Condensed Summary here.

Part 4 Chemistry Extended Summary:

Sn1 Reactions: Step 1.5-Rearrangement

Sn1 reactions involve the departure of a leaving group as the first step; the departure of a leaving group forms a carbocation, or a carbon with a positive charge. Carbocations can be stabilized through hyperconjugation, where charge is distributed over a larger area. However, when a carbocation exists, there is also the possibility of rearrangement.

Rearrangement is when an atom or group attached to a carbon adjacent to the carbocation shifts to become attached to the carbocation. This results in the carbon adjacent to the original carbocation to gaining a positive charge. An alternate way to think of this concept is to have two cups, one of which is full of water. The empty cup represents a carbocation, and the full cup represents the adjacent carbon with a group (the water) attached to it. When you pour the water from the full cup into the empty one, the cup that was originally full is now empty, and thus has a positive charge on it, while the cup that was originally empty that is now full has the group attached to it.

In this part, Cathy and Danielle switch roles, rearranging the structure of the band. A rearrangement occurs when an atom or group attached to a beta carbon (a carbon adjacent to the carbocation) shifts to become attached to the carbocation; in this case, consider instruments as groups attached to the beta carbons. When the band rearranges, instruments change owners, and the spotlight (and charge) goes onto Danielle, who becomes the lead singer, stabilizing Cathy. Danielle has essentially become the carbocation of the molecule. Rearrangement occurs due to a driving force to put the charge onto a carbon that has a better ability to handle the charge, such as a tertiary carbocation as opposed to a secondary carbocation. Danielle isn't as shy as Cathy is, making her more capable of handling the charge.

Note: Rearrangement does not have to occur in an Sn1 reaction. An Sn1 reaction can commence from start to finish without any rearrangement.