Interpreting the infrared spectrum of Pent-1-ene (1-pentene)

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 spectrum of pent-1-ene

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

Pent-1-ene, , , (1-pentene)

Interpretation of the spectrum of pent-1-ene

The most prominent infrared absorption lines of pent-1-ene

The most characteristic absorption is ~1660 cm^-1 due to the C=C vibration (stretching).

You don't find this C=C absorption band in the infrared spectra of saturated alkanes.

There are also characteristic lines due to various C-H vibration absorptions group at wavenumbers ~3100, ~2900, ~1290-1420 and ~900 cm^-1.

The absorption band at 915 to 905 cm^-1 is due to C-H vibrations of the =CH₂ group.

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