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.

Expanding the Scope of an Electrophilic Aromatic Substitution Discovery Experiment Including Hydride and Methyl Shifts

“…The mechanisms include both hydride and alkyl shifts, including a ring contraction. 35,36 Kjonaas and Tucker showed that the conversion of an alcohol to an alkyl halide undergoes a hydride shift in the formation of the product. 37 Field and coworkers investigated the conversion of an alkene to 1,2dichloro-1,1,2,2-tetraphenylethane.…”

“…Two groups independently presented a valuable update to the classic dehydration of 2-methyl-cyclohexanol laboratory, including an examination of the formation of five previously unidentified products via carbocation rearrangements. The mechanisms include both hydride and alkyl shifts, including a ring contraction. , Kjonaas and Tucker showed that the conversion of an alcohol to an alkyl halide undergoes a hydride shift in the formation of the product . Field and co-workers investigated the conversion of an alkene to 1,2-dichloro-1,1,2,2-tetraphenylethane.…”

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

“…In 2011, two separate reports , attempted to clarify the mechanistic details of the dehydration of 1 . In addition to the previously reported species listed above, 4-methylcyclohexene ( 6 ), a cyclopentene ( 7 ), ethylidenecyclopentane ( 8 ), and vinylcyclopentane ( 9 ) were also detected via GC–MS analysis along with possibly other unidentified C 7 H 12 species.…”

“…Concurrently, Clennan and Clennan carried out the dehydration reaction using sulfuric acid, but did not isolate the products via distillation. They detected the same alkene isomers (excluding 9 ) as described above and several methylcyclohexanols by GC–MS analysis.…”

“…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 .…”