Advanced Organic pre-university/college Chemistry: The 13C NMR spectrum of benzene C6H6

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Interpreting and explaining the 13C NMR spectrum of benzene

[Author ©  Dr Phil Brown GRIC, PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 & AP honors chemistry courses: Molecular spectroscopy of benzene [spectrum page updated RE-EDIT]

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 C-13 NMR spectroscopy - spectra index

Introductory note on the 13C NMR spectrum of benzene

Students and teachers please note that my explanation of the carbon-13 NMR spectrum of benzene is designed for advanced, but pre-university, chemistry courses.

The description does not involve the chemical shift δ spin-spin coupling effects for benzene and the relative size of the carbon-13 NMR shifts does not give the ratio of the carbon atoms in the different non-equivalent chemical environments of the benzene molecule.

The most common solvent used for investigating the C13 NMR spectrum of compounds like benzene, is CDCl3 and other deuterated solvents.

C-13 nmr spectrum of benzene analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of benzene 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 shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the 13C atoms in an organic molecule - benzene here.

aromatic benzene C6H6, skeletal formula (c) doc b , structural/displayed formula

The molecular structure and naming of aromatic compounds

Interpreting the C-13 NMR spectrum of benzene C6H6

As you can see from the diagram above there is only one chemical shift line in the C-13 NMR spectrum of benzene (at 128 ppm) indicating only one chemical environment for all carbon atoms.

C6H6

So, all six ring carbons are chemically equivalent to each other due to the high symmetry of the benzene molecule.

So all you see is one singlet 13C NMR chemical shift at 128 ppm.

Take care in presenting the C-13 NMR spectrum of benzene as evidence of the true aromatic structure i.e. the symmetrical hexagon of carbon atoms with a C-C bond order of 1.5 (), with (i) the circular common pi bonding ring of electrons and (ii) the carbon atoms of the theoretical Kekule structures shown on the right. In both cases, due to the high symmetry of the benzene molecule structures, all carbon atoms are in the same chemical environment and a single chemical shift is expected for both (i) and (ii).

This C-13 NMR spectrum of benzene fits in with modern view of it being a completely symmetrical planar hexagonal molecule with all the carbon atoms in the same chemical environment - all bound by the same symmetrical delocalised electron system of the pi orbitals (diagram below)

explaining the 13C carbon-13 NMR spectrum of benzene diagram of the rings of pi orbitals of benzene aromatic compounds aromaticity above and below a hexagonal ring of carbon atoms .

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 Selected isomers of molecular formula C6H6 (not usually encountered at pre-university level, but interesting!)

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