2-Bromopropane mass spectrum

Isopropyl derivatives were introduced by Pettitt and Stouffer [287] and later studied by other workers [288]. They are prepared by reaction with 2-bromopropane in the presence of sodium hydride in dimethyl sulphoxide. The reaction scheme and the preparation procedure were given in Chapter 4 (see p. 64). Except for Arg, all amino acids under study provided the expected derivatives. The hydroxyl group of Hypro was, however, not protected. The derivatives were found to be stable for a reasonable period of time and were analysed on 3% of OV-17. The extension of this promising one-step method to all protein amino acids did not fulfill expectations, however [288]. Some amino acids (Gly, Gin, Asp and Asn) did not provide detectable derivatives and the others led to multiple peaks. Moreover, significant amounts of by-products were produced, which may interfere. Arg provided a single peak, the mass spectrum of which was identical with that of Orn both derivatives resulted from lactam formation. Isoprop derivatives of 23 common amino acids were separated on 5% of-Carbowax 20M on silanized Chromosorb G with temperature programming (50-240°C). 

Let s look first at the mass spectrum of 1-bromopropane, shown in Figure 13.4. The relative heights of the M and M + 2 peaks are about equal, so we can conclude that the compound contains a bromine atom. Electron bombardment is most likely to dislodge a lone-pair electron if the molecule has any, because a molecule does not hold onto its lone-pair electrons as tightly as it holds onto its bonding electrons. Thus, electron bombardment dislodges one of bromine s lone-pair electrons. 

The mass spectrum of 1-bromopropane. The M and M + 2 peaks are about equal... 

The weakest bond in the molecular ion is the one most apt to break. In this case, the C — Br bond is the weakest. When the C—Br bond breaks, it breaks heterolytically, with both electrons going to the more electronegative of the atoms that were joined by the bond, forming a propyl cation and a bromine atom. As a result, the base peak in the mass spectrum of 1-bromopropane is at miz = 43 (M - 79 - 43 or [M-H2] - 81 = 43). 

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. 

Figure 11-24 Mass spectrum of 1 -bromopropane. Note the neariy equal heights of the peaks at m/z =122 and 124, owing to the almost equal abundances of the two bromine isotopes.

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