Infra-red Spectroscopy

In the data sheet that is provided in the exam the following absorptions are present to indicate Alcohols, aldehydes, ketones, carboxylic acids and esters.

Note: The datasheet also provides information on absorption for other bonds O–H free and N–H, however these won’t be in the chains and rings exam so just ignore them!

Alcohols, esters

Aldehydes, ketones, carboxylic acids, esters

Hydrogen bonded in carboxylic acids

Hydrogen bonded in alcohols, phenols

Some points of note
An absorption or peak is actually a ‘trough’ in the graph.
The scale at the bottom of this axis is ‘back to front’, it starts at a larger value and moves towards a smaller number.

Infrared spectrum of a carboxylic acid



Notice the large absorption (or peak) between 2500-3300 and the absorption between 1680-1750 which indicates the presence of O-H and C=O bonds.

Infra-red Spectroscopy is used to identify polar covalent bonds – most importantly the functional groups of the organic molecules. Often however questions may ask for chemical methods of identification. For these long answer questions infra-red spectroscopy is not a suitable method of identification unless it is specifically asked for in the question.