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Chapter 14: Q.21558-14-73P (page 569)

Question: The 1 H NMR spectrum of N,N-dimethylformamide shows three singlets at 2.9, 3.0, and 8.0 ppm. Explain why the two groups are not equivalent to each other, thus giving rise to two NMR signals.

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01

 1H NMR

The number of signals in a PMR spectrum is equal to the number of kinds of protons present in the molecule. The splitting of the NMR signal is caused by spin-spin coupling

02

Non-equivalent proton in NMR

The structure for N,N-dimethylformamide places the two CH3 groups in different environments and different chemical shifts. One CH3 group is cis to the O atom, and one is cis to the H atom.

This gives rise to two different absorptions for the CH3 groups.

Representation of cis

It should be noted that spin-spin splitting is observed only between non-equivalent neighboring protons.

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(A) Treatment of \({\bf{C}}{{\bf{H}}_{\bf{2}}}{\bf{ = CHCOC}}{{\bf{H}}_{\bf{3}}}\) with one equivalent of HCl forms compound A. A exhibits the following absorptions in its \({}^{\bf{1}}{\bf{H}}\)NMR spectrum: 2.2 (singlet, 3 H), 3.05 (triplet, 2 H), and 3.6 (triplet, 2H) ppm. What is the structure of A?

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