Interpreting the infrared spectrum of benzaldehyde C₆H₅CHO

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 benzaldehyde

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

 benzaldehyde C₇H₆O, C₆H₅CHO , ,  

Interpretation of the infrared spectrum of benzaldehyde

The most prominent infrared absorption lines of benzaldehyde (an aromatic aldehyde - where the aldehyde group is directly attached to the benzene ring)

At wavenumbers 3000 to 3080 cm^-1 you get the C-H vibrations from the benzene ring.

At wavenumbers 2650 to 2880 cm^-1 you get the C-H vibration absorptions from the aldehyde group.

At ~1700 cm^-1 you get the characteristic C=O stretching vibrations of the carbonyl group.

This is a very characteristic infrared absorption band for aldehydes and ketones, but slightly lower value that you see in the infrared spectra of aliphatic aldehydes and ketones.

Between wavenumbers 1440 and 1625 cm^-1 you get several absorption bands due to C=C vibrations in the benzene ring.

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