Interpreting the infrared spectrum of propylamine (1-aminopropane)

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 propylamine

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

Interpretation of the infrared spectrum of propylamine

The most prominent infrared absorption lines of propylamine

At ~wavenumbers 3500 to 3300 cm^-1 are 'twin peaks' for N-H bond stretching vibrations, these twin bands are characteristic of primary 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 propylamine molecules (diagram below).

The hydrogen bonding interferes with the N-H stretching vibrations often producing a broad absorption band.

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 occur at 1220 to 1020 cm^-1.

Around 3000 to 2800 are absorptions due to C-H stretching vibrations and tend to overlap with the N-H stretching vibration absorptions.

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