How to recognize carbocation rearrangements?
In organic chemistry, when students learn reactions oftentimes hydride and alkyl shifts happen that student might not recognize. Here are the ways to be aware of them:
1. Any reactions that go through a carbocation are capable of rearrangements such as SN1, E1, and alkene reactions with acids.
2. Rearrangements happen at the carbocation stage.
3. The shift happens if the neighboring carbon would make for a more stable, more substituted carbocation. For example if a secondary carbocation is located next to a carbon that is tertiary, rearrangement will take place.
4. If the hydrogen is present, do hydride shift. If there are only carbons, do an alkyl shift.
5. Often, the professor asks students for mechanism and gives reactant and product. Looking at the product if it's structure has changed from that of the reactant, rearrangement took place. For example, if you see a cyclopentane expanding to a cyclohexane, or a ring becoming smaller, then a rearrangement definitely took place.
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