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Advanced Organic Chemistry: Infrared spectrum of butan-2-ol  (2-butanol)

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Interpreting the infrared spectrum of butan-2-ol  (2-butanol, sec-butyl 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 butan-2-ol

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Infrared spectroscopy - spectra index

C4H10O CH3CH(OH)CH2CH3 infrared spectrum of butan-2-ol liquid film wavenumbers cm-1 functional group detection fingerprint pattern identification of 2-butanol sec-butyl alcohol doc brown's advanced organic chemistry revision notes 

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

Butan-2-ol    C4H10O    alcohols and ether structure and naming (c) doc b    alcohols and ether structure and naming (c) doc b    alcohols and ether structure and naming (c) doc b 

Interpretation of the infrared spectrum of liquid film butan-2-ol

The most prominent infrared absorption lines of butan-2-ol

The most characteristic absorption bands are the broad O-H stretching vibration bands at wavenumbers ~3550 to 3230 cm-1, the breadth is caused by interference of hydrogen bonding interactions, common to all hydrogen bonded molecules with a hydroxyl group.

infrared spectrum of ethanol diagram of intermolecular hydrogen bonding forces between liquid alcohol molecules doc brown A level organic chemistry revision notes R-OHδ+llllδ:O-R ... etc.

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

There are C-O stretching vibration absorption bands at wavenumbers ~1350 to 1030 cm-1.

C-H stretching vibration absorptions appear at wavenumbers 1470 to 1370 cm-1.

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

Extra comment on the infrared spectrum of butan-2-ol vapour

infrared spectrum of butan-2-ol vapour vapor gaseous phase

In the vapour state there is no hydrogen bonding between the butan-2-ol molecules so the broad band at wavenumbers ~3550 to 3230 cm-1 (for H bonded molecules) doesn't show up.

Instead, you get a sharper narrower band at wavenumbers 3670 to 3580 cm-1, characteristic of a non-hydrogen bonded hydroxyl molecule.

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