Interpreting the infrared spectrum of 1,4-dimethylbenzene

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 1,4-dimethylbenzene

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Spectra obtained from a liquid film of 1,4-dimethylbenzene. The right-hand part of the of the infrared spectrum of 1,4-dimethylbenzene, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of 1,4-dimethylbenzene and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of 1,4-dimethylbenzene.

Interpretation of the infrared spectrum of 1,4-dimethylbenzene

The most prominent infrared absorption lines of 1,4-dimethylbenzene

Strong absorption at wavenumbers 3080 to 3030 cm^-1 due to =C-H aryl C-H stretching vibrations.

Strong C-H stretching vibrations from the alkyl groups (CH₂ and CH₃) at wavenumbers 2975 to 2845 cm^-1.

C=C benzene ring vibrations with peaks close to wavenumbers 1600 and 1500 cm^-1.

C-H side-chain alkyl vibrations are observed at wavenumbers 1470 to 1370 cm^-1.

Other aryl C-H vibration absorptions occur at wavenumbers 770 to 735 cm^-1 characteristic of a disubstituted benzene ring.

The absence of other specific functional group bands will show that particular functional group is absent from the 1,4-dimethylbenzene molecular structure.