Rearrangement reactions McLafferty

Example The observed KER values and also peak shapes may change dramatically as Eor decreases. Here, the chain length of a leaving alkene increases with an increase of the reacting alkyl substituent. Nevertheless, the mechanism of the reaction - McLafferty rearrangement of immonium ions (Chap. 6.7) - remains unaffected. [58]... 

It seems likely that PVAc containing CH3-C = O groups, with more acidic protons, should be more effective in McLafferty-type rearrangement reactions and intermolecular proton-transfer to PMMA chains, even if the distances between PVAc-PMMA and PMMA-PMMA are comparable, i.e., PMMA and PVAc chains in the coalesced blend are very intimately mixed. Making use of the above proposal permits the approximate reaction scheme shown in Scheme 2 to be written. 

A concrete example for the competition between the rearrangement reaction and the simple benzyUc cleavage is shown in Scheme 8. In the El mass spectrum of 3-(n-butyl)phenol m-23, the McLafferty reaction of gives rise to the base peak at... 

Look for abundant OE+ ions in the spectrnm (e.g., m/z 60, 74, 92,...) these ions are formed by the loss of a stable nentral molecnlar species via specific rearrangement reactions (e.g., McLafferty rearrangement, see Section 6.5.2) or nonspecific reactions (e.g., loss of water), and can provide a clue to the presence of certain functional gronps. 

Ethyl or longer-chain phenyl ethers, in addition, would produce an ion of m/z 94 (C6H60+ ) via McLafferty-type rearrangement [reaction (6.29)]. The ion of m/z 93 (C6H7N+) is also prodnced from ionized 2-alkyl and 4-alkylpyridines via the McLafferty rearrangement. The m/z 105 ion (CeHsCO" "), which is produced via a-cleavage, is evidence for the presence of phenyl ketones and esters and amides of benzoic acids. Esters of benzoic acids will also produce a characteristic ion of m/z 122 as a result of the McLafferty rearrangement ... 

Figure 6.17. Warhaftig diagram The upper portion represents the probability of a particular ion at a specified energy. The lower portion represents the rate constant of a unimolecular fragmentation of the ABCD+ radical cation as a function of internal energy the steep curve is for a direct cleavage reaction, and the shallow curve is for a rearrangement reaction. (Redrawn from F. W. McLafferty and F. TureCek, Interpretation of Mass Spectra, 4th ed.)...

A very recent experiment, involving butanoic acid clustered with argon, has provided one of the most spectacular examples of reaction selectivity we have so far encountered. In the ion source of a mass spectrometer, an isolated butanoic acid ion will undergo a McLafferty rearrangement reaction... 

McLafferty, Fred Warren, 732 McLafferty rearrangement. 416, 732 Mechanism (reaction), 139 acetal formation, 717-718 acetylide alkylation, 272 acid chloride formal ion with SOCl2, 795... 

Example Isotopic labeling does not only reveal the original position of a rearranging atom, but can also reveal the rate-determining step of multi-step reactions by its marked influence on reaction rates. Thus, the examination of H/D and isotope effects led to the conclusion that the McLafferty rearrangement of aliphatic ketones (Chap. 6.7) rather proceeds stepwise than concerted. [68]... 

For carboxylic acid ethyl and longer aliphatic chain esters the McLafferty rearrangement can also occur on the alkoxy branch (R ) of the molecular ion. It then competes as a second alkene loss with the reaction at R ... 

Alkene loss via McLafferty rearrangement at the alkoxy group of aliphatic and aromatic carboxylic acid esters competes with yet another reaction path, where two hydrogens instead of one as in the normal McLafferty product are transferred to the charge site. This second pathway leading to alkenyl loss has early been noticed [94] and became known as McLafferty rearrangement with double hydrogen transfer (r2H) ... 

Note The McLafferty rearrangement and the RDA reaction have several features in common i) both belong to the rearrangement type of fragmentations, although the name conceals this fact in case of the latter, ii) both represent pathways for alkene loss from molecular ions, and iii) both are highly versatile in structure elucidation. 

As with the McLafferty rearrangement and the retro-Diels-Alder reaction before, the occurrence of the ortho elimination is not restricted to molecular ions. It may equally well proceed in even-electron species. 

Kingston, D.G.I. Bnrsey, J.T. Bursey, M.M. Intramolecular Hydrogen Transfer in Mass Spectra. II. McLafferty Rearrangement and Related Reactions. Chem. Industry 1974, 74, 215-245. 

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