Halogen compounds isotope abundance

The mass spectra ofbromacil and terbacil and their A-methyl derivatives show only weak molecular ions, but they have the isotope patterns expected from bromine and chlorine compounds, respectively. The major ions arise from [M—55] and [M—56] and also reflect the halogen substituent. Less abundant, but highly characteristic ions, corresponding to [M-99Y and [M-98], are formed by retro Diels-Alder fragmentations. 

Halogenated compounds are particularly easy to identify b their mass spectra because both chlorine and bromine occur naturally as mixtures of two abundant Isotopes. Chlorine occurs as Cl (75.8 Mi) and - Cl (24.2%) bromine occurs as Br (50.7%) and Br (49.3%). At what masses do the molecular ions occur foi the following formulas What are the relative percentages of each molecular ion ... 

The mass spectra of chlorine- and bromine-containing compounds clearly show the abundance ratios of the stable isotopes 35C1 37C1 = 3 1 and 79Br 81Br = 1 1 in the molecular ions and those ionic fragments which contain halogens (see Section 9-11). 

The isotopic distributions of several elements commonly found in organic compounds are shown in Table 13.2. From the isotopic distributions, we see why the M -b 1 peak can be used to determine the number of carbon atoms in a compound It is because the contributions to the M + 1 peak by isotopes of H, O, and the halogens are very small or nonexistent. This formula does not work as well in predicting the number of carbon atoms in a nitrogen-containing compound because the natural abundance of is relatively high. 

Identification of chloro and bromo compounds is a relatively simple matter because of the unique isotopic pattern. The presence of fluoro and iodo compounds, although not easy, can be inferred from the conspicuously low [M - -1]/[M] ratio, which is due to the fact that F and I are monoisotopic. The molecular ion in aliphatic chlorides is visible only in lower monochlorides. With an increase in the number of chlorine atoms, the abundance of the molecular ion decreases further. The t-cleavage to expel a halogen atom often produces an abundant ion (e.g., the base peak in the mass spectrum of r-butyl chloride is C4H9+). The a-cleavage is of low consequence in alkyl chlorides, but the loss of an alkyl radical can be prominent when the alkyl chain is longer than four carbons the product is a flve-membered ring halonium ion ... 

For most elements in organic compounds, such as C, H, O, S, P, and the halogens, the mass (of the most abundant isotopes) and valence are either both even or both odd, so even molecular weights always result. Nitrogen is a major exception the atomic weight is 14, but the valence is 3. This phenomenon has led to the nitrogeii rule in mass spectrometry, as expressed in this exercise. 

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