Dehydration Reaction

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 Zaitzevís 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 alcoholís
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 itís 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 fractionís 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