15.10
Massenspektrometrie: Fragmentierung von Alkylhalogeniden
Chlorisotope kommen als ^35Cl und ^37Cl im Verhältnis 3:1 vor, während Bromisotope als ^79Br und ^81Br im Verhältnis 1:1 vorliegen. Das Massenspektrum…
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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Q1: Why do alkyl halides show two molecular ion peaks in mass spectrometry?
Alkyl halides display two molecular ion peaks because chlorine and bromine exist as multiple isotopes. Chlorine has isotopes 35Cl and 37Cl in a 3:1 ratio, while bromine has 79Br and 81Br in a 1:1 ratio. These isotopic differences create a molecular ion peak [M] and a molecular ion plus two peak [M+2], with relative heights reflecting the isotopic abundance ratios of the halide.
Q2: What is the base peak in alkyl halide mass spectra and how does it form?
The base peak results from heterolytic cleavage of the carbon-halogen bond in the molecular ion. During this process, bonding electrons transfer to the more electronegative halogen atom, forming a stable cation and a halogen atom. This fragmentation pattern is characteristic of alkyl halides and produces the most intense peak in the mass spectrum.
Q3: Why do alkyl chlorides undergo alpha cleavage but alkyl bromides do not?
Alkyl chlorides undergo alpha cleavage because the C-Cl and C-C bonds have similar bond strengths, allowing the positive charge to be shared between two atoms, stabilizing the resulting cation. In alkyl bromides, the C-C bond is weaker than the C-Br bond, making alpha cleavage unfavorable and preventing this fragmentation pathway from occurring.
Q4: How do 2-chloropropane and 1-bromopropane differ in their mass spectra?
2-Chloropropane displays two molecular ion peaks with relative heights in a 3:1 ratio, reflecting chlorine's isotopic abundance. In contrast, 1-bromopropane exhibits two peaks with relative heights in a 1:1 ratio, corresponding to bromine's isotopic distribution. These distinct peak patterns allow chemists to identify the halide present in a compound.
Q5: What is the difference between fragmentation patterns in fluoroalkanes and iodoalkanes?
Fluoroalkanes show limited fragmentation due to the strong C-F bond and possess only one isotope (19F), resulting in no isotopic peaks. Iodoalkanes, with their weak C-I bond and single isotope (127I), predominantly fragment by losing iodine, producing simpler spectra with minimal isotopic splitting compared to chlorine and bromine-containing compounds.
Q6: How is the molecular ion formed in alkyl halides during mass spectrometry?
Alkyl halides form molecular ions by losing a non-bonding electron from the halogen atom. This ionization process creates the molecular ion species that subsequently fragments through various pathways, including heterolytic cleavage and alpha cleavage, generating the characteristic fragmentation patterns observed in mass spectra of alkyl halides.
Q7: How can isotopic peak ratios help identify halogens in unknown compounds?
The relative heights of molecular ion and M+2 peaks directly correspond to isotopic abundance ratios. A 3:1 peak ratio indicates chlorine, while a 1:1 ratio indicates bromine. These distinctive patterns serve as diagnostic tools in mass spectrometry, allowing analysts to identify which halogen is present in mass spectrometry aldehyde and ketone fragmentation studies and other analytical applications.