A common Sophomore Organic Chemistry laboratory experiment that has great potential for further research is the acid catalyzed dehydration of simple alcohols. The classic dehydration of 2-methylcyclohexanol experiment that was introduced in Journal of Chemical Education in 1967 Taber(1967)JCE:44,p620. The rather simple procedure of distilling an alcohol with an aqueous acid has spawned several investigations that have resulted in formal journal articles. At the same time, the experiment has retained its popularity in the Sophomore Organic Chemistry laboratory curriculum. In one line of inquiry it has been observed that a mixture of 2-methylcyclohexanol diastereomers gives rise to a mixture of three isomeric alkenes Todd(1994)JCE:71,p440; Feigenbaum(1987) JCE:64, p273; Cawley (1997) JCE:74l, p102. Explaining the presence of the three alkene products requires an intense synthesis of information communicated in a typical SOC textbook. The continued popularity of this experiment is corroborated by the observation that Googling the phrase “Dehydration of 2-Methylcyclohexanol” on January 13th, 2008 returned no less than 20 hits for online student handouts and/or guides for this SOC laboratory experiment. Moreover, this experiment provides fertile ground for experimentation and innovation that has not yet been fully explored. At Dominican University, the SOC students performed this experiment during the Fall 2007 semester with not only the dehydration of 2-methylcyclohexanol (Aldrich 153087) but also the 4-methyl (Aldrich 153095) and 3-methyl (Aldrich 139734) positional isomers. The reaction products were submitted to GC-FID analysis.
As predicted from the Journal of Chemical Education articles, three methylcyclohexene products were observed. Their relative abundance measured by peak height was 80, 16, and 4%. The alkene products represented by these peaks apparently correspond to 1-methycyclehexene, 3-methycyclehexene, and methylenecyclohexane respectively. [pic]
The dehydration of 4-methylcyclohexanol produce two products, that can be distinguished by our current GC column, at 90 and 10% with retention times that match 3-methycyclehexene and 1-methycyclehexene respectively. My current theory is that the retention times 3 and 4-methycyclohexene could not be distinguished with GC column and temperature program. However, there is still the issue of how 1-methycyclehexene is produced from 4-methylcyclohexanol. [pic]
The dehydration of 3-methylcyclohexanol yields two products, that can be distinguished by our current GC column, at 80 and 20% with retention times that match 3-methylcyclohexene and 1-methycyclehexene respectively. [pic]
Samples of 1-methyl and 3-methyl cyclohexenes purchased from Aldrich chemical confirmed two of compound assignments for the dehydration of 2-methylcyclohexanol. Obviously, it remains to separate the 3 and 4-methylcyclohexene by GC.
There are several advantages of studying the dehydration of methylcyclohexanols in the first semester of Organic Chemistry: 1) The experiment involves reactions that are typically studied during first semester: E1, E2, and the 1,2-hydride shift. It is a time-tested protocol that has been run in hundreds of labs by thousands of students.
2) Analysis of the experiment involves the understanding of all three mechanisms mentioned previously and how they may compete with each other. In other words, it is a simple experiment that demands a rather involved interpretation of results.
3) It shows that textbooks “rules” such as the Zaitzev’s rule in this case, are not necessarily rules as such, but rather astute observations of general trends that can vary experimentally depending on the reactant and the reaction conditions.
4) Analytically, we are observing/measuring the presence of 3 known methylcyclohexene and methylenecyclohexane products that can be separated and detected by Gas...