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Advanced Organic Chemistry: Carbon-13 NMR spectrum of benzoic acid

Interpreting the Carbon-13 NMR spectrum of benzoic acid

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing C-13 NMR spectra of benzoic acid

13C nmr spectrum of benzoic acid C7H6O2 C6H5COOH analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of benzoic acid C13 13-C nmr doc brown's advanced organic chemistry revision notes 

TMS is the acronym for tetramethylsilane, formula Si(CH3)4, whose 13C atoms are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in 13C NMR spectroscopy and all other 13C resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the 13C atoms in an organic molecule - benzoic acid here.

Benzoic acid (benenecarboxylic acid)C7H6O2C6H5COOH  ,  (c) doc b , (c) doc b , (c) doc b

Interpreting the C-13 NMR spectrum of benzoic acid

As you can see from the diagram above there are 5 different chemical shift lines in the C-13 NMR spectrum of benzoic acid indicating 5 different chemical environments of the 7 carbon atoms of benzoic acid.

C6H5COOH

(Note the 2 different colours indicating the aromatic benzene ring and hydroxyl group chemical environments of the 7 carbon atoms in benzoic acid).

BUT, strictly speaking, there are 5 different chemical environments, 4 for the benzene ring carbon atoms and 1 for the carboxyl group carbon atom.

13C chemical shifts (a) to (e) on the C-13 NMR spectrum diagram for benzoic acid.

Note:

(i) The close proximity of the 13C chemical shifts for the aromatic carbon atoms of the benzene ring - just about separated at high resolution.

(ii) The effect of two highly electronegative oxygen atoms on the carboxylic acid group carbon atom, considerably increasing its 13C chemical shift compared to the benzene ring carbon atoms. This carbon atom cannot be equivalent to any of the benzene ring carbon atoms.

(iii) Carbon atoms C1 and C4 are also not equivalent to each other or any other ring carbons.

(iv) Due to the symmetry of a monosubstituted benzene compound like benzoic acid and free rotation of the -COOH group, carbon atoms C2 and C6 are equivalent to each other (same chemical shift) as are C3 and C5.

The carbon-13 NMR spectra provides direct evidence of 5 different carbon atom environments for the 7 carbon atoms in the benzoic acid molecule, deduced from the presence of 5 different 13C NMR chemical shifts (ppm).


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Links associated with benzoic acid

INDEX of all AROMATIC COMPOUND chemistry revision notes

The physical and chemical properties of benzoic acid and selected derivatives

C-13 NMR spectroscopy index

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