McLafferty rearrangement

The McLafferty rearrangement is a chemical reaction that can only take place under the conditions of mass spectrometry . This is a response that is similar to the ester - pyrolysis or ene reaction . The reaction is named after its discoverer, Fred McLafferty . A substrate containing double bonds , which is present as a radical cation at the beginning of the reaction , splits off a neutral molecule (an alkene in the example below ), whereby the remaining fragment is again present as a radical cation. With an aldehyde (R ¹ = H, R ² = H or organyl group , e.g. alkyl) or a ketone (R ¹ = organyl group, R ² = H or organyl group) the reaction proceeds as follows:

Suitable substrates

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

For C = O bonds, aldehydes , ketones , esters , carboxylic acids , lactones , lactams and amides are suitable starting materials; suitable C = N-containing compounds are, among others. a. Hydrazones , Schiff's bases and semicarbazones , for S = O only sulfonic acids come into question as potential starting compounds. As a rule, the McLafferty rearrangement takes place in a long cascade of decay reactions. In other words, the substrates can be present from the start or can only arise through other decomposition reactions.

mechanism

Initially, the hydrogen atom is transferred from the γ position to the radical heteroatom via a six-membered transition state . The then triply bonded (hetero) atom now carries a positive charge, while the carbon atom at the γ position is now present as a radical . Since primary or secondary radicals are thermodynamically and kinetically unstable, a bond shift occurs and the neutral leaving molecule (here: an alkene) is split off. The radical is now right next to a double bond, which is better able to stabilize it through mesomeric effects. This better stabilization is the mainspring for the cleavage of the small molecule.

literature

• Manfred Hesse, Herbert Meier, Bernd Zeeh : Spectroscopic methods in organic chemistry . 5th, revised. u. exp. Edition. Thieme Georg Verlag, Stuttgart 1995, ISBN 3-13-576105-3 .
• Zerong Wang: Comprehensive Organic: Name Reactions and Reagents . Wiley Verlag, 2009, ISBN 978-0-471-70450-8 , pp. 1856-1859 .