13.13 • Uses of 13C NMR Spectroscopy
The information derived from 13C NMR spectroscopy is extraordinarily useful for structure determination. Not only can we count the number of nonequivalent carbon atoms in a molecule, we can also get information about the electronic environment of each carbon and find how many protons are attached to each. As a result, we can address many structural questions that go unanswered by IR spectroscopy or mass spectrometry.
Here’s an example: how do we know that the E2 reaction of an alkyl halide follows Zaitsev’s rule (Section 11.7)? Does treatment of 1-chloro-1-methylcyclohexane with a strong base give predominantly the trisubstituted alkene 1-methylcyclohexene or the disubstituted alkene methylenecyclohexane?
1-Methylcyclohexene will have five sp3-carbon resonances in the 20 to 50 δ range and two sp2-carbon resonances in the 100 to 150 δ range. Methylenecyclohexane, however, because of its symmetry, will have only three sp3-carbon resonance peaks and two sp2-carbon peaks. The spectrum of the actual reaction product, shown in Figure 13.22, clearly identifies 1-methylcyclohexene as the product of this E2 reaction.