Time as a Determinate of Final Product in a Dehydration Reaction
Robert Simack, Department of Chemistry and Biochemistry, University of Alaska Fairbanks, Fairbanks, Alaska.
Abstract: This study involved acid dehydration of 2-methylcyclohexanol. The results varied depending on the time elapsed after initial reaction. I attempted to prove the Evelyn Effect, which stated that over a period of time the products of the aforementioned reaction will beobserved to change volume so that those products formed by a cis isomer of 2-methylcyclohexanol will form first. However, once all molecules in the cis isomer undergo reaction the remaining trans configured 2-methylcyclohexanols will proliferate during the latter period of the reaction. I also postulated as to the possible formulation of 1-ethylcyclopentene, and to the cause of such an event.
Introduction: After researching acid-catalyzed dehydration reactions (McMurray) and background on the Evelyn Effect (Clausen) I hypothesize that the cis isomer of 2-methylcyclohexanol will react via an E1 type process forming 1-methylcyclohexene according to predictions from Zaitzev’s rule (Lehman). This should be due to the fact that the cis isomer has 2 anti-coplanar hydrogens. These two hydrogens should make the molecule more reactive. The trans isomer, with only one anti-coplanar hydrogen, should be slower to react and will form a 3-methylcyclohexene. In addition the 1-ethylcyclopentene will be formed from both the cis and trans isomers but only if the hydroxyl group is in an equatorial position. In that position electrons from the ring may attack the alcohol directly from behind pushing it off the ring and forming a five-membered ring instead.
Results ; Discussion: An NMR (300MHz) spectra of the original reagent and the three fractions provided a huge amount of information in support of my hypothesis. Both cis and trans isomers were present in the spectra for the original material as well as for the first two fractions. The alcohol’s hydrogen showed up at approximately 3.79 and 3.1 for cis and trans respectively. In the spectra for pure starting material (ref: Jim Starr /Steve Standish NMR 24 March, 2000) cis isomers of starting material comprised only 25% of the sample compared to 75% of trans as observed in the integration of peaks. In the spectra for fraction one a 3:1 ratio of trans to cis was observed. In the spectra of fraction two the cis isomer nearly disappeared; the ratio was roughly 6:1 trans/cis. Finally, in the spectra of the third fraction the cis isomer was absolutely imperceptible while the integration of trans was nearly twice that of the integration from fraction one. These spectra show that cis reacted first and was quickly consumed by the reaction leaving trans isomers to finish the reaction. Because it is known that the reaction with cis starting material caused both 3-methylcyclohexene and 1-methylcyclohexene I postulated that the foremost product of the latter stages of the reaction must be 3-methylcyclohexene, which is the sole product of the trans reaction (McMurray, chap. 11.12). In addition to the cis and trans peaks the peaks for both 3-methylcyclohexene and 1-methylcyclohexene could be found on the spectra at 5.7 and 5.4 respectively. The NMR showed that the integration of 1-methylcyclohexene dropped only slightly throughout the reaction while the integration of 3-methylcyclohexene increased nearly tenfold. The findings from the spectra prove the hypothesis that the cis reaction will go the fastest followed by the trans because as the cis is consumed it’s peak at 3.79 will decrease as well as the peak for 1-methylcyclohexene due to termination of that products formation.
Also, peaks for 1-ethylcyclopentene begin to show in the spectra for the second fraction and increase in size (area beneath the peak) by the spectra of the third fraction. At the root of this phenomena is steric hinderance. Both the cis and trans isomers will form 1-ethylcyclopentene (fig. 1). However, because of steric hinderance the trans isomer is favored to form the 1-ethylcyclopentene. This fact will explain why more of the pentene shows up in the third fraction.
Finally, a tiny peak showed at 4.6 in every fraction’s spectra indicating the presence of methylenecyclohexane. This product formed from the original product by acid catalyst.
Experimental: An apparatus was constructed with a round bottom flask topped by a claisen adaptor in which was placed a thermometer and a condensing tube. In the apparatus 150mmole of 2-methylcyclohexanol was mixed with 5mL H3PO4 and distilled. The distilled liquid was collected in three tubes, at approximately 4mL per tube, labeled fraction 1, 2 and 3. Each fraction was placed in a centrifuge tube and combined with 4mL saturated NaHCO3. The aqueous layer was removed and MgSO4 was added for a final separation. The solid and aqueous layers were then removed and the final product was combined with CDCL3 in an NMR tube in preparation for spectra. The liquid remaining in the original apparatus was put through the separation process described above. However, instead of CDCL3 as a spectrum reagent we used CH2CL2. Also, an NMR was not performed on the remaining liquid but instead a GC.
Figure 1: Reactions of cis and trans isomers of 2-methylcyclohexanol during acid-catalyzed dehydration.
Clausen, Tom, ?Organic Chemistry 324 Lecture,? Univeristy of Alaska, Fairbanks, March 20, 2000.
Lehman, John W., Operational Organic Chemistry, 3rd ed., New Jersey: Prentice-Hall, Inc., 1999.
McMurry, John, Organic Chemistry, 4th ed., California: Brooks/Cole Publishing, 1996.