Ethyl chloride, mass spectrum

The gas phase pjnrolysis of alkyl hahdes has been extensively reviewed 58>, and in general the unimolecular gas phase reactions of alkyl halides parallel their reactivity in a mass spectrometer. For example, ethylchloride yields ethylene and HCl on thermolysis 5 >, and the ethylene ion in the mass spectrum of ethyl chloride is significantly more intense than the molecule ion. 1,2-dichloroethane also eliminated HCl thermolytically and the corresponding ion is the base peak in its mass spectrum. Elimination of HCl is also common to the mass spectra and thermochemistry of chloroprene dimers.Although in this case the major ion at mje 91 had no definite analog in the thermochemistry. This is probably due to the fact that mje 91 was a tropylium ion which would not be stabihzed as a neutral. 

Let us now consider in detail the radiolysis of ethyl chloride. Although it is well known that the relative intensities of ionic fragments observed in the low pressure mass spectrum of a compound do not necessarily represent the abundances of these ions in a radiolysis system at higher pressures (owing to a difference in the time scale of events), such a spectrum still provides a useful indication of possible primary ionic dissociation modes. The primary ions observed in the mass spectrum of ethyl chloride and their relative abundances are shown below. 

Examine the isotopic mixture of chlorine. The molecular ion of ethyl chloride (see Figure 2.2) is revealed as two peaks separated by 2 Da and with an approximate intensity ratio corresponding to the relative abundances of the two isotopes (3 1). The lower mass peak of this doublet at 64 Da is referred to as P and corresponds to C2H5CP5, and the higher mass peak at 66 Da is (P + 2) and corresponds to C2H5CP . A pair of peaks separated by two masses and with an intensity ratio of 3 1 is therefore characteristic of one chlorine atom in a molecule. Likewise, a molecular ion containing bromine appears as two peaks separated by two mass units and with nearly equal intensities (Figure 2.3 is the mass spectrum of ethyl bromide). 

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