SN1 w/ rearrangement [alkyl shift]

S_N1 w/rearrangement [alkyl shift] Definition:

The 1,2-alkyl shift can occur during the substitution reactions if an unstable carbocation is formed adjacent to a quaternary carbon.

S_N1 rearrangement [alkyl shift] Explained:

We have already talked about rearrangements, but with alkenes (Rearrangement [alkyl shift]). And all of this can be applied to alkyl halides in S_N1 reactions.

The same rule applies: if a carbocation can be achieved a more stable form it will do so. The reactivity of carbocations goes from tertiary to secondary to primary (primary alkyl halides usually do not occur in reactions). In S_N1 reactions we usually have secondary or tertiary alkyl halides which can give a secondary or tertiary carbocation. If we have secondary carbocation which has quaternary carbon in adjacent to positively charged carbon there will always be a rearrangement. The tertiary carbocation is lower in energy than the secondary carbocation.

As we know an S_N1 process must always exhibit two basic steps:

1. Loss of a leaving group
2. Nucleophilic attack

But we must consider whether any of the other three possible steps will occur.

In the following example, we have alkyl halide with a good leaving group which means that the mechanism will not begin with a proton transfer. This will be the last step of the mechanism. And the order will be as follows: loss of a leaving group, carbocation rearrangement, nucleophilic attack, and proton transfer (deprotonation).

In the next example, we will see a rare case where loss of the leaving group occurs simultaneously with a carbocation rearrangement. There are alkyl halides which cannot lose its leaving group because that would generate a primary carbocation. And for this process is needed too high energy to form that carbocation.