Paracetamol is 4-acetamidophenol and may be represented by the following formula:

Paracetamol Structure

In some publications, it is described as 4-hydroxyacetanilide or N-acetyl-p-aminophenol and in the US Pharmacopoeia it is known as acetaminophen.

Paracetamol is a white, odourless crystalline powder with a bitter taste, soluble in 70 parts of water (1 in 20 boiling water), 7 parts of alcohol (95%), 13 parts of acetone, 40 parts of glycerol, 9 parts of propylene glycol, 50 parts of chloroform, or 10 parts of methyl alcohol. It is also soluble in solutions of alkali hydroxides. It is insoluble in benzene and ether. A saturated aqueous solution has a pH of about 6 and is stable (half-life over 20 years) but stability decreases in acid or alkaline conditions, the paracetamol being slowly broken down into acetic acid and p-aminophenol.

Mixtures of paracetamol and aspirin are stable in dry conditions, but tablets containing these two ingredients, particularly in the presence of moisture, magnesium stearate, or codeine, produce some diacetyl-p- aminophenol when stored at room temperature, and this latter compound is hydrolyzed in the presence of moisture to paracetamol and p-aminophenol.

A detailed review (456 references) of the chemical and physical properties of paracetamol together with stability data and analytical methods has been published: Fairbrother J.E. (1974): Acetaminophen. From 'Analytical Profiles of Drug Substances', Vol. 3. Editor: Klaus Florey. Publisher: Academic Press, New York and London, pp1-109