A rearrangement is a reaction in which C-C ? bonds (or C-X, X - heteroatom) have both broken and formed in the course of a reaction involving just a single substrate. It is often more difficult to draw a reasonable mechanism for a rearrangement than for any other kind of reaction. When you draw a mechanism for a rearrangement reaction, it is especially important to determine exactly which bonds are being broken and which are formed. Do this by numbering the atoms in the starting materials, identifying the atoms by the same numbers in the products, and making a list of bonds that are made and broken in the reaction. A list of such intra-molecular rearrangements is shown in this essay together with some examples.
Base promoted rearrangements
Rearrangement reactions under basic conditions usually involve addition of a nucleophile to a carbonyl group followed by cleavage of one of the C-C a bonds. The reaction is simply an addition-elimination reaction in which the leaving group is an alkyl group. The difference between this and most addition-elimination reactions of carbonyl compounds, of course, is that R- is a horrid leaving group (unless of course it is somehow stabilized, e.g., as an enolate). RP can act as a leaving group in a rearrangement reaction because it migrates to an electrophilic atom a to the carbonyl group instead of leaving the compound entirely. The adjacent atom may be either a a-bond electrophile or a T-bond electrophile.
A. Migration from C to C
Benzylic acid rearrangement:
Favorsky rearrangement:
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