McLafferty rearrangement aldehydes

A A-dimethylamines, A -ethylamines cycloalkanols, cyclic ethers, ethylene ketals, aliphatic aldehydes (McLafferty rearrangement) anhydrides, lactones, carboxyhc acids... 

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. 

Above C4, aliphatic aldehydes undergo the McLafferty rearrangement, resulting in an observed m/z 44 ion, provided the a-carbon is not substituted. Substitution on the a-carbon results in a higher m/z peak. (See the proceeding text.)... 

Aldehydes are distinguished from alcohols by the loss of 28 and 44 Daltons from the molecular ion. The M — 44 ion results from the McLafferty rearrangement with the charge remaining on the olefinic portion. 

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. 

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. 

The McLafferty rearrangements is the mass spectrometric analog of the Norrish type II photochemical cleavage of ketones. The relationship between these two processes is easily the most extensively studied of aU mass spectral photochemical correlations. In addition to the well known reaction of aldehydes, ketones, and esters, Eq. (62), many analogous y-hydrogen... 

The molecular ion peak of an aliphatic aldehyde is usually observable, although at times it may be fairly weak. Principal modes of fragmentation include a-cleavage and y8-cleavage. If the carbon chain attached to the carbonyl group contains at least three carbons, McLafferty rearrangement is also observed. 

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