Description: The Pinacol rearrangement involves the shift of an alkyl group accompanied by loss of water and the conversion of an alcohol to a ketone (or aldehyde).
Notes: A strong acid such as H2SO4 is commonly used for this reaction. Other strong acids can also be used. The reaction is named after “Pinacol”, which is another name for 2,3-dimethyl butanediol (the molecule shown, above left).
Notes: The second example shows how this reaction may occur with ring expansion.
Mechanism: Protonation of the alcohol (Step 1, arrows A and B) converts it into an oxonium ion. Loss of water (Step 2, arrow C) results in a carbocation. Alkyl shift then occurs, accompanied by formation of the new C-O π bond (Step 3, arrows D and E), which is then deprotonated to give the neutral ketone (Step 4, arrow F).
Notes: Note that acid is catalyltic in this reaction. The driving force for this reaction is formation of the strong C-O π bond; note that it can occur even where tertiary carbocations will form. Note that it is also reasonable to draw a mechanism showing the formation of the C-O π bond happening at the same time as the alkyl shift and loss of the leaving group.