Organic Chemistry

560 CHAPTER 14 NMR Spectroscopy

PROBLEM 32

Explain why the chemical shift of the OH proton of a carboxylic acid is at a higher fre-

quency than the chemical shift of an OH proton of an alcohol.

PROBLEM 33

Which would show the signal for the OH proton at a greater chemical shift, the NMR

spectrum of pure ethanol or the NMR spectrum of ethanol dissolved in

PROBLEM 34

Propose a mechanism for base-catalyzed proton exchange.

PROBLEM 35

Identify the compound with molecular formula responsible for the NMR

spectrum in Figure 14.29.

C 3 H 7 NO 1 H

1 H CH 2 Cl 2?

1 H

87654 3210

(ppm)

frequency

Figure 14.29

1 HNMR spectrum for Problem 35.

14.16 Use of Deuterium in NMR Spectroscopy

Because deuterium signals are not seen in an NMR spectrum, substituting a deu-

terium for a hydrogen is a technique used to identify signals and to simplify NMR

spectra (Section 14.14).

If after an alcohol’s NMR spectrum is obtained, a few drops of are added

to the sample and the spectrum is taken again, the OH signal can be identified. It will

be the signal that becomes less intense (or disappears) in the second spectrum because

of the proton exchange process just discussed. This technique can be used with any

proton that undergoes exchange.

If the NMR spectrum of were compared with the NMR

spectrum of the signal at the highest frequency in the first spectrum

would be absent in the second spectrum, indicating that this signal corresponds to the

methylene group. Deuterium substitution can be a helpful technique in the analysis of

complicated NMR spectra. 1 H

CH 3 CD 2 OCH 3 ,

1 H CH 3 CH 2 OCH 3 1 H

RHO + DDO RDO + DHO

1 H D 2 O

1 H

1 H

(^1) H