Cetirizine hydrochloride is an antihistamine known as a major metabolite of hydroxyzine. It is used in the creation of many allergy-related medications for conditions including allergies, hay fever, angioedema and urticaria. It is usually available over the counter in most countries. In the United States, it is the main active ingredient in Zyrtec.
Cetirizine hydrochloride does not occur naturally, but is synthesised using a chemical process involving four primary steps. In the first step, 4'chloro diphenyl methyl piperazine is isolated and combined with a base solution that allows the methyl-based compound to react with other compounds more easily. 2-chloroethoxy ethyl acetate, a more simple compound made of chlorine and oxygen segments connected to the ethyl group, is then added to the base solution and reacts with the diphenyl methyl piperazine to form the third compound, an ester that is made of the two linked compounds bonded by a nitrogen connection.
This new compound is then hydrolyzed. Hydrolyzing should not be confused with hydration. It involves the breaking up of water molecules into different parts. The chemical components, hydrogen and oxygen in a 2-to-1 ratio, are divided and separated into simple hydrogen atoms and a few simple molecules known as hydroxide anions. In the case of cetirizine hydrochloride, like many other synthetic chemicals, hydrolysis is performed by introducing water, not dealing with water already present in the chemical. The water molecules immediately split on encountering the compound, both breaking up the water and changing the compound. This is commonly done to esters; the water molecule attacks the carbon atoms and tends to create a compound in the carboxylic acid group.
The end result is the basic form of cetirizine, which is then converted to a hydrochloride version by adding 2HCl. The final compound's chemical formula is C21H25ClN2O3•2HCl, and it resembles a chain of the original compounds now bonded to each other in a long chain. The chemical is manufactured as a white, crystalline powder that is water-soluble and can be placed in capsules for oral administration, or contained in a syrup for more immediate effect.