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.

Figure 13.22  The ¹³C NMR spectrum of 1-methylcyclohexene, the E2 reaction product from treatment of 1-chloro-1-methylcyclohexane with base.

Problem 13-23

We saw in Section 9.3 that addition of HBr to a terminal alkyne leads to the Markovnikov addition product, with the Br bonding to the more highly substituted carbon. How could you use ¹³C NMR to identify the product of the addition of 1 equivalent of HBr to 1-hexyne?