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Isotope patterns for -Cl and -Br

isotope patterns

The following two examples of haloalkanes mass spectra show characteristic isotope patterns of monohalogenated molecules, these are highlighted in the green boxes:

MS of 2-chloropropane

The first MS is of 2-chloropropane.
Note the isotope pattern at 78 and 80 that represent the M amd M+2 in a 3:1 ratio.
Loss of 35Cl from 78 or 37Cl from 80 gives the base peak a m/z = 43, corresponding to the secondary propyl cation.
Note that the peaks at m/z = 63 and 65 still contain Cl and therefore also show the 3:1 isotope pattern.

The second MS is of 1-bromopropane.  

Mass spectrum of 1-bromopropane

Note the isotope pattern at 122 and 124 represents the M and M+2 in a 1:1 ratio.
Loss of 79Br from 122 or 81Br from 124 gives the base peak a m/z = 43, corresponding to the propyl cation.
Note that other peaks, such as those at m/z = 107 and 109 still contain Br and therefore still show the 1:1 isotope pattern.

Note: the isotope patterns for polyhalogenated molecules (such as having both -Cl and -Br or with multiple -Cl or -Br) give different (but still characteristic isotope patterns).

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary