Alkyl bromides mass spectrum

A study of alkyl- or aryl-substituted 1,2-dithiolium salts leads to similar conclusions. One group of these salts undergoes thermolysis to a dithiolyl radical, and in the mass spectrum the parent ion is the dithiolyl ion, which often loses a hydrogen atom. However, sometimes the parent ion is stable enough to be observed in the spectrum. This is the case for 3,5-diphenyl-l,2-dithiolium bromide. Other salts, such as 3- or 4-phenyl-1,2-dithiolium bromide, probably first expel a proton, giving a carbene intermediate. " ... 

This mode of fragmentation is most important for alkyl fluorides and chlorides and is less important for alkyl bromides and iodides. In the mass spectrum of 1-bromohexane, the peak corresponding to the loss of hydrogen bromide at m/e = 84 is very weak. However, for 2-chloroheptane, the peak corresponding to the loss of hydrogen chloride at m/e = 98 is quite intense. 

The corresponding cyclic ion can be observed at m/e =135 and 137 in the mass spectrum of 1-bromohexane and at m/e = 105 and 107 in the mass spectrum of 2-chloroheptane. Such fragmentation is important only in the mass spectra of long-chain alkyl chlorides and bromides. 

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