Interpretation McLafferty rearrangement

Examples The mass spectrum of methyl heptanoate perfectly meets the standard, and thus all peaks may be explained by following Scheme 6.39 (Fig. 6.26a). In principle, the same is true for the isomeric methyl 2-methylhexanoate, but here care has to be taken not to interpret the mass spectmm as belonging to an ethyl ester. This is because the 2-methyl substituent resides on the ionic products of McLafferty rearrangement and y-cleavage, shifting both of them 14 u upwards. Nevertheless, a look at the a-cleavage products, [M-OMe], and [COOMe], m/z 59, reveals the methyl ester. 

Nitriles. The molecular ion peak is weak or non-existent in aliphatic nitriles but strong in aromatic compounds. Interpretation of the spectrum is often difficult since skeletal rearrangements are common and the resulting ion series (m/z 41, 55, 69, etc.) overlaps with that arising from hydrocarbons. Thus in the McLafferty rearrangement ... 

The McLafferty rearrangement in certain carboxylic acids initially forms the ion (CH2 = C(OH)2)t, which then undergoes metastable ion decomposition to lose a hydroxyl radical. With the labelled ions [CD2 = C(OH)(OD)]t and [CH2=C(OH)(OD)]t, the isotope effect 7oh/ od has been measured as 0.38 [526, 753], 0.56 [342] and 0.42 [407], i.e. OD- loss was more probable than OH - loss. This observation has been interpreted [407, 526] in terms of the rate-determining step... 

Methyl n-propyl ketone and methyl n-butyl ketone have been subjected to 3 Mev y-rays (Pitts and Osborne, 1961). Two of the many products, acetone and an olefin (ethylene from the first ketone and propylene from the second), could have been formed by an intramolecular hydrogen rearrangement analogous to the McLafferty rearrangement in mass spectrometry and the Norrish type II photochemical process (Section D). Essentially the same interpretation has been mentioned briefly by Ausloos and Paulson (1958). 

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.)...

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