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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