Interpreting the infrared spectrum of Methylbenzene (Toluene)

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 infrared spectra of methylbenzene

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Spectra obtained from a liquid film of methylbenzene. The right-hand part of the of the infrared spectrum of methylbenzene, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of methylbenzene and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of methylbenzene.

Methylbenzene  C₇H₈, C₆H₅CH₃ , ,

Interpretation of the infrared spectrum of Methylbenzene (Toluene)

The most prominent infrared absorption lines of methylbenzene

Absorption at wavenumbers ~3000 to 3100 cm^-1 is typical of (benzene ring) C-H stretching vibrations of aromatic compounds (~3050 cm^-1 for methylbenzene).

Around wavenumber 2940 cm^-1 is an absorption band due to C-H vibrations in the side-chain methyl group of methylbenzene.

In the wavenumber 1440 to 1625 cm^-1 region are band of absorptions due to vibrations of the bonds in the benzene ring of methylbenzene.

The complex infrared absorption pattern around 700 cm^-1 is typical of a benzene compound with just one substituent group in the benzene ring (like methylbenzene).

The absence of other specific functional group infrared absorption bands shows that particular functional group is absent from the methylbenzene molecular structure.