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Alkyl Rearrangements

Yesterday I mentioned how rearrangement reactions tend to cause lots of problems for students. And it’s all a matter of understanding how to draw ONE arrow.

The driving force for rearrangements is to go from a less stable carbocation to a more stable carbocation.

(Rearrangements ONLY occur with carbocations)

Rearrangements can occur through the “migration” of a hydrogen to give a more stable carbocation.

Here’s another twist: what if there’s no hydrogen to migrate? Like C-3 of the molecule below?

(A carbon with four carbon bonds attached is called a “quaternary” carbocation, by the way). If we can’t migrate a hydrogen, we can migrate a carbon. The result will still be a tertiary carbocation (more stable than secondary!)

Just like with the migrations of hydrogens, we can show these reactions by drawingone arrow. Like this.

Notice how we went from a secondary carbocation to a tertiary carbocation, except we’ve migrated carbon.

Keep this reaction in mind… later on, you’ll see other examples when we talk about reactions of alkenes where carbocations form.

Tomorrow: let’s wrap up the week!

Thanks for reading! James
P.S.  I don’t want to hog your time, but there’s a bunch more examples in the Reaction Guide. You can see some specific examples of the SN1 reaction with rearrangements here.