Interpreting the infrared spectrum of benzene C₆H₆

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 spectrum of benzene

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

Interpretation of the spectrum of benzene C₆H₆

The most prominent infrared absorption lines of benzene

There are characteristic =C-H stretching vibrations peaking at ~3030 to 3080 cm^-1

Characteristic benzene ring stretching vibrations peaking at ~1480 cm^-1.(moderate C=C absorption) and peaking at ~1800 cm^-1 (weak C=C absorption).

(I have also seen the 1480 cm^-1 absorption attributed to a =C-H bending vibration)

Most of the peaks in the fingerprint region are due to C-H vibrations of the C-H bond.

The absence of other specific functional group bands will show that particular functional group is absent from the benzene molecular structure.