Question

NMR SPECTROSCOPY

Although a monosubstituted benzene ring has three different types of hydrogens, their chemical shifts are so close, that they frequently appear as a single peak.

         A 1,4-disubstituted benzene ring in which the two substituents are different has a characteristic pattern of aromatic hydrogen absorption. What would you predict this pattern to be? (Hint – There are two different kinds of hydrogens.)

Answer 1

• A 1,4-disubstituted benzene ring with two different types of substituents have two different types of hydrogen environment and hence different characteristic pattern of aromatic hydrogen absorption.
• Let these 2H's be Ha and Hb. Using the splitting pattern formula we see that Ha peaks will be a doublet which is double the intensity because there are 2 Ha of the same pattern.
• Similarly, Hb peaks will be doublet which is double the intensity because there are 2 Hb of the same pattern.
• Because Ha and Hb are at different electronic environments, the chemical shifts will be different but pattern of the peaks will be same.