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#1 2009-02-27 02:02:01


Dehydration of 2-methylcyclopentanol to methylcyclopentene

Yahoo!Answers in chemistry wrote:

Q1. Dehydration of 2-methylcyclopentanol gives several products. Write the intermediate that immediately precedes them.

Re: "I hate dehydration questions." I agree with you.

Here is why. In the dehydration of 2-methylcyclopentanol, the products are listed as methylcyclopentene and methylenecyclopentane. Okay, but when I searched for the products of the reaction, the major product is listed as methylcyclopentene and the minor as 3-methylcyclopentene.

It actually isn't dehydration reactions that are problematic, it is rearrangement reactions. They are notorious in their unpredictability and variability. The implication the problem is inferring is that the secondary carbocation rearranges to a tertiary carbocation and that the tertiary carbocation gives rise to the methylenecyclopentane. Certainly methylenecyclopentane (and methylcyclopentene) would come from the tertiary carbocation. However, that doesn't account for the actual minor product of this reaction, see below.


I don't have the data to know the percentages of the actual products, but from other rearrangement reactions, the tertiary carbocation cation probably forms about 50% of the time. The unrearranged makes up about the other 50%. The unrearranged carbocation gives both 1- and 3-methylcyclopentene.

In A Guide to Organic Chemistry Mechanisms, I tried to provide actual data for rearrangement reactions so you could judge how much rearrangement actually took place.


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