The ^1 H NMR spectrum of 2 -chloroacetamide (CICH2 CONH2) shows three signals at 4.02,7.35, and

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The ^1 H NMR spectrum of 2 -chloroacetamide (CICH2 CONH2)...

The ${ }^{1} \mathrm{H}$ NMR spectrum of 2 -chloroacetamide $\left(\mathrm{CICH}_{2} \mathrm{CONH}_{2}\right)$ shows three signals at $4.02,7.35,$ and 7.60 ppm. What protons give rise to each signal? Explain why three signals are observed.

Key Concepts

1H NMR Spectroscopy Basics

1H NMR spectroscopy detects the magnetic environments of hydrogen nuclei in a molecule. Each distinct set of protons in chemically different environments gives rise to a separate signal in the spectrum. This technique relies on the fact that protons, depending on the atoms or functional groups to which they are attached, experience different electronic surroundings that shift their resonance frequency.

Chemical Shifts and Electronegativity

Chemical shifts in 1H NMR are sensitive to the electron density around the protons. Protons near electron?withdrawing groups, such as halogens or carbonyl groups, are deshielded and appear downfield (i.e., at higher ppm values) compared to those in electron-rich environments. Understanding these shifts helps in assigning which protons in a molecule give signals at specific chemical shift values.

Proton Equivalence and Magnetic Non-equivalence

In NMR, protons that are in identical chemical environments (i.e., equivalent) will produce the same signal. In molecules with as few as two types of protons, differences in their immediate surroundings can render them chemically non-equivalent. Even within a functional group, slight differences—such as those induced by hydrogen bonding or restricted rotation—can cause protons to experience distinct magnetic environments, resulting in separate signals.

Amide Proton Exchange and Non-equivalence

Protons attached to heteroatoms in amide functional groups can be involved in hydrogen bonding and exchange processes. Although they might appear similar, these protons can be non-equivalent due to different levels of hydrogen bonding or interactions with other parts of the molecule. Slow exchange at the NMR timescale can lead to the resolution of distinct signals for each amide proton.

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