Name: mass spectra of alkyl halides
Uploaded: 2025-04-16T15:46:54.000Z
Duration: 1 min 22 s
Channel: JoVE

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&#8208;chloropropane shows two molecular-ion peaks with relative heights in a 3:1 ratio.
Similarly, 1&#8208;bromopropane exhibits two peaks with relative heights in a 1:1 ratio.&#160;&#160;
Notably, the base peak is formed by the heterolytic cleavage of the molecular ion&#39;s C&#8211;X bond, forming a cation and a halogen atom.
Moreover, alkyl chlorides undergo &#945; cleavage producing two peaks in the mass spectra. This happens because the C&#8211;Cl and the C&#8211;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 &#945; cleavage as the C&#8211;C bond is weaker than the C&#8211;Br bond.

mass spectrometry: alkyl halide fragmentation

Mass Spectrometry: Alkyl Halide Fragmentation

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&nbsp; 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.

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 ...

mass-spectrometry-alkyl-halide-fragmentation

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Analytical Chemistry

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Mass Spectrometry Fragmentation Methods

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mass spectra of alkyl halides

Mass spectra of alkyl halides show clear isotope patterns from the halogen atom. Chlorine has two common isotopes, 35Cl and 37Cl, in a 3:1 ratio. Bromine has two common isotopes, 79Br and 81Br, in a 1:1 ratio. These isotope ratios create two molecular ion peaks, [M] and [M + 2], with relative heights that match the halide abundance.

This pattern is easy to see in examples such as 2-chloropropane and 1-bromopropane. 2-Chloropropane gives peaks in a 3:1 ratio. 1-Bromopropane gives peaks in a 1:1 ratio. Alkyl halides usually form molecular ions by losing a non-bonding electron from the halogen.

The base peak often comes from heterolytic cleavage of the carbon-halogen bond. In heterolytic cleavage, both bonding electrons go to the more electronegative atom. That leaves a cation and a halogen atom. Alpha-cleavage appears only in alkyl chlorides, not in alkyl bromides. In alkyl chlorides, the C-Cl and C-C bonds are similar in strength, and this helps form a stable cation with the positive charge shared between two atoms.

Alkyl bromides behave differently because the C-C bond is weaker than the C-Br bond. Fluoroalkanes also show limited fragmentation because the C-F bond is very strong. They also have only one isotope, 19F, so they do not show isotopic peaks. Iodoalkanes have a weak C-I bond and one isotope, 127I. They usually fragment by losing iodine, which gives simpler spectra with less isotopic splitting.

Mass Spectra of Alkyl Halides

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key terms and concepts

Learn how chlorine and bromine isotope ratios shape alkyl halide mass spectra, and why chlorides, bromides, fluorides, and iodides fragment differently.

Mass Spectra of Alkyl Halides | JoVE K12

analytical chemistry

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Mass Spectra of Alkyl Halides

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