Dehydration of Methylcyclohexanol Isomers in the Undergraduate Organic Laboratory and Product Analysis by Gas Chromatography−Mass Spectroscopy (GC−MS)

Abstract

Dehydrations of cis- and trans-2-methylcyclohexanol mixtures were carried out with 60% sulfuric acid at 78−80 °C as a function of time and the products were identified by gas chromatography−mass spectroscopy (GC−MS) analysis. The compounds identified in the reaction mixtures include alkenes, 1-, 3-, and 4-methylcyclohexenes and 1-ethylcyclopentene, as well as the alcohols, 1-, 3-, and 4-methylcyclohexanols, and residual starting material. Independent reactions under identical conditions of the pure cis- and trans-2-methylcyclohexanol isomers, 1-, 3- and 4-methylcyclohexanol were also carried out. These reactions reveal that the cis-isomer reacts predominately to form 1-methylcyclohexene whereas the trans-isomer reacts to give a complicated mixture consisting of the isomeric 1-, 3-, and 4-methylcyclohexanols, 1-, 3-, and 4-methylcyclohexenes, and the ring contraction product, 1-ethylcyclopentene. In the experiment, each student carries out a dehydration of the cis- and trans-2-methylcyclohexanol mixture or of one of the pure isomers and the results from the GC−MS analyses of the whole class are pooled. The analysis reinforces lecture material and provides an appreciation of the complexity of elimination−dehydration reactions that proceed via carbocation intermediates and can undergo intricate rearrangements such as hydride and alkyl shifts. The reaction procedure is straightforward and the GC−MS analysis simple enough so it can easily be used in the first semester of undergraduate organic chemistry laboratory.