The Dehydration of 2-Methylcyclohexanol Revisited: The Evelyn Effect David Todd Juniata CollegeHuntingdon, PA 16652-21 19
Some vears a m there was introduced the acid-catalvzed dehydrition of 5-methylcyclohexanol as a student experiment ( I ) . The exoeriment has been widelv used. as it can serve &t only &introduce the student td gas cLomatograohv. but serves also to drive home the validitv of the ~ a k s z vrule. The percent of l-methylcyclohexen~,of the total alkenes, is usually around 75-80.'
2-Methylcydohexanol cis- and tram)
3WlhylI- ethylmtyenp cydohexene cydohexene cydohexane
Recently the author used this experiment at Pomona College, and, in checking it out to ensure that the stock batch of alcohol to be used gave the expected results, the procedure had to be interrupted during the slow distillation by an urgent summons to lunch with the Chemistry Deoartment secretam M e r lunch I reohced the orieinal reckiver, with about third of the exp&ted volume oidistillate in it, by a fresh receiver-purely on the basis of curiosity to see whether or not the earlier distilling material would differ in comwsition from the later distilline material. The two fractions were worked up separately, &d GC analysis showed a distinct difference in the ratio of 1methyl- to 3-methylcyclohexene.The entire class was then asked to collect the ~roductas two fractions, and it became clear that the pictu;e is quite consistent. he early part of the distillate is distinctly enriched in the higher boiling isomer, l-methylcyclohexene @.p. 110 'C), and the later distilling material is relatively higher in the lower boiling 3-methyl isomer 6.p. 102.5 T I . It should be noted that if the reaction went faster than the distillation took place, one would expect exactly the opposite result. It follows that apparently the alkenes distill
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essentiallv as fast as thev are formed. and. moreover. the l-methyl isomer is form& faster than'is the 3-isomer.' Alogical explanation of this observation follows from two facts. First, the starting alcohol is fairly close to a 50-50 mixture of cis and trans isomer^.^ Second, the rate of dehydration of the cis isomer was shown some years ago to be about 30 times greater than that of the trans isomer (2). It then appears that the reaction proceeds mainly by a trans mechanism, and, following the Zaitsev rule, 1methylcyclohexene is preferentially formed in the early stages of the reaction. Indeed, if only about 10%of the total distillate is collected as the first fraction, one finds that the alkene is about 93% l-methvlcvclohexene: at the other end of the distillation (when one &es to the trouble to collect four or five successive fractions) one finds a value as low as 55% of l-methyl isomer. Clearly this reaction proceeds only partly by a trans mechanism and one can reasonably argue--particularly in the light of the formation of some methylenecyclohexane,' which can only arise via a hydride shift mechanism-that both E l and E2 mechanisms are involved. At the cost of very little extra lab time the student can dehydrate 0.2 mol of 2methylcyclohexanol (using 85% HsPOa),collect about 5-8 mL of a first distillate, then colled the rest in a second receiver. The observation of the Evelyn effect can then lead to some interesting class discussion. I leave it to the reader to deduce the name of the wretary. Literature Cited 1. Taber, R. L:Chempion. W. C. J. Ckem. Edue. 1881.44, 620. 2. Vavon, G.:Barbier,M.BuU.Soe. Chim 141 1851,49,567:Ckem.Abetr 1881,25,4?3@.
'It IS worth mentlonlng that some years ago Prof James Cason of me lolal alkenes 1s fomd, by NMR analysts, that about methylenecyclohexane. Th~smarenal (b.p 103 'C)e not separao e. on the usual GC columns used, from 3-methylcyclohexene. 'The Aldrich material used for the last two years is 47.1% cis and 52.6% trans.
