15 Consider the spectral data for 4-chlorobenzoic acid (Figs. 16.7 and 16.8). th

Question

15 Consider the spectral data for 4-chlorobenzoic acid (Figs. 16.7 and 16.8). the spectral data for , chioobernzoic acid (Figs 16 ad6.8 In the functional group region of the IR spectrum, specify the absorptions associated with the carbonyl component of the carboxyl group. What func- tional group is responsible for the broad absorption in the region of about 2800 cm -1, and why is the absorption broad In the 'H NMR spectrum, assign the various resonances to the hydrogen nuclei responsible for them. For the 13C NMR data, assign the various resonances to the carbon nuclei a. b. C NMR data,assign the various c.

Explanation / Answer

a) Carbonyl group gives a peak around 1750 cm-1 O-H stretching frequency comes around 2800 cm-1. In the case of highly molecules with O-H groups, another factor in the absorption is the degree of Hydrogen bonding. Hydrogen bonding will alter the electron cloud, which in turn alters the resonant frequency of that particular bond. Different molecules will have slightly different Hydrogen bonding states, which will lead to slightly different absorption frequencies and thus the broad band. If, however, the O-H groups are not Hydrogen-bonded for any reason, you will not see the broad peak.

b) 1H NMR: 13.8 ppm carboxylic acid proton. 7.9 ppm, 2 protons, ortho protons which are attached to carbon ortho to carboxylic acid group. 7.7 ppm, 2 protons, ortho protons which are attached to carbon ortho to chlorine group.

c) 13C NMR: 128.5, 129.8 ppm (two carbons which are ortho to chlorine group) 131.1, 138.2 ppm, 2 carbons, which are ortho to carboxylic acid group.

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