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.
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.
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.
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:
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% methylenecyclohexane and not the 4% previously reported.
The dehydration of methylcyclohexanols provides a fecund problem to explore. The key is to develop methods to determine the distribution of alkene products in terms of % total alkenes. There are four possible positional isomers:
Two of the alkene positional isomers contain an asymmetric carbon.
The obvious place to start is by studying how the alcohol structure affects the product distribution of alkenes. There are 5 positional isomers of methylcyclohexanol:
Three of the alcohols are present in cis and trans diastereomer pairs:
In addition there are 4 entaniomer pairs among the alcohol starting materials. Most of them are commercially available, for a price.
Besides the structure of the alcohol, what other variables may be explored?
The last installment of this series will explore the logistics of “dehydration of methylcycohexanols” as a collaborative experiments. The most straightforward collaboration would be to perform the “dehydration of methylcycohexanols” experiment in the same way and compare the relative yield of alkenes as measured by GC from different starting alcohols. Comparisons could be made with past data or concurrently collected data from different institutions. This may be seem fairly straightforward, but there will most likely be discrepancies that could will need to be explored. One aspect to make note of would be the source and composition of the methylcyclohexanols used a starting materials. Sigma-Aldrich has
An experimental variable that is hard to control is rate of heating. Students who crank up the hot plate to get done quickly (even though they were told not to) may get different results than those students who go slowly and maintain an even temperature. Different GC columns and methods may also give results that need to be corroborated.
Remember! This is just a sample.
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