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When Do Carbocation Rearrangements Happen?

September 18, 2019

 

When Can Carbocation Rearrangements Happen?

Carbocation Rearrangements can occur in reactions whose mechanism goes through carbocation (C+) intermediates. These reactions are SN1, E1 and the reaction of alkenes (c=c) with an acid.

 

How do I know if the carbocation rearrangement will happen during my mechanism?

If you have a carbocation and the neighboring carbon is more substituted, you need to do the rearrangement. Most often we have a secondary carbocation in our intermediate that is next to either a tertiary or quaternary carbon.

How do I know what to move during a carbocation rearrangement?

Scenario 1

 If you have a secondary carbocation in our intermediate that is next to either a tertiary carbon, you will move the hydrogen from the tertiary carbon to the carbocation. Thus, tertiary carbon will now miss a bond and become a carbocation itself. 

 

Scenario 2 

If you have secondary carbocation in our intermediate that is next to a quaternary carbon, then you will have to move the carbon group to the carbocation (since there are no hydrogens on a quaternary carbon). This will result in a completely different molecule. The second scenario results in expansion and contraction of rings among other possibilities.

 

How do I know if the carbocation rearrangement happened given the reactant and the product?

An easy way to spot a rearrangement is by comparing product to the reactant. If the product has a different connection of carbons, such as methyl group in a different place, or the nucleophile added in a different place than expected, most likely a rearrangement has occurred.

Here are three ways to spot a rearrangement:

1. Ring has expanded. For example, we have a cyclopentane as the reactant and it became a cyclohexane.

2. Ring has contracted. For example, we have a cyclohexane as the reactant and it became a cyclopentane.

3. Alkyl groups moved.

 

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