FREE ESSAY ON TIME AS A DETERMINATE OF FINAL PRODUCT IN A DEHYDRATION REACTION

TIME AS A DETERMINATE OF FINAL PRODUCT IN A DEHYDRATION REACTION

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.
Bibliography
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.