Interpreting the infrared
spectrum of ethanol (ethyl
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
honors organic chemistry
courses involving molecular spectroscopy analysing infrared spectra
mobile phone in 'landscape' orientation?
Infrared spectroscopy - spectra index
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.
Revision notes on the structure and naming
(nomenclature) of aliphatic ALCOHOLS and ETHERS
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
The hydrogen bonding interferes with the O-H
Ethanol gives a peak-trough at 3391 cm-1
for O-H stretching vibrations.
C-H stretching vibration absorptions are observed
for ethanol at wavenumbers ~3010 to 2850 cm-1. Ethanol
gives a peak-trough of 2981 cm-1 for C-H stretching
The C-O stretching band for primary alcohols is
~1050 to 1075 cm-1.
Ethanol gives a trough-peak at 1055 cm-1
for the C-O stretching vibrations.
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 groups are absent from the
comparing the IR, mass, 1H
NMR and 13C NMR
spectra of isomers of C2H6O
Extra note 1. The
uses of the infrared spectrum of ethanol.
(a) Determining the
ethanol concentration in petrol (gasoline)
Ethanol is now a common additive to petrol - anywhere from 0.2% to 22%,
and gives the fuel cleaner burning properties.
Ethanol can be directly added to the hydrocarbon molecules in petrol e.g.
blending 'normal' petrol with bioethanol.
An infrared analytical technique can be used in the quality control
of fuels for road vehicles.
The infrared spectrum of the fuel is analysed using an infrared
spectrometer and the relative absorption of peaks unique to ethanol
in the mixture can me used to monitor the ethanol concentration in the fuel
- this can be done in real-time as the petrol blend is manufactured.
Determining the concentration of ethanol vapour in a breathalyser test
Infrared spectrometry is an analytical technique that can be applied to
monitor-measure the alcohol vapour content in human breath - a breathalyser
test with an infrared breath-alcohol analyzer.
The instrument involved, measures the absorption of selected wavelengths
of infrared radiation after passage through a known volume of the breath
The instrument essentially behaves as an infrared spectrometer with a
very accurate and selective infrared optical band filter system because both
ethanol and hydrocarbons like alkanes, both absorb infrared strongly due to
C-H stretching vibrations. O-H stretching wavenumbers would not be used
because traces of water would interfere with the analysis. Some analysers
also use the C-O stretching vibration absorption since C-O bands will be
absent in hydrocarbon spectra.
Quite 'simply', the higher the concentration of ethanol vapour in the
sample the more infrared energy is absorbed at certain wavenumber unique to
ethanol, and the lower percent transmittance gives the concentration of
ethanol in the breath of the person being tested.
Extra note 2. The
infrared spectrum of ethanol vapour
The principal difference is the position of the O-H
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
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
explaining the infrared spectrum of ethanol and complications due to hydrogen
bonding Explanatory diagram of the infrared spectrum of the ethanol molecule. Listing wavenumber data of the prominent main peaks troughs in the infrared spectrum of ethanol. How to explain the infrared spectrum of ethanol. Use of the infrared spectrum of ethanol, identification of ethanol from its infrared spectrum - wavenumber fingerprint pattern to identify the ethanol molecule. The uses of the infrared spectrum of the ethanol molecule explaining the peaks-trough of the transmittance of the infrared spectrum of ethanol
The mass spectrum of Ethanol (ethyl alcohol)
The H-1 NMR spectrum of
Ethanol (ethyl alcohol)
The C-13 NMR spectrum Ethanol (ethyl alcohol)
The chemistry of ALCOHOLS
revision notes INDEX
Infrared spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
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