McLafferty rearrangement

Mechanism and requirements

Mechanistically the transformation proceeds through a six-membered cyclic transition state. The process typically begins with an ionized parent species (for example a radical cation created by electron ionization). A γ-hydrogen shifts intramolecularly to the carbonyl oxygen at the same time as cleavage occurs between the adjacent carbon atoms. The immediate products are an enol (or enol radical cation) and an alkene or neutral radical fragment; the enol form often tautomerizes to a carbonyl in subsequent steps.

Key features

• Requires a hydrogen three atoms removed from the carbonyl (a γ-hydrogen).
• Proceeds through a six-membered cyclic transition state, favoring concerted transfer and cleavage.
• Common in ketones and aldehydes, but analogous processes are seen for esters, amides and other carbonyl-containing systems.
• Produces predictable fragment ions that aid structural interpretation of mass spectra.

Variations and scope

There are variants broadly described as "McLafferty-type" rearrangements. These include pathways in which a proton rather than a hydrogen atom is effectively transferred (ionic analogues) and cases observed under tandem mass spectrometry where collision-induced dissociation reveals similar rearrangement behavior. The precise pathway (radical versus ionic) depends on the ionization method and the electronic structure of the substrate.

Analytical importance and examples

Analytically, the McLafferty rearrangement is a diagnostic fragmentation: its presence in a spectrum indicates both a carbonyl functional group and the existence of an appropriately positioned hydrogen. Interpreters of mass spectra use this information to narrow possible structures and to confirm the location of substituents along a carbon chain. It often competes with other fragmentation modes, such as α-cleavage, and the observed pattern helps distinguish alternative structural proposals.

Historical note and notable facts

The rearrangement is named after the spectrometrist who first recognized the pattern in mid-20th-century mass spectra. Since that recognition it has become a central concept taught in organic mass spectrometry courses and incorporated into many spectral-prediction tools. Its simplicity and diagnostic value make it one of the standard fragmentations chemists look for when interpreting mass spectral data.