Interpreting the Carbon-13 NMR spectrum of benzoic acid  (re-edit)

 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

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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.

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).