In the Laboratory
The Acid Catalyzed Dehydration of an Isomeric 2-Methylcyclohexanol Mixture A Kinetic and Regiochemical Study of the Evelyn Effect John J. Cawley* Department of Chemistry, Villanova University, Villanova, PA 19085 Patrick E. Lindner Dartmouth College, Hanover, NH 03755 Background
as known up to this time.
The above-titled dehydration reaction has been a staple item (1) on the organic chemistry lab menu for decades. Recently a regiochemical study relative to fractions collected at various time intervals has been reported (2) in this Journal. We decided to investigate the kinetics as well as the regiochemistry in our laboratories because (i) the cited (3) ratio of rate constants for dehydration of the isomeric 2-methylcyclohexanols and (ii) the cited (2) NMR results of Professor James Cason, which claimed an 8 rel.% of methylenecyclohexane, seemed paradoxical to us. Further, the recent finding (4) that in a closely related 5-membered ring system E1 elimination products “track” with but are not in the exact thermodynamic magnitude of preference made us question that both E1 and E2 mechanisms had to be invoked (2) to explain the Evelyn Effect. In light of the preceding background material, we decided to prepare a new experiment for the second semester organic chemistry laboratory once we had garnered all the necessary data. This experiment is a kinetic and regiochemical study as it relates to the “Evelyn Effect” (2).
Typical Procedure
Philosophy This experiment is designed for an elite group of second semester organic chemistry laboratory students. It is intended to be an introduction to research. The interaction between the instructor (research director) and these students (researchers) is meant to be dynamic from start to finish. Setting up the experiment and gathering the data is not difficult, but involving the researchers in finding out why they are treating the raw data in the fashion proposed will require interactive meetings of some duration. When all the results are derived, the search for mechanism begins. The research director may want to relate the meaning of Ockham’s Razor (5) to the researchers at the outset. In any case they should be reminded that they are looking for a consistent answer as well as that elusive simplest consistent answer. It may be that the formulating the simplest consistent answer will be the longest part of the experiment. Nevertheless, it is the most important part and will require the greatest output of the instructor to elicit from the students the “best” answer consistent with the facts,
N OTE: Make sure that you prelabel all test tubes, corks, and Erlenmeyer flasks before you begin the experiment.
The Reaction Add 6 mL of chilled 85% phosphoric acid to 25 mL (0.203 mol) of chilled 2-methylcyclohexanol, contained in a 50-mL graduated Erlenmeyer flask. Swirl the flask to mix the contents, record the initial volume (Vi), and quickly transfer to a 100-mL round bottom flask. Add a couple of boiling chips. Arrange the flask for distillation in your apparatus. Make sure the condenser water is running before heating. When the first drops of liquid reach the thermometer collect a 100-µL sample of the reaction solution and deliver it into a 10-mL test tube containing 5 mL of 3 N NaOH. Cork and set aside. Distill slowly, collecting the product into one of the 10-mL test tubes containing a small amount of K2CO3 (all of which are in an ice bath) until 4 min has elapsed. Switch to a second 10-mL test tube, tightly corking the previous one. Collect another 100-µL sample of reaction solution, treating it as you did before. Continue to collect until 8 min has elapsed. Switch to a third 10-mL test tube, tightly corking the previous one and collect another 100-µL sample of reaction treating it as you did before. Continue this operation at the 16-, 24-, and 28-min elapsed time marks. Make sure you record the volume of each collected sample. Dismantle your apparatus, clean up the equipment and make sure your work area is neat and clean before continuing. Experimental Procedure for Collecting the Appropriate Experimental Data In principle the experimental data could be collected in four different ways, as follows: 1. reactant and product data by NMR 2. reactant and product data by capillary vapor pressure chromatography (VPC) 3. reactant data by NMR and product data by capillary VPC 4. reactant data by capillary VPC and product data by NMR
Since most undergraduate labs will more likely have access to NMR than capillary VPC, only option 1 will be
*Corresponding author.
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In the Laboratory
presented here. Nevertheless, capillary VPC has certain advantages over NMR, such as the advantage that all the product “enes” including cyclohexene from the reactant impurity, cyclohexanol, are separable by capillary VPC, whereas in the NMR experiment the vinyl protons of cyclohexene are coincident with part of the vinyl proton pattern of 3-methylcyclohexene. For the capillary VPC experiment a Carbowax 20 M column is required for the reactant alcohols and a methylsilicone column (SP-2100) for the product enes.
