Ketones McLafferty rearrangement

In addition to fragmentation by the McLafferty rearrangement, aldehydes and ketones also undergo cleavage of the bond between the carbonyl group and the a carbon, a so-called a cleavage. Alpha cleavage yields a neutral radical and a resonance-stabilized acyl cation. 

Characteristic fragment ions Aliphatic ketones also give abundant McLafferty rearrangement ions at m/z 58, 72, 86, and so forth. Methyl ketones produce an abundant ion at m/z 43. Low-intensity ions at m/z 31, 45, 59, 73, and so on reveal oxygen in the unknown ketone and are especially useful in distinguishing ketone spectra from isomeric paraffin spectra. Subtract 43 from the mass of the rearrangement ion to determine R. 

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]... 

Henion, J.D. Kingston, D.G.I. Mass Spectrometry of Organic Compounds. IX. McLafferty Rearrangements in Some Bi-cyclic Ketones. J. Am. Chem. Soc. 1974, 96.2532-2536. 

Many distonic radical ions are involved in unimolecular dissociation reactions. For example, the McLafferty rearrangement of carbonyl compounds proceeds by C—C bond cleavage of distonic ions. ° Alternatively, bond-cleavage reactions of cyclic molecular ions may generate distonic ions (see above).For example, C—C bond cleavage of cyclic ketone molecular ions (e.g., 61 " ) generates 62 ". ... 

McLafferty rearrangements are common for aliphatic aldehydes and ketones, providing that an alkyl group of at least three carbons long is attached to the carbonyl group. Odd-electron ions are formed which are useful in the analysis of the spectrum. 

Isotope effects on the McLafferty rearrangement have been determined from the peak heights in the El mass spectra for a variety of ketones and other compounds. Intramolecular [554, 555, 556, 913] and inter-molecular [194, 907] isotope effects for this rearrangement via a 6-membered cyclic transition state have fallen in the region 1—2. The literature on this rearrangement, including isotope effects has been thoroughly reviewed [205],... 

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 McLafferty rearrangement is a characteristic fragmentation of ketones and aldehydes as long as they have y hydrogens. It is equivalent to a cleavage between the a and fi carbon atoms, plus one mass unit for the hydrogen that is transferred. 

The mass spectra of the two isomers also differ. Like ketones, aldehydes also undergo alpha cleavage and McLafferty rearrangements. 

Some aliphatic aldehydes and ketones undergo McLafferty rearrangement. 

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). 

The evidence for the mechanisms of the mass-spectrometric and photochemical reactions leading to the eliminations of an olefin from a ketone [equation (120)] have been summarized (Section VIIDl). If it is accepted that the structure of the fragment ion from this process has an enolic structure, it is possible to discuss the mechanism of the reaction theoretically. The reaction appears to consist of two parts, first the transfer of hydrogen and second, the elimination of olefin. There has been considerable conjecture as to whether these parts of the mass-spectrometric McLafferty rearrangement are stepwise or concerted. Prom their self-consistent field calculations, Boer et al. (1968) have concluded the reaction is step-wise. From perturbation and valence-electron molecular orbital calculations, Dougherty (1968b) has concluded the reaction is concerted. The above results depend on the adjustable parameters fed into the equations one set of parameters may eventually prove to be better. 

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