Interpreting the infrared spectrum of methyl 2-hydroxybenzoate

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 methyl 2-hydroxybenzoate (methyl salicylate)

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Spectra obtained from a liquid film of methyl 2-hydroxybenzoate. The right-hand part of the of the infrared spectrum of methyl 2-hydroxybenzoate, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of methyl 2-hydroxybenzoate and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of methyl 2-hydroxybenzoate.

For more see The molecular structure and naming of aromatic compounds

Interpretation of the infrared spectrum of methyl 2-hydroxybenzoate (methyl salicylate)

The most prominent infrared absorption lines of methyl 2-hydroxybenzoate (methyl salicylate)

The O-H stretching vibrations produce a broad band, due to hydrogen bonding, from 3800 to 2600 peak at ~3200 cm^-1. The hydrogen bonding can be both intermolecular and intramolecular (mostly the latter, since methyl 2-hydroxybenzoate is a liquid at room temperature).

There are several arene C-H stretching vibrations peaking at 3071 cm^-1, ( -H)

The peak at ~2955 cm^-1 corresponds to the C-H stretching vibrations of the CH₃ group.

The very characteristic carbonyl C=O stretching vibrations peaks at 1680 cm^-1.

There are several peaks due to C-O stretching vibrations around 1300 to 1250 and 1150 to 1100 cm^-1 (where the O is directly bonded to the benzene ring as in a phenol).

There is also a C-O stretching vibration at ~1200 cm^-1 for the C-O of the ester linkage

stretching vibration absorptions of the benzene aromatic ring at ~1600 - 1450 cm^-1  

The absence of other specific functional group bands will show that a particular functional group is absent from the methyl 2-hydroxybenzoate molecular structure.

The carbonyl group C=O is clearly distinguished in the spectrum of methyl 2-hydroxybenzoate, but this grouping occurs in many other types of organic compound.