The NMR Experiment For the product enes, make up 5% by volume deuterochloroform (DCCl 3) solutions (5) of the 4-, 8-, 16-, 24-, and 28-min ene mixtures. For the vinyl hydrogen region integrate the areas, paying special attention to the =C–H vs. the =CH2 environments. Don’t forget to normalize by multiplying by two (2) for the 1methylcyclohexene vinyl hydrogen area. The data will give you the product ratio for the 0–4-, 4–8-, 8–16-, 16–24-, and 24–28-min fractions. Also for the product enes, mix the correct proportional volumes for the 4- and 8-min fractions, the 4-, 8-, and 16-min fractions, the 4-, 8-, 16-, and 24-min fractions, and the 4-, 8-, 16-, 24-, and 28-min fractions. These samples will yield the kinetic data for product formation. Perform the NMR experiment as before. Don’t forget to “scale” the areas of the different fractions to take care of the fact that the reactant solution is being concentrated because the products are being distilled out as formed. These areas then become the quantity of product formed at the 4-, 8-, 16-, 24-, and 28-min times. For the reactant alcohols add approximately 1 mL of chloroform, CHCl3, to each test tube; then add 10 µL of cyclohexanol to each test tube as an internal standard for the NMR experiment. For each sample, separate the chloroform layer and evaporate it using a water aspirator. Take up the reactant alcohols in sufficient DCCl3 for the NMR experiment. For the hydroxyl proton region, integrate the three signals. Then “scale” the areas of the reactant alcohol fractions to take care of the fact that the reactant solution is being concentrated because the products are being distilled out as formed. Finally, normalize to the cy-
clohexanol hydroxyl proton.
Data Treatment Calculate the rate constant and half-life for the decrease in each reactant alcohol. For the mechanism, try to conceptualize one bridging ion for each reacting isomer that allows you to form the three isomeric enes in each case. Further, remember nearly anti “minor” transition states (E2) when considering mechanisms of product formation from any bridging ions that form in the rate-determining step. Assign relative rate constants for the production of the individual enes from each bridging carbocation intermediate using fxc’s and fxt ’s. Then check to make sure your relative rate constants make sense in proportion to the production of these enes with time. For the Instructor The mechanism, in the parlance of the variable transition state theory, is most probably “E2-like”. For simplicity it may be easier to conceptualize two separate bridging ions, one from the cis alcohol and one from the trans alcohol, as follows in Figures 1 and 2, which then proceed with E2 anti restraints to give product enes. That picture will account for all the facts without having to invoke any free 2° or 3° carbonium ions. Conclusions It will take a lot of “give and take” between the instructor and the lab students until each student is able to accomplish what is being asked in the data treatment section. The potential for student growth in chemical knowledge and in sophistication is maximized when the instructor and students take full advantage of the exchanges that necessarily take place over the course of this experiment. In this way the student is given an excellent introduction to research. Epilogue In our labs the ratio of rate constants (cis/trans) for dehydration of a 36.6/63.4 cis/trans mixture of alcohols in 85% phosphoric acid was determined to be 8.4/1.0 [ trans alcohol +H+ ]
[ cis alcohol +H+ ]
kc
Hc Hb
kt
Ha
Hb
+
CH2
+ + H2O
+ H2O CH2
Ha Hc
- Ha+ fa c
-Hc+ fcc o
fbc -Hb+
1-methylcyclohexene + 3-methylcyclohexene +
kc = 0.32 min fa c = 0.94 fb c = 0.063 fc c = 0.000
- Ha+
methylenecyclohexane
{1
Figure 1. The reaction of cis-2-methylcyclohexanol.
fa
t
t
+
fb -Hb
fct
-Hc+ o
1-methylcyclohexene + 3-methylcyclohexene + methylenecyclohexane
kt = 0.038 min {1 fa t = 0.75 fb t = 0.21 fct = 0.04
Figure 2. The reaction of trans-2-methylcyclohexanol.
Vol. 74 No. 1 January 1997 • Journal of Chemical Education
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In the Laboratory
(0.32/0.03), not 30/1 (3). When we went back to the original article (3) and did our own calculation, based on both reactions being run at 160 °C in 3% sulfuric acid in di-n-butyl ether, we obtained a value of 13/1.0, which seems reasonable in that solvent system and for that temperature. The cited (2) NMR results of Professor James Cason were not reproduced by us. We found that 2.1 rel. % (not 8 rel. %) of the total enes is methylenecyclohexane, over 27 min. In fact the maximum methylenecyclohexane was determined to be 3.7 rel. % for a fraction taken between 8 and 28 min reaction time. We calculate 4 rel. % of the total enes is methylenecyclohexane, over ∞ time for reaction of pure trans alcohol. Lastly, the claim (2) that both E1 and E2 mechanisms are involved appears to be incorrect. Only 0.9 rel. % methylenecyclohexane appears in 4 min, approximately two half-lives for the cis alcohol, which would appear to be the most likely candidate to lead to a free 3° carbonium ion via hydride migration from an initially formed 2° carbonium ion, utilizing an E1 pathway. This result clearly surprised us! Since the reaction conditions are clearly not E2, it is likely that the mechanism is neither E1 nor E2 but rather “E2-like”, exhibiting first order kinetics. Literature Cited 1. Taber, R. L.; Champion, W. C. J. Chem. Educ. 1967, 44, 620. 2. Todd, D. J. Chem. Educ. 1994, 71, 440. 3. Vavon, G.; Barbier, M. Bull. Soc. Chim. [4] 1931, 49, 567; Chem. Abstr. 1931, 25, 4234. 4. Reinecke, M. G.; Smith, W. B. J. Chem. Educ. 1995, 72, 541. 5. Hoffmann, R., et al. Abstracted in Catalyst 1995, 80, 119.
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