description has so far not been communicated. Herein, similar to other dehydration reactions, , we present computational and modern spectroscopic techniques to clarify the mechanistic description of the acid-catalyzed dehydration of 1 .…”