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

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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.

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

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.

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).