Infra Red Spectrum - 1

Infrared spectrum 1: ethanol, CH₃CH₂OHWhen reading a spectrum, start with the region above 1500 cm^-1, where the absorptions identify functional groups. Then check the fingerprint region only if you need to compare with a known spectrum.Key absorptions• A strong, broad absorption at about 3200 to 3550 cm^-1. This is the O-H stretch of an alcohol. It is broad because the O-H groups in the sample are hydrogen bonded to each other, and the hydrogen bonds vary in strength.• Absorptions just below 3000 cm^-1 (about 2850 to 2980 cm^-1), caused by C-H bonds. Almost every organic compound has these, so they are not useful on their own.• A strong absorption at about 1050 cm^-1, caused by the C-O bond. This is in the fingerprint region.What is missing matters tooThere is no strong, sharp absorption at about 1700 to 1750 cm^-1, so ethanol has no C=O bond. This rules out aldehydes, ketones and carboxylic acids.ConclusionA broad absorption near 3350 cm^-1 together with no absorption near 1700 cm^-1 shows that the compound is an alcohol. The spectrum on its own does not say which alcohol it is; for that, the molecular formula, a mass spectrum or a comparison of the fingerprint region is needed.