Dehydration of Methylcyclohexanol

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.

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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:

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 Chromatography. I believe that the product mixtures can also be analyzed by NMR.
5. The reaction lends itself to an inquiry format that involves the study different reactants and reaction conditions on the ratio of products. In fact, this experiment, in my opinion, is an ideal candidate for a multi-institution collaborative study that combines and interprets student data.

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:

• I. methylenecyclohexane (Aldrich, Acros, 1192-37-6);
• II. racemic 3-methyl-1-cyclohexene (Acros, 591-48-0);
• III. 1-methyl-1-cyclohexene (Aldrich, Acros 591-49-1)
• IV. racemic 4-methyl-1-cyclohexene (Aldrich, Acros 591-47-9).

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:

• I. cyclohexanemethanol (Aldrich 100-49-2);
• II. 1-methylcyclohexanol (Aldrich 590-67-0);
• III. racemic cis&trans 2-methylcyclohexanol (Aldrich 583-59-5)
• IV. racemic cis&trans 3-methylcyclohexanol (Aldrich 591-23-1)
• V. cis&trans 4-methylcyclohexanol (Aldrich 589-91-3).

Three of the alcohols are present in cis and trans diastereomer pairs:

• cis 2-methylcyclohexanol (Aldrich 7445-70-1)
• trans 2-methylcyclohexanol (Aldrich 7445-52-9)
• cis 3-methylcyclohexanol (5454-79-5)
• trans 3-methylcyclohexanol (7443-55-2)
• cis 4-methylcyclohexanol (Aldrich 7731-28-4)
• trans 4-methylcyclohexanol (Aldrich 7731-28-4).

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?

1. One variable for this reaction that could be investigated is the nature of the catalytic acid. Aqueous acids, such as the 85% H3PO4 typically used for this experiment, contain some water which is also product of the reaction. I may also add that, the amount of acid is not always in catalytic proportion to the substrate. In my current protocol 0.075 moles of acid is used to dehydrate 0.2 moles of alcohol. Non-aqueous acids may give different results. Acidic resins are an interesting substitute for aqueous acids. For example, John Ludeman and Kurt Field of Bradley University presented a poster at the 2006 ACS Great Lakes Regional Meeting on the use of Dowex 50WX2-100, Amberlite IRC-50S, and Amberlyst 15, for the dehydration of alcohols.
2. Another variable would be the reaction conditions. In the current paradigm, the alkene is distilled away from the reaction mixture. Presumably, it is being distilled away as it is formed. An ad-hoc observation is that students seem to get somewhat different product ratios if they distill is carefully or if they “crank up the heat” and distill it quicker. What if the reaction mixture was refluxed to equilibrium before distillation? Would we see more thermodynamic products?
3. Reaction conditions could be changed in other ways too. Microwave irradiation is currently being explored as an alternative to heating reactions. Possibly, sonication could also be performed on the alcohol.
4. Another avenue to explore may be different strategies to push the reaction towards the products other than distilling off the alkene. For example, removing water with molecular sieves may be tried.

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

• 1-methylcyclohexanol #M38214;
•  2-methylcyclohexanol #66320, #215295, #178829, #24113, & #153087,
• 3-methylcyclohexanol #139734;
• 4-methylcyclohexanol #66360, #104183, #104191, & #153095;
• as well as just plain methylcyclohexanol #66370.

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.