Grob Fragmentation
An elimination reaction that splits a neutral aliphatic chain into three fragments: a positive ion (electrofuge), an unsaturated neutral fragment, and a negative ion (nucleofuge). Positive ion may be carbenium, carbonium, or acylium ions, neutral fragments may be alkene, alkyne, or imine; negative fragments may be tosyl or hydroxyl ions. The reaction is named after the Swiss chemist Cyril A. Grob, who first described it in 1955.
An electrofuge is a leaving group, which does not retain the bonding pair of electrons from its previous bond with another species.
A nucleofuge is a leaving group, which retains the lone pair from its previous bond with another species.
The reaction can proceed through either a stepwise or concerted mechanism, but the concerted pathway is preferred. For concerted pathway the centers must have anti-periplanar (Stereoelectronic) for donation into antibonding orbitals. Grob fragmentations can be affected by the presence of nucleophiles or bases.
Mechanism of Grob Fragmentation
Mechanism takes place in the following steps-
1. Formation of a positive ion (electrofuge) at position 1
2. Concerted cleavage of the C-C bond between positions 2 and 3
3. Expulsion of the negative ion (nucleofuge) from the other end of the chain
4. Formation of an unsaturated neutral fragment spanning positions 3 and 4
