Interpreting the Carbon-13 NMR spectrum of propanoic acid (propionic 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 propanoic acid

email doc brown

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 - propanoic acid here.

Interpreting the C-13 NMR spectrum of propanoic acid

As you can see from the diagram above there are 3 different chemical shift lines in the C-13 NMR spectrum of propanoic acid indicating 3 different chemical environments of the 3 carbon atoms of propanoic acid.

CH₃CH₂COOH

(Note the 3 different colours indicating the 3 different chemical environments of the 3 carbon atoms in propanoic acid).

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

Note the decreasing effect on the ¹³C chemical shift as the carbon atom is further from the more electronegative oxygen atoms in propanoic acid.

The carbon-13 NMR spectra provides direct evidence of 3 different carbon atom environments for the 3 carbon atoms in the propanoic acid molecule, deduced from the presence of 3 different 13C NMR chemical shifts (ppm).