Overview

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.

Procedure

Benzene protons produce a singlet around 7.3 ppm. Substituents influence the splitting and shift the signal downfield or upfield, depending on the nature of the substituent.

To illustrate, consider 1-bromo-4-ethylbenzene with four nonequivalent sets of protons.

The more electronegative substituent, bromine, deshields the two equivalent ortho protons, producing a signal far downfield.

The protons ortho to the electropositive substituent have slightly lower chemical shifts. Both signals appear as doublets due to the interaction with the adjacent proton.

The observed splitting pattern is typical of para substitution.

An upfield quartet signal arises from benzylic protons coupling with three methyl protons.

The upfield triplet peak represents methyl protons, split by two adjacent benzylic protons.

The six nonequivalent sets of carbons exhibit six characteristic ¹³C signals.

The peak with the largest chemical shift arises from the quaternary ring carbon bonded to the alkyl group.

The peak around 120 ppm represents the quaternary ring carbon bonded to bromine.

Other downfield signals indicate the unsubstituted ring carbons, while the upfield signals represent benzylic and alkyl carbons.