In the last post we saw how certain carbocations can sometimes rearrange (through hydride shifts)to give more stable carbocations.
However, sometimes there are situations where a hydride shift would not lead to a more stable carbocation, such as in this case. If a hydride shift occurred, we’d be going to a less stable (primary) carbocation.
You might note something with this example, however: it is possible for a more stable tertiary carbocation to be formed if an alkyl group migrates instead!
The most common situation where alkyl shifts can occur is when a quaternary carbon (that’s a carbon attached to 4 carbons) is adjacent to a secondary carbocation. How does this work? Well, the pair of electrons from the C-C bond can donate into the empty p orbital on the carbocation (side note: this means they have to be aligned in the same plane). In the transition state, there are partial bonds between the carbon being transferred and each of the two adjacent carbon atoms. Then, as one bond shortens and the other lengthens, we end up with a (more stable) tertiary carbocation.
Rearrangements can potentially occur any time a carbocation is formed. That includes SN1 reactions (and as we’ll later see, elimination and addition reactions).
It doesn’t always have to be a methyl group that moves. One interesting example is when a carbocation is formed adjacent to a strained ring, such as a cyclobutane. Even though the CH3 could potentially migrate in this case, it’s favorable to shift one of the alkyl groups in the ring, which leads to ring expansion and the formation of a less strained, five-membered ring.
Here’s an example of an SN1 where an alkyl shift leads to ring expansion.
Having gone through two types of rearrangements in substitution reactions, the next series of posts will cover a different class of reactions: elimination reactions.