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15.22 The peaks at m/z 186, 188 and 190 in a 1:2:1 ratio suggest the presence of two bromine atoms. Subtracting two from 186 leaves a mass of 28, corresponding to C₂H₄. Therefore the formula is C₂H₄Br₂. The unknown is one of the two isomers with this formula, 1,1-dibromoethane or 1,2-dibromoethane. Determining which of these two isomers is correct is more difficult. Both are expected to lose a bromine atom to give at m/z 107 and 109, the base ions in the spectrum. However, only 1,2-dibromoethane can fragment Br Br + to give CH₂Br with m/z 93 and CH₂Br + CH₂Br 95. The presence of these peaks in the spectrum indicate that the m/z 93, 95 unknown is 1,2-dibromoethane. m/z 186, 188, 190 15.23 Both the ketones produce fragment ions at m/z 43 and 85 due to the cleavage of the bonds to the carbonyl carbon. However, the fragment ions resulting from the McLafferty rearrangement are different for each of these ketones. In the case of 3-methyl-2-pentanone, McLafferty rearrangement produces an ion with m/z 72, while in the case of 4-methyl- 2-pentanone the ion has m/z 58. Therefore, the first spectrum is that of 4- methyl-2-pentanone and the second spectrum is that of 3-methyl-2- pentanone. 3-methyl- + H H . O 2-pentanone CH₃C CH₂ CH₃ C CH-CH₂ CH CH₃ CH₃ m/z = 100 m/z = 72 + H 4-methyl- + H 2-pentanone CH₃C CH₂ CH₃ C CH₂ CH₃ m/z = 100 m/z = 58 233