Infra Red Spectrum - 3

Infrared spectrum 3: ethanoic acid, CH₃COOHEthanoic acid is a carboxylic acid. The COOH group contains both an O-H bond and a C=O bond, so its spectrum shows both absorptions.Key absorptions• A very broad absorption from about 2500 to 3300 cm^-1. This is the O-H stretch of the carboxylic acid. It is much broader than the O-H absorption of an alcohol because of strong hydrogen bonding between the acid molecules. It is so broad that it overlaps the C-H absorptions near 2900 to 3000 cm^-1.• A strong, sharp absorption at about 1710 cm^-1 from the C=O bond (carboxylic acids absorb at 1700 to 1725 cm^-1).• A C-O absorption in the fingerprint region, at about 1200 to 1300 cm^-1.Comparing with the last two spectra• Alcohol: O-H absorption (broad, 3200 to 3550 cm^-1) but no C=O.• Ketone: C=O absorption but no O-H.• Carboxylic acid: both, and the O-H absorption is very broad and at lower wavenumber.ConclusionA spectrum with a very broad band from 2500 to 3300 cm^-1 and a strong absorption at about 1700 to 1725 cm^-1 belongs to a carboxylic acid. This is the product expected when ethanol is heated under reflux with excess acidified potassium dichromate(VI).