McLafferty rearrangement

The McLafferty rearrangement is a mass spectrometry fragmentation reaction that involves the transfer of a hydrogen atom through a six-membered transition state, leading to the cleavage of a molecule into two distinct fragments. This rearrangement is particularly significant in the field of organic chemistry and analytical chemistry, as it helps in the identification and structural elucidation of organic compounds.

The McLafferty rearrangement typically occurs in molecular ions that contain at least one gamma hydrogen (a hydrogen atom three bonds away from a radical site or a site of unsaturation). The process involves the migration of this gamma hydrogen to the electronegative atom (usually oxygen), resulting in the formation of a double bond and the cleavage of the molecule into two parts. This reaction is facilitated by the presence of a six-membered transition state, which provides the necessary geometry for the hydrogen transfer.

The rearrangement is named after Fred W. McLafferty, who first described this reaction in 1959. It has since become a fundamental tool in mass spectrometry, particularly in the analysis of organic compounds. The McLafferty rearrangement allows for the determination of the structure of unknown compounds and the identification of functional groups within a molecule.

In practical applications, the McLafferty rearrangement is often observed in the mass spectra of compounds containing carbonyl groups, such as ketones, aldehydes, and carboxylic acids. The rearrangement leads to the formation of a neutral molecule (often an alkene) and a positively charged fragment, which can be detected and analyzed using mass spectrometry.

The significance of the McLafferty rearrangement extends beyond its utility in mass spectrometry. It also provides insight into the mechanisms of chemical reactions and the stability of ionized molecules. Understanding the conditions under which the rearrangement occurs and the factors that influence its outcome is crucial for interpreting mass spectral data and for advancing the field of organic chemistry.