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Interpreting the infrared
spectrum of ethanol (ethyl
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
Spectra obtained from a liquid film of ethanol. The right-hand part of the of the
infrared spectrum of ethanol, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of ethanol and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of ethanol.
Ethanol C2H6O,
,
,
,
Revision notes on the structure and naming
(nomenclature) of aliphatic ALCOHOLS and ETHERS
Interpretation of
the infrared spectrum of ethanol
The most prominent infrared absorption lines of ethanol
The most distinct feature in the infrared spectrum
of alcohols is the broad absorption band centred around wavenumbers
3400 to 3230 cm-1 due to O-H stretching vibrations, but
broadened by intermolecular hydrogen bonding (diagrams below).
The intermolecular hydrogen
bond ROδ--Hδ+ǁǁǁ:Oδ-R
in alcohols
The hydrogen bonding interferes with the O-H
stretching vibrations.
CH3CH2-O–Hδ+llllδ–:O-CH2CH3
... etc.
C-H stretching vibration absorptions are observed
for ethanol at wavenumbers ~3010 to 2850 cm-1
C-H bending vibration absorptions at ~1040 cm-1.
The C-O stretching band for primary alcohols is
~1050 to 1075 1000 cm-1.
The O-H bending deformation band for primary
alcohols is ~1350 to 1260 cm-1.
The absence of other specific functional group bands
will show that particular functional group is absent from the
ethanol
molecular
structure.
See also
comparing the IR, mass, 1H
NMR and 13C NMR
spectra of isomers of C2H6O
below.
Extra note on the
infrared spectrum of ethanol vapour
The principal difference is the position of the O-H
stretching vibration.
In liquid films of alcohols like ethanol, the O-H is
hydrogen bonded with other ethanol molecules and the O-H stretching
vibrations occur wavenumbers at 3500 to 3200 cm-1.
However, in ethanol vapour, the molecules are free and the
O-H is NOT hydrogen bonded with other ethanol molecules and the O-H
stretching vibrations occur at higher wavenumbers at 3670 to 3580 cm-1.
You often get sharper peaks in the infrared spectrum of
vapours compared to liquid films, but to be honest that doesn't seem to be
the case with the two spectra I obtained from the internet.
See also comparing the IR, mass,
1H NMR and
13C NMR spectra of
isomers of C2H6O
below.
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Key words & phrases: image and diagram of the
infrared spectrum of ethanol (ethyl alcohol), complete infrared absorption
spectrum of ethanol, comparative spectra of ethanol, prominent peaks/troughs for
identifying functional groups in the infrared spectrum of ethanol, important
wavenumber values in cm-1 for peaks/troughs in the infrared spectrum of ethanol,
revision of infrared spectroscopy of ethanol, fingerprint region analysis of
ethanol, how to identify ethanol from its infrared spectrum, identifying organic
compounds like ethanol from their infrared spectrum, how to analyse the
absorption bands in the infrared spectrum of ethanol C2H6O CH3CH2OH C2H5OH
Associated links
The chemistry of ALCOHOLS
revision notes INDEX
Infrared spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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