Interpreting the infrared spectrum of methylamine

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 methylamine

email doc brown

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

Interpretation of the infrared spectrum of methylamine

The most prominent infrared absorption lines of methylamine

A broad absorption band from @3600 to 3200 cm-1 due to N-H stretching vibrations - the breadth is caused by hydrogen bonding interactions between methylamine molecules.

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 methylamine molecules (diagram below).

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.

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