Mass spectra McLafferty rearrangement

McLafferty rearrangement A molecular rearrangement that occurs under certain ionization conditions which results in the production of characteristic ions in the mass spectrum of the analyte from which it has been generated. 

Example The NR mass spectrum of acetone closely resembles its 70 eV El mass spectrum (Chap. 6.2.1), thereby demonstrating that the molecular ion basically retains the structure of the neutral (Fig. 2.29). [144] However, the isomeric CsHeO" ions formed by McLafferty rearrangement of 2-hexanone molecular ion are expected to have enol structure (Chap. 6.7.1), and thus the corresponding NR mass spectrum is easily distinguished from that of acetone. 

Fig. 6.22. 70 eV El mass spectrum of butanal. The base peak at m/z 44 is caused by the product of McLafferty rearrangement. Note that here the peak at m/z 57 cannot result from [C4H9], but is due to [M-CHs]. Spectrum used by permission of NIST. NIST 2002. 

Following the above general description of the McLafferty rearrangement, the peak at m/z 44 in the mass spectrum of butanal can be explained by C2H4 loss from the molecular ion. The process may either be formulated in a concerted manner (a) or as a stepwise process (b) ... 

Example In the El mass spectrum of 3,3-dimethyl-2-butanone no fragment ion due to McLafferty rearrangement can be observed, because there is no y-hydrogen available (Fig. 6.23). Instead, the products are exclusively formed by simple cleavages as evident from their odd-numbered m/z values. The highly stable tert-butyl ion, m/z 57, predominates over the acylium ion at m/z 43 (Chap. 6.6.2). 

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. 

Example The base peak in the El mass spectrum of (3-methylpentyl)-benzene is formed by McLafferty rearrangement of the molecular ion (Fig. 6.28). As long as pentene loss may occur, there is not much difference to spectra of other isomers such as 2-methylpentyl-, 4-methylpentyl, or n-hexyl. Reference spectra are needed to distinguish those isomers, because differences are mainly due to peak intensities, whereas only minor peaks might appear or vanish depending on the isomer. 

Fig. 6.28. El mass spectrum of (3-methylpentyl)-benzene. McLafferty rearrangement and benzylic cleavage are clearly dominating. In the low-mass range carbenium ions and the aromatic fragments are present. Spectmm used by permission of NIST. NIST 2002.

Example Ethyl loss clearly predominates methyl loss in the El mass spectrum of 2-(l-methylpropyl)-phenol. It proceeds via benzylic bond cleavage, the products of which are detected as the base peak at m/z 121 and m/z 135 (3 %), respectively (Eig. 6.34a). The McLafferty rearrangement does not play a role, as the peak at m/z 122 (8.8 %) is completely due to the isotopic contribution to the peak at m/z 121. From the HR-El spectrum (Fig. 6.34b) the alternative pathway for the formation of a [M-29] peak, i.e., [M-CO-H]", can be excluded, because the measured accurate mass of this singlet peak indicates CgHgO". HR-MS data also reveal that the peak at m/z 107 corresponds to [M-CHs-CO]" and that the one at m/z 103 corresponds to [M-C2H5-H20]. Although perhaps unexpected, the loss of H2O from phenolic fragment ions is not unusual. 

Note Regardless of whatever the exact mechanism and regardless of whatever the correct name, the McLafferty rearrangement of onium ions is one of those processes allowing to reliably track ionic structures through a mass spectrum. 

Alkyl-2-methoxypyrazines exhibit a base peak at miz 124 in the mass spectrum. The peak corresponds to a molecular ion in 2-methoxy-3-methylpyrazine (24a) and to a fragment ion, resulting from a McLafferty rearrangement of an alkyl group, in 3-isopropyl-2-methoxypyrazine (24b) and 3-5cc-butyl-2-methoxy pyrazine (24d) (97). 

The McLafferty rearrangement, as it is known, is useful for structure identification. For example, the mass spectrum of 2-methylbutanal shows a peak at m/e 58, while that of 3-methylbutanal shows a peak at m/e 44. 

Carboxylic acids. Monocarboxylic acids normally show the molecular ion in the spectrum. Cleavage of bonds adjacent to the carbonyl group (a-cleavage) results in formation of fragments of mass M — 17 (OH) and M — 45 (C02H). Characteristic peaks arise from the McLafferty rearrangement. 

The peaks at m/z 56 and 61 in the mass spectrum of butyl acetate (Fig. 3.85) can be explained by the above rearrangements. The mass spectrum of ethyl buta-noate, Fig. 3.86, shows two important peaks due to odd-electron ions at m/z 88 and 60, resulting from two successive McLafferty rearrangements. 

The mass spectra of oxyphenbutazone is reproduced in Figure 5. According to Locock et al (7) the drug undergoes McLafferty rearrangement to give a radical ion at m/e 268. A metastable ion is present in the spectrum to indicate that this ion is a direct fragmentation from the molecular ion with the loss of the elements of butene (Scheme I). 

McLafferty Rearrangement of Ketones and Aldehydes The mass spectrum of butyraldehyde (Figure 18-4) shows the peaks we expect at m/z 72 (molecular ion), m/z 57 (loss of a methyl group), and m/z 29 (loss of a propyl group). The peak at m/z 57 is from cleavage between the /3 and y carbons to give a resonance-stabilized carbocation. This is also a common fragmentation with carbonyl compounds like the other odd-numbered peaks, it results from loss of a radical. 

The mass spectrum of butyraldehyde shows the expected ions of masses 72, 57, and 29. The base peak at m/z 44 results from the loss of ethylene via McLafferty rearrangement. 

The mass spectrum of pentanoic acid is given in Figure 20-7. The base peak at m/z 60 corresponds to the fragment from loss of propene via the McLafferty rearrangement. The strong peak at m/z 73 corresponds to loss of an ethyl radical with rearrangement to give a resonance-stabilized cation. 

The mass spectrum of pentanoic acid shows a weak parent peak, an even-numbered base peak from the McLafferty rearrangement, and another strong peak from loss of an ethyl radical. 

Describe the 13C NMR and H NMR spectra of D. For the NMR spectrum, include the spin-spin splitting patterns, peak areas, and positions of the chemical shifts. Give two significant absorptions that you might see in the IR spectrum. Would you expect to see products of McLafferty rearrangement in the mass spectrum of D Of alpha cleavage ... 

In the mass spectrum of palustrine (73), the side chain is the first fragment to be eliminated by a cleavage from the molecular ion (M = 309), after which by a McLafFerty rearrangement, one of the H atoms attached to C-2 appears at the lactam oxygen fragment ion, m/z 250. From this the position... 

Figure 19.19 Mass spectrum of 5-methyl-2-hexanone. The peak at m/z = 58 is due to McLafferty rearrangement. The abundant peak at m/z = 43 is due to cleavage at the more highly substituted side of the carbonyl group. Note that the peak due to the molecular ion is very small.

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