Interpreting the infrared spectrum of benzoic acid  (re-edit)

 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 benzoic acid

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

Interpretation of the infrared spectrum of benzoic acid

The most prominent infrared absorption lines of benzoic acid

A most characteristic absorption band is the broad O-H stretching vibration bands at wavenumbers ~3300 to 2500 cm^-1, the breadth is caused by interference of hydrogen bonding interactions, common to all hydrogen bonded molecules with a hydroxyl group.

The intramolecular hydrogen bond C₆H₅-C=O^δ--H^δ+ǁǁǁ:O^δ-O-C₆H₅ in benzoic acid.

The 'peak' of the O-H stretching vibration is at ~3580 cm^-1, but the stretching O-H vibrations overlap the C-H aryl stretching vibrations at wavenumbers 3080 to 3030 cm^-1.

The 2nd characteristic band is the absorption due to the carbonyl (C=O) stretching vibrations wavenumbers 1700 to 1680 cm^-1 for aryl carboxylic acids like benzoic acid.

The carbonyl group 'peak' is at 1760 cm^-1 for benzoic acid.

There are aryl C-H ( -H) stretching vibration bands at wavenumbers ~1100 to 1000 cm^-1 (for monosubstitution benzene compounds).

There are quite prominent aryl C-H deformation vibration absorptions at wavenumbers 770 to 690 cm^-1, you can see strong absorption at a little over wavenumber 700 cm^-1 (typical for monosubstitution benzene compounds like benzoic acid).

The presence of strong absorption at wavenumbers for C=O and O-H stretching vibrations are very indicative of a carboxylic acid group in the benzoic acid molecule, but not proof or exclusive e.g. you can have a hydroxy-ketone!

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