Products: 1-methylcyclohexene, 3-methylcyclohexene, methylenecyclohexane 1a) Which kind of mechanism can better account for the product mixture...

Products: 1-methylcyclohexene, 3-methylcyclohexene, methylenecyclohexane

1a) Which kind of mechanism can better account for the product mixture obtained from the dehydration of cis and trans 2-methylcyclohexanol: E1, E2, or a combination of the two? [Keep in mind that the actual mechanism may be none of these.]

b) Write detailed mechanisms explaining the formation of all the observed products.

Solutions

Expert Solution

The dehydration of cis and trans 2-methylcyclohexanol in presence of acid (phosphoric acid) gives 1-methylcyclohexene as a major product, 3-methylcyclohexene as a minor product, while very little methylenecyclohexane is formed. The reaction proceeds through E1 mechanism, in which formation of carbocation is RDS i.e. Rate Determining Step and second step is deprotonation of carbocation leads to elimination product.

In this case, the higher yield of 1-methylcyclohexene over 3-methylcyclohexene due to greater thermodynamic stability of the more highly substituted alkene. The rule that "the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the β-carbon having the fewest hydrogen substituents" is known as the Zaitsev's rule.

Under kinetic control, a significant yield of 3-methylcyclohexene is expected when 2-methylcyclohexanol is dehydrated. The kinetic product is known as the Hofmann product.

The detailed mechanism is shown in image below:

queen_honey_blossom answered 1 year ago

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