Mass spectra chlorine-bromine isotope intensities

If sulfur or silicon, is present, the M + 2 will be more intense. In the case of a single sulfur atom, 34S contributes approximately 4.40% to the M + 2 peak for a single silicon in the molecule, 30Si contributes about 3.35% to the M + 2 peak (see Section 2.10.15). The effect of several bromine and chlorine atoms is described in Section 2.10.16. Note the appearance of additional isotope peaks in the case of multiple bromine and chlorine atoms. Obviously the mass spectrum should be routinely scanned for the relative intensities of the M + 2, M + 4, and higher isotope peaks, and the relative intensities should be carefully measured. Note that F and I are monoisotopic. 

Due to the distinctive mass spectral patterns caused by the presence of chlorine and bromine in a molecule, interpretation of a mass spectrum can be much easier if the results of the relative isotopic concentrations are known. The following table provides peak intensities (relative to the molecular ion (M+) at an intensity normalized to 100%) for various combinations of chlorine and bromine atoms, assuming the absence of all other elements except carbon and hydrogen.1 The mass abundance calculations were based on the most recent atomic mass data.1... 

How does the mass spectrum give evidence of isotopes in the compounds of bromine, chlorine, and carbon Assuming the molecular ion of each of these compounds is of 100% ahundance, what peaks (and in what intensity) would appear around that mass number (a) C2H5BrO, (b) CgOi (c) Ce BrCl Give in cases (a) and (c) a possible structure for the compound. What compound is (b) ... 

Even with a lower-resolution mass spectrometer we can get hints about the elemental composition. Molecular ions (M) have higher weight peaks at M+1, M+2 due to minor isotopes, which can tip you off to their presence. Common examples are shown below (with the intensity of the taller peak set at 100%). Hydrogen, nitrogen, oxygen, fluorine, and iodine have no significant isotope peaks (<0.4%). The presence of chlorine or bromine is important to identify in the mass spectrum. 

Both chlorine ( Cl 76% and Cl 24%) and bromine Br 51% and Br 49%) exist as two isotopes. Consider the differences in mass numbers for the isotopes—any compound containing either Ci or Br will have molecular ions 2 u apart. The lightest isotopomer of ClBrj is Ci Br3 at 272 u and the heaviest is Cl Brj at 280 u. Three other molecular masses are possible, giving rise to a total of five peaks in the mass spectrum shown in Figure 8.38. The differences in the relative intensities of these peaks are a consequence of the differences in the percent abundance for each isotope. 

Examine the isotopic mixture of chlorine. The molecular ion of ethyl chloride (see Figure 2.2) is revealed as two peaks separated by 2 Da and with an approximate intensity ratio corresponding to the relative abundances of the two isotopes (3 1). The lower mass peak of this doublet at 64 Da is referred to as P and corresponds to C2H5CP5, and the higher mass peak at 66 Da is (P + 2) and corresponds to C2H5CP . A pair of peaks separated by two masses and with an intensity ratio of 3 1 is therefore characteristic of one chlorine atom in a molecule. Likewise, a molecular ion containing bromine appears as two peaks separated by two mass units and with nearly equal intensities (Figure 2.3 is the mass spectrum of ethyl bromide). 

If more than one chlorine atom or bromine atom is present in a molecule, distinctive isotope cluster patterns are seen in the mass spectrum. Figure 10.8 gives a graphical representation of the isotope peak intensity patterns for Cl and Br. The numerical values for the peak ratios are given in Table 10.8. The patterns arise as follows ... 

As mentioned earlier, molecules containing chlorine or bromine produce easily characterized spectra. This observation is one of the most useful aspects of mass spectrometry for organic chemists. Table 15.1 lists the natural isotopic ratio for Cl to Cl (75.77 24.23), which is essentially a 3 1 ratio. The ratio for Br to Br (50.69 49.31) is close to a 1 1 ratio. Chlorine and bromine are the only elements frequently found in organic molecules that have significant M + 2 isotopes. There are other elements (osmium and mercury, for example) that have M -b 2 isotopes, but we are less likely to deal with mass spectra of such compounds. Therefore, if a mass spectrum shows an M -b 2 peak that is one-third the intensity of the molecular ion (M ) as shown in Figure I5.14a, then we know there is a chlorine in the molecule. If a spectrum has an M -b 2 peak that is the same intensity as the molecular ion, we can conclude that the molecule has a bromine (see Rg. 15.14b). 

Flnorine and iodine are isotopically pure. However, chlorine (75.53% Cl 24.47% Cl) and bromine (50.54% Br 49.46% Br) each exist as a mixture of two isotopes and give rise to readily identifiable isotopic patterns. For example, the mass spectrum of 1-bromopropane (Figure 11-24) shows two peaks of nearly equal intensity at m/z = 122 and 124. Why The isotopic composition of the molecule is a nearly 1 1 mixture of CH3CH2CH2 Br and CHjCHjCHj Br. Similarly, the spectra of monochloroalkanes exhibit ions two mass units apart in a 3 1 intensity ratio, because of the presence of abont 75% and 25% R Cl. Peak patterns such as these are useful in revealing the presence of chlorine or bromine. 

---