Chapter 14.11, Problem 14P

Interpretation Introduction

Interpretation:

The reason due to which eight lines are observed for ${\text{H}}_{\text{a}}\text{and}{\text{H}}_{\text{b}}$ protons in the ${}^{\text{1}}\text{H}$ NMR spectrum of phenylalanine is to be discussed.

Concept Introduction:

For the compounds in which vicinal protons are not equivalent, a proton $H_a$ is coupled to $H_b,H_c,H{d}_{}$ with unequal coupling constants. The original signal for $H_a$ is split into $(n+1)$ peaks by $n{H}_b$ protons. Each of these lines is further split into $(n+1)$ peaks by $n{H}_c$ protons and each of these lines into $(n+1)$ peaks by $n{H}_d$ protons and so on..

Chemical-shift non-equivalence can occur when two environments are stereo chemically different.

Protons that yield diastereomers on being replaced by some test group are diastereotopic and can have different chemical shifts.