Chlorine isotopes exist as 35Cl and 37Cl in a 3:1 ratio, while bromine isotopes exist as 79Br and 81Br in a 1:1 ratio. The mass spectrum of alkyl halides typically produces two distinct molecular ion peaks, the molecular ion peak, [M], and the molecular ion plus two, [M + 2] peak. The relative heights of these two peaks are proportional to the isotopic abundance ratios of the halide. For example, 2‐chloropropane and 1‐bromopropane display two peaks with relative peak heights in a 3:1 and 1:1 ratio, respectively.
Alkyl halides typically lose a non-bonding electron from the halogen to form molecular ions. The base peak results from the heterolytic cleavage of the carbon–halogen bond, where the bonding electrons go to the most electronegative atom, forming a cation and halogen atom. Interestingly, the ⍺-cleavage is observed only in alkyl chlorides, not alkyl bromides. In alkyl chlorides, the C–Cl and C–C bonds exhibit similar strengths, while in alkyl bromides, C–C bond is weaker than the C–Br bond. Consequently, the alkyl chlorides form a stable cation, where the positive charge is shared between the two atoms.
While less commonly discussed, fluoroalkanes and iodoalkanes exhibit unique fragmentation behavior. Fluoroalkanes, due to the strength of the C–F bond and the presence of only one isotope (19F), show limited fragmentation and lack isotopic peaks. Iodoalkanes, with their weak C–I bond and a single isotope (127I), predominantly fragment by losing iodine, resulting in simpler spectra with less isotopic splitting.
The mass spectrum of alkyl halides is distinctive because of the existence of two Cl isotopes in a 3:1 ratio and two Br isotopes in a 1:1 ratio.
This isotopic effect generates two different molecular ions, observed in the mass spectra of corresponding alkyl halides in the form of a molecular-ion peak and a molecular-ion-plus-two peak, differing by two units.
For example, 2‐chloropropane shows two molecular-ion peaks with relative heights in a 3:1 ratio.
Similarly, 1‐bromopropane exhibits two peaks with relative heights in a 1:1 ratio.
Notably, the base peak is formed by the heterolytic cleavage of the molecular ion's C–X bond, forming a cation and a halogen atom.
Moreover, alkyl chlorides undergo α cleavage producing two peaks in the mass spectra. This happens because the C–Cl and the C–C bonds have similar strengths, and the cation formed is stable as the positive charge is shared between two atoms.
Conversely, alkyl bromides do not undergo α cleavage as the C–C bond is weaker than the C–Br bond.
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