18.4: NMR Spectroscopy of Benzene Derivatives
Simple unsubstituted benzene has six aromatic protons, all chemically equivalent. Therefore, benzene exhibits only a singlet peak at δ 7.3 ppm in the 1H NMR spectrum. The observed shift is far downfield because the aromatic ring current strongly deshields the protons. Any substitution on the benzene ring makes the aromatic protons nonequivalent, and the protons split each other. The peak is, therefore, no longer a singlet and the splitting pattern and their associated coupling constants depend on the degree of substitution on the ring. The nature of the substituents on the benzene ring either increases or decreases the ring proton's chemical shift values. Additionally, an electron-withdrawing substituent moves the proton's chemical shift farther downfield, while an electron-donating group moves the signal upfield. Monosubstituted benzene has a more complex 1H NMR spectrum in the aromatic region due to several splittings between protons on adjacent carbons as well as coupling between protons that are more than one C–C bond in the ring system. A disubstituted ring shows a typical doublet pattern if the ring's substituents have a para relationship.
In 13C NMR spectroscopy, the aromatic carbons exhibit signals between δ 110-160. Substituted benzene exhibits six peaks corresponding to six nonequivalent sets of protons. The extent of the signal shift depends on the type of ring substituents. A quaternary ring carbon shows the highest shifts compared to other ring carbons. Benzylic and alkyl carbons of the substituents are observed in the upfield region.