Interpreting the infrared spectrum of cinnamic acid

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 cinnamic acid

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

Interpretation of the infrared spectrum of cinnamic acid

Cinnamic acid has two functional groups, an aliphatic carboxylic acid group and an alkene group.

The most prominent infrared absorption lines of cinnamic acid (wavenumber bands in cm^-1)

From ~3400 to ~2300 cm^-1 there two broad overlapping bands from O-H and C-H stretching vibrations.

The bands are complicated by hydrogen bonding affecting the O-H vibration frequencies (as part of the carboxylic acid functional group) and the C-H vibrations vary due to different origins i.e. alkene C-H vibrations from the CH=CH group and arene C-H vibrations from the benzene ring.

The peak at ~1680 cm^-1 is due to the C=O stretching vibrations of the carbonyl group (of the carboxylic acid functional group).

The C=O absorption is clearly distinguished from the alkene C=C stretching absorption band which peaks at ~1630 cm^-1.

According to internet data there are two peaks for the arene C=C stretching vibrations of the benzene ring at ~1580 and ~1500 cm^-1.

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