26 5.7 How to Interpret a Mass Spectrum?
The Nitrogen Rule:
The nitrogen rule of mass spectrometry says that a compound with an odd number of nitrogen atoms has an odd-numbered molecular weight. The logic behind the rule comes from the fact that nitrogen is trivalent, thus requiring an odd number of hydrogen atoms. The presence of nitrogen in a molecule is often detected simply by observing its mass spectrum. An odd-numbered molecular ion usually means that the unknown compound has one or three nitrogen atoms, and an even-numbered molecular ion usually means that a compound has either zero or two nitrogen atoms.
Mass Spectrum of Triethylamine
Presence of One Chlorine Atom:
The fact that some elements have two common isotopes gives their mass spectra a distinctive appearance. Chlorine, for example, exists as two isotopes, 35Cl and 37Cl, in roughly a 3 : 1 ratio. In a sample of chloroethane, three out of four molecules contain a 35Cl atom and one out of four has a 37Cl atom. In the mass spectrum of chloroethane, we see the molecular ion (M) atm/z = 64 for ions that contain a 35Cl and another peak atm/z = 66, called the M + 2 peak, for ions containing a 37Cl. The ratio of the relative abundance of M:M +2 is about 3 : 1, a reflection of the isotopic abundances of chlorine.
Figure XX: Mass Spectrum of Chloroethane
Presence of One Bromine Atom:
In the case of bromine, the isotopic distribution is 50.7% 79Br and 49.3% 81Br. In the mass spectrum of 1-bromohexane (FIGURE 12.13) the molecular ion appears atm/z = 164 for 79Br-containing ions and the M + 2 peak is at m/z = 166 for 81Br-containing ions. The ions at m/z = 135 and 137 are informative as well. The two nearly equally large peaks tell us that the ions at thosem/z values still contain the bromine atom. The peak at m/z = 85, on the other hand, does not contain bromine because there is not a large peak at m/z = 87.
Mass Spectrum of 1-Bromohexane
Examples
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?
Answer:
Both mass spectra show molecular ions at M+ = 98, corresponding to C7H14, but they differ in their fragmentation patterns. Sample A has its base peak at m/z = 69, corresponding to the loss of a CH2CH3 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 CH3 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.
Exercises
One spectrum is that of 2-methyl-2-pentene; the other is of 2-hexene. Which is which? Explain.
