Interpreting the infrared spectrum of 2-methylpropan-1-ol (isobutyl alcohol)

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 2-methylpropan-1-ol

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Spectra obtained from a liquid film of 2-methylpropan-1-ol. The right-hand part of the of the infrared spectrum of 2-methylpropan-1-ol, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of 2-methylpropan-1-ol and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of 2-methylpropan-1-ol.

Interpretation of the infrared spectrum of 2-methylpropan-1-ol

The most prominent infrared absorption lines of 2-methylpropan-1-ol

The most characteristic absorption is the broad O-H stretching vibration band at wavenumbers ~3500 to 3230 cm^-1, the breadth is caused by hydrogen bonding interactions, common to all hydrogen bonded molecules with a hydroxyl group e.g. alcohols and carboxylic acids.

R-O–H^δ+llll^δ–:O-R ... etc.

C-H stretching vibration absorption occurs ~2900 cm^-1, wavenumbers common to any molecule with alkyl groups.

C-C-C skeletal vibrations show absorptions at wavenumbers ~1175 to 1140 cm^-1 and 840 to 790 cm^-1 for a -C(CH₃)₂ grouping.

There are C-O stretching vibration and C-H deformation vibration absorption bands of wavenumbers ~1350 to 1030 cm^-1, common to aliphatic alcohols

The absence of other specific functional group bands will show that a particular functional group is absent from the 2-methylpropan-1-ol molecular structure.