Interpreting the Carbon-13 NMR spectrum of phenol C₆H₅OH

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 phenol

C-13 NMR spectroscopy - spectra index

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 - phenol here.

Interpreting the C-13 NMR spectrum of phenol

As you can see from the diagram above there are 4 different chemical shift lines in the C-13 NMR spectrum of phenol indicating 4 different chemical environments of the 6 carbon atoms of phenol.

¹³C chemical shifts (a) to (d) on the C-13 NMR spectrum diagram for phenol.

Numbering of benzene ring carbon atoms.

As a consequence of the symmetry of phenol:

Resonance (b) is for the two equivalent carbon atoms 2 and 6.

Resonance (c) is for the two equivalent carbon atoms 3 and 5.

Carbon atom 1, resonance (a) and carbon atom 4, resonance (d), resonance are not equivalent to each other or to any other carbon atoms in the benzene ring.

The carbon-13 NMR spectra provides direct evidence of 4 different carbon atom environments for the 6 carbon atoms in the phenol molecule, deduced from the presence of 4 different 13C NMR chemical shifts (ppm).