[doublek1229]

Hi guys, I am new to the forum.  I am currently in Organic Chemistry 2, and am hoping to get into a pharmacy program.  I graduated from UConn with a degree in psychology and am taking a couple of extra science courses so I am able to apply.

In our lab we converted 4-methylcyclohexanol to 4-methylcyclohexene (alcohol to alkene) through alcohol dehydration.  We used phosphoric acid and a small amount of sulfuric acid and distilled the reactants.  I am writing my lab report and have to write the discussion section.

I believe that this is an E1 reaction is that correct?  I know that in the mechanism the OH is a poor leaving group and is protonated to form water, which then detaches from the compound leaving you with an alkene product.  Can someone help explain to me if this is indeed E1, and how I could tell for sure?  Thanks!

[doublek1229]

This is what I have come up with so far...

In this experiment, an acid catalyzed dehydration was performed and 4-methylcyclohexanol was converted into 4-methylcyclohexene.  This process was an E1 (unimolecular reaction), because it showed first order kinetics as the breaking of the C-H bond occurred after the rate limiting step.  The primary reactant, 4-methylcyclohexanol is a secondary alcohol, and was therefore required highly concentrated phosphoric and sulfuric acid coupled with high heat to react.  (As opposed to tertiary alcohols which are react more easily under E1 conditions.) 

Is the purpose of the phosphoric acid / sulfuric acid to protonate the OH leaving group?  Thanks for any *delete me*

[orgopete]

This is indeed a good question. In order to come up with an answer, let us imagine the following. If you take hexan-1-ol, cyclohexanol, and 2,2-dimethylbutan-2-ol and treated each with the same mixture of phosphoric acid and a small amount of sulfuric acid. Let us further assume that kinetics of the tertiary alcohol are unimolecular (E1) and the primary alcohol are bimolecular (E2). What are the kinetics of the secondary alcohol and how would you know for sure?

I don't know that you could know for sure. If you had 2- and 3-methylcyclohexanols and treated them as before, you could begin to infer whether the reactions had carbocation intermediates from the product distribution. However, I don't know that you could exclude E1 and E2 mechanisms competing in the reactions leading to the products.

In Williamson's Macroscale and Microscale Organic Experiments, the reaction mechanism is shown as an E1 reaction. I don't know what proof was used for it though.

[Smrt guy]

The mechanism should be E1.  At high temps, the protonated hydroxyl would leave, giving the carbocation intermediate.  It could be proven by detecting the intermediate which may be possible or by kinetic isotope effect (not that it matters for the point of this discussion).