Interpreting the infrared spectrum of propylbenzene (1-phenylpropane)

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 propylbenzene

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

Interpretation of the infrared spectrum of propylbenzene

The most prominent infrared absorption lines of propylbenzene

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 690 cm^-1 characteristic of a monosubstituted benzene ring.

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