McLafferty rearrangement

The McLafferty rearrangement is a chemical reaction that can occur only under the conditions of mass spectrometry. It is a reaction that is similar to the ester or pyrolysis of the ene reaction. The reaction is named after its discoverer Fred McLafferty. A double bond -containing substrate, which is present at the beginning of the reaction as a radical cation, which splits a neutral molecule from ( in the example below is an alkene ), the remaining fragment is again as a radical cation. With an aldehyde (R1 = H, R2 = H or Organylgruppe such as alkyl ) or a ketone (R1 = Organylgruppe, R2 = H or Organylgruppe ), the reaction proceeds like this:

Suitable substrates

Typical substrates for the rearrangement are compounds having at least one double bond. The double bond can be between two carbon atoms in alkenes (C = C) between a carbon atom and a hetero atom ( C = N, C = O) or between two hetero atoms ( S = O).

For C = O bonds, aldehydes, ketones, esters, carboxylic acids, lactones, lactams and amides are suitable starting materials, suitable C = N-containing compounds include hydrazones, Schiff bases and semicarbazones are eligible for S = O only sulfonic acids as potential starting compounds in question. Usually the McLafferty rearrangement proceeds embedded in a long cascade of decomposition reactions, i.e., the substrates can be present from the beginning, or only occur with other decomposition reactions.

Mechanism

First runs over a six -membered transition state from the transfer of a hydrogen atom from the γ - position to the radical heteroatom. Then the triple-bonded ( hetero) atom then carrying a positive charge, while the carbon atom is now present at the γ - position as a radical. Since primary and secondary radicals are thermodynamically and kinetically unstable, a bond shift and the neutral molecule takes place finish ( here: an alkene ) is cleaved. The radical is now right next to a double bond, which is better able to stabilize it by mesomeric effects. This better stabilization is the driving force for the cleavage of the small molecule.