Gamma-hydrogen rearrangement

The base peak in methyl isovalerate and indeed of all methyl esters up to C26 is the rearrangement ion mjz 74 which is directly analogous to the mjz 60 in the free acid spectra and is formed by the same gamma hydrogen rearrangement, i.e. R... 

The TMS equivalent to the m/z 74 in methyl esters is found in cases where the gamma hydrogen rearrangement is possible, e.g. isovaleric acid (Fig. 5.15). 

Esters Rather weak intensity Base peak3 at m/e equal to the mass of R-C=0 peaks at m/e equal to the mass of 0=C-0R, the mass of OR and R McLafferty rearrangement possible in case of (1) presence of a beta hydrogen in R (peak at m/e equal to the mass of R-C( OH)OH, and (2) presence of a gamma hydrogen in R peak at m/e equal to the mass of (CH2=C( OH)OR) loss of 42 (CH2=C=0) in case of benzyl esters loss of ROH via the ortho effect in case of o-substituted benzoates... 

If the carbon alpha to the carbonyl is substituted, then the rearrangement ion is shifted by the mass of the substituent. The lack of a gamma hydrogen prevents the formation of this ion in propionic acid. Formic, acetic and propionic acids all show prominent M— 1 peaks by loss of a proton from the molecular ion, and an ion at mlz 45 due to the carboxyl group. 

Unsubstituted saturated A -acylglycine methyl esters are characterized by the ion at mjz 131, which results from a McLafferty rearrangement when the alkyl chain contains a gamma hydrogen. 

The fragment ions which are common to most of the spectra are miz 217,232 and 245. When the aliphatic side chain has a gamma hydrogen, a McLafferty rearrangement involving the quinoxalinol hydrogen takes place to give mjz 232. 

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