Question 10.1: Assume that you have two unlabeled samples, one of methylcyc...
Using Mass Spectra to Identify Compounds
Assume that you have two unlabeled samples, one of methylcyclohexane and the other of ethylcyclopentane. How could you use mass spectrometry to tell them apart? The mass spectra of both are shown in FIGURE 10.6.
S t r a t e g y
Look at the possible structures and determine how they differ. Then think about how any of these differences in structure might give rise to differences in mass spectra. Methylcyclohexane, for instance, has a -CH_3 group, and ethylcyclo-pentane has a -CH_2CH_3 group, which should affect the fragmentation patterns.
The mass spectra of both samples show molecular ions at {M^+} = 98, corresponding to C_7H_{14}, but the two spectra differ in their fragmentation patterns. Sample A has its base peak at m/z = 69, corresponding to the loss of a CH_2CH_3 group (29 mass units), but B has a rather small peak at m/z = 69. Sample B shows a base peak at m/z = 83, corresponding to the loss of a CH_3 group (15 mass units), but sample A has only a small peak at m/z = 83. We can therefore be reasonably certain that A is ethylcyclopentane and B is methylcyclohexane.