Is the McLafferty rearrangement of ketones concerted or stepwise? The application of kinetic isotope effects

Abstract

Intramolecular I3C and 'H isotope effects have been measured for unimolecular losses of ethene (theMcLafferty rearrangement) from metastable molecular ions of 2-ethyI-l-phenylbutan-l-one, kthylpentan-Zone and heptan-4one. Primary and secondary deuterium isotope effects are observed at the y-(terminal) and fl-positions, respectively. Large primary isotope effects occur at fl-positions and for the y positions of 3-ethylpentan-2-one and heptan-4one. The carbon isotope effects in the cases of the doubly isotopically labelled CH,COCH(C,H,)(13CH,CH,) and CD,COCD(C,L),)('3CD,CD,) are 1.17 (AO.01) and 1.04 (+0.01), respectively. All of these isotope effects are consistent with a stepwise mechanism in which more than one step is rate determining.

The influence of structural features on facile McLafferty‐type, even‐electron rearrangements in tandem mass spectra of carboxylate anions

“…The McLafferty rearrangement14 is one the most‐studied processes in the mass spectrometry literature,4, 15 including complex studies as to whether the rearrangement is synchronous or step‐wise 16, 17. Although most examples of the McLafferty rearrangement involve radical cations, cases involving EE ions have been known for a long time 4–10, 12, 15.…”

McLafferty rearrangement of the radical cations of butanal and 3‐fluorobutanal: A theoretical investigation of the concerted and stepwise mechanisms

“…In particular, an extensive experimental investigation, utilizing both 13 C and 2 H kinetic isotope effects, clearly indicated that the McLafferty rearrangement of ketone radical cations is stepwise. 9 However, Stone et al 7 proposed a concerted mechanism for the loss of acetaldehyde from the benzyl ethyl ether radical cation, which is also a McLafferty type reaction, in collisional activation spectra measurements. A few years later, the same group of authors 8 confirmed this observation by using Rice-Ramsperger-Kassel-Marcus (RRKM) theory on the same reaction.…”

Roaming-Mediated CH2NH Elimination from the Ionization of Aromatic Ethylamines

“…More evidence for this rearrangement reaction was found with the decomposition of ionized aliphatic aldehydes, ketones, esters, amides and other derivatives with observation of elimination of vinyl hydrocarbons . The origin of γ‐hydrogen transfer has been established by extensive experiments involving deuterium and heavy‐atom labeling . McLafferty proposed the electronic mechanism for this type of rearrangement reaction, involving γ‐hydrogen transfer.…”