Explaining and interpreting the infrared spectrum of ethanol (ethyl alcohol)

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.

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 RO^δ--H^δ+ǁǁǁ:O^δ-R in alcohols

The hydrogen bonding interferes with the O-H stretching vibrations and this produces a wider range of O-H stretching vibrations.

Ethanol gives a peak-trough at 3391 cm^-1 for O-H stretching vibrations.

CH₃CH₂-O–H^δ+llll^δ–:O-CH₂CH₃ ... etc.

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 vibrations.

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 ethanol molecular structure.

See also comparing the IR, mass, ¹H NMR and ¹³C NMR spectra of isomers of C₂H₆O below.

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.

Hydrocarbon molecules do not give an absorption band due to the O-H group, as in ethanol.

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.

(b) 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 sample.

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 for the purpose of estimation, so the lower percent transmittance of that wavenumber 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 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.with a much sharper peak than in the liquid film.

You often get sharper peaks in the infrared spectrum of vapours compared to liquid films.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 isomers of C2H6O NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original ethanol (ethyl alcohol) and methoxymethane (dimethyl ether) image sizes.
INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, the most striking difference is the broad O-H stretching band ~3400 cm-1, found in the infrared spectrum of alcohols, but absent in the infrared spectrum of ethers. You can clearly see this (hydrogen bonded) O-H stretching vibration band on the left of the infrared spectrum of ethanol - it overlaps with the C-H stretching vibrations of ethanol ~3000 cm-1. You can see an even sharper division of the OH band from the CH band when you examine the infrared spectrum of ethanol vapour.
MASS SPECTRA: Both ethanol and methoxymethane show some similarities in their mass spectra, but their base ion peaks are quite different - for ethanol it is m/z 31 and for methoxymethane it is m/z 45.
1H NMR SPECTRA: The 1H NMR spectra of ethanol and methoxymethane are quite significantly different. Ethanol gives 3 peaks in the proton ratio 3:2:1 (3 different chemical environments), whereas methoxymethane only gives one 1H chemical shift peak (all 6 protons in the same chemical environment).
13C NMR SPECTRA: The 13C NMR spectra of ethanol and methoxymethane are different. Ethanol gives two 13C resonances, but methoxymethane only one (2 different 13C chemical environments and a 13C single chemical environment).

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