Advanced level pre-university organic chemistry: 13C NMR spectrum 1,3,5-trimethylbenzene (mesitylene)

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Interpreting and explaining the 13C NMR spectrum of 1,3,5-trimethylbenzene (mesitylene)

[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 and AP honors chemistry courses: Molecular spectroscopy analysis of 1,3,5-trimethylbenzene [spectrum page updated RE-EDIT]

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 Links associated with 1,3,5-trimethylbenzene (mesitylene)

See also Isomers of molecular formula C9H12 (only aromatic hydrocarbons)

Introductory note on the 13C NMR spectrum of 1,3,5-trimethylbenzene (mesitylene)

Students and teachers please note that my explanation of the carbon-13 NMR spectrum of 1,3,5-trimethylbenzene (mesitylene) is designed for advanced, but pre-university, chemistry courses.

The description does not involve the chemical shift δ spin-spin coupling effects for 1,3,5-trimethylbenzene (mesitylene) 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 1,3,5-trimethylbenzene (mesitylene) molecule.

The most common solvent used for investigating the C13 NMR spectrum of compounds like 1,3,5-trimethylbenzene (mesitylene), is CDCl3 and other deuterated solvents.

C9H12 C-13 nmr spectrum of 1,3,5-trimethylbenzene (mesitylene) analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of 1,3,5-trimethylbenzene (mesitylene) C13 13C 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 shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the 13C atoms in an organic molecule - 1,3,5-trimethylbenzene (mesitylene) here.

structural formula 1,3,5-trimethylbenzene mesitylene molecular structure molecular formula C9H12 C6H3(CH3)3

The molecular structure and naming of aromatic compounds

Interpreting the C-13 NMR spectrum of 1,3,5-trimethylbenzene (mesitylene)

As you can see from the diagram above there are 3 different 13C chemical shift lines in the C-13 NMR spectrum of 1,3,5-trimethylbenzene (mesitylene) indicating 3 different chemical environments of the carbon atoms.

On the C-13 NMR spectrum diagram for 1,3,5-trimethylbenzene you can see chemical shifts (a) and (b) are for the benzene ring carbon atoms and one chemical shift (c) for the side-chain carbon atom of the methyl groups.

Ring carbon atoms 1, 3 and 5 are equivalent and so are 2, 4 and 6, due to the symmetry of the molecule.

The 13C NMR chemical shift for the aromatic ring carbon atoms tend to be much greater than those of the carbon atoms of alkyl groups.

(c) The 3rd 13C NMR chemical shift is for the three carbon atoms of the methyl groups, which are all equivalent to each other.

The carbon-13 NMR spectra a provides direct evidence of only 3 different carbon atom environments for the 9 carbon atoms of the 1,3,5-trimethylbenzene (mesitylene) molecule from 3 different 13C chemical shifts (ppm) due to the high degree of symmetry of the molecule.

The relatively small number of 13C NMR chemical shifts is due to the very high symmetry of the 1,3,5-trimethylbenzene molecule (mesitylene).

Adding D2O to the sample under investigation, dissolved in CDCl3, makes no difference to the 13C NMR spectrum of 1,3,5-trimethylbenzene (mesitylene) because 13C NMR does not detect proton resonances!

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Links associated with 1,3,5-trimethylbenzene (mesitylene)

See also Isomers of molecular formula C9H12 (only aromatic hydrocarbons)

The mass spectrum of 1,3,5-trimethylbenzene

The H-1 NMR spectrum of 1,3,5-trimethylbenzene

The infrared spectrum of 1,3,5-trimethylbenzene

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