Thermal degradation decarbonylation

Processes such as decarbonylation, decarboxylation, elimination of water, and several other reactions may also occur prior to ionization, i.e., as non-mass spectral reactions, typically as a result of thermal degradation upon heating of the sample to enforce evaporation. In such a case, the mass spectrum obtained is not that of the analyte itself, but of its decomposition product(s). Sometimes, those thermal reactions are difficult to recognize, because the same neutral loss may also occur by a true mass spectral fragmentation of the corresponding molecular ion. 

Thermal degradation prior to ionization can cause decarbonylation or decarboxylation of the analyte. Decarbonylation, for example, is observed from a-ketocarboxylic acids and a-ketocarboxylic acid esters, whereas decarboxylation is typical behavior of P-oxocarboxylic acids such as malonic acid and its derivatives and di-, tri-, or polycarboxylic acids. 

Enthalpy barriers for the decarbonylation and dethiocarboxylation of y-thiobutyrolactone (364) have been calculated as 378 and 404 kJ moP respectively, which accords with the experimental results which saw CO as the major and COS as the minor thermal degradation products.Phenyl and 4-nitrophenyl chlorothionoformates (365 X = H, NO2) reacted with phenolates in aqueous dioxane with /3 uc = 0.55 and 0.47, respectively, from which it was concluded that a concerted mechanism prevailed. 

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