18.1 : ベンゼン誘導体のNMR分光分析

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.