Advanced Organic Chemistry: Infrared spectrum of dimethylamine

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Interpreting the infrared spectrum of dimethylamine (N-methylmethanamine)

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 dimethylamine

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Infrared spectroscopy - spectra index

infrared spectrum of dimethylamine C2H7N CH3NHCH3 wavenumbers cm-1 functional group detection fingerprint pattern identification of N-methylmethanamine doc brown's advanced organic chemistry revision notes 

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

Dimethylamine, N-methylmethanamine, C2H7N, (c) doc b , (c) doc b , (c) doc b

Interpretation of the infrared spectrum of dimethylamine

The most prominent infrared absorption lines of dimethylamine

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

Secondary aliphatic 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 dimethylamine molecules (diagram below).

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

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

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

There are also characteristic bands due to N-H vibrations at wavenumbers ~1650 to 1580 cm-1, often sharp, so do not confuse with the C=O vibrations from carbonyl compounds.

Absorption due to C-N vibrations, characteristic of C-N bonds in aliphatic amines like dimethylamine occur at 1250 to 1020 cm-1.

There are other N-H vibration absorptions at wavenumbers ~910 to 665 cm-1,

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

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