Interpreting the infrared spectrum of ethylamine (ethanamine)

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 ethylamine

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

Interpretation of the infrared spectrum of ethylamine

The most prominent infrared absorption lines of ethylamine

At wavenumbers ~3500 to 3300 cm^-1 is a broad band for N-H bond stretching vibrations,  characteristic of amines.

Primary amines have a highly polar bond (^δ-N-H^δ+) and the intermolecular forces are increased by permanent dipole - permanent dipole attractions including hydrogen bonding (N-H^δ+llll^δ-N-H) between the ethylamine molecules (diagram below).

The hydrogen bonding interferes with the N-H stretching vibrations producing the broad band peaking at around ~3400 cm^-1.

There are also characteristic bands due to N-H vibrations at wavenumbers ~1650 to 1580 cm^-1.

Absorption due to C-N vibrations, characteristic of C-N bonds in aliphatic amines like ethylamine occur at 1220 to 1020 cm^-1.

Around 3000 to 2800 cm^-1 are absorptions due to C-H stretching vibrations - they overlap with the N-H stretching vibrations.

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