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Advanced Organic Chemistry: Infrared spectrum of methylamine

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

infrared spectrum of methylamine CH5N CH3NH2 wavenumbers cm-1 functional group detection fingerprint pattern identification of methylamine doc brown's advanced organic chemistry revision notes 

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.

Methylamine  CH5N   (c) doc b   (c) doc b   (c) doc b   (c) doc b

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-Hδ+) and the intermolecular forces are increased by permanent dipole - permanent dipole attractions including hydrogen bonding (N-Hδ+llllδ-N-H) between the methylamine molecules (diagram below).

hydrogen bonding in aliphatic amines stronger intermolecular force bonds bonding higher boiling points compared to alkanes with no hydrogen bonding

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.


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