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?
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.
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.
At Transformation Tutoring NYC we love organic chemistry and are happy to tutor it to our students in Brooklyn, Manhattan and online. Our organic chemistry tutors from NYU and Columbia have helped hundreds of students ace their classes and continue on the pre-health track. Call us today at 646-407-9078 to schedule your organic chemistry tutoring session today.