Monohydric

Monohydric is an adjective used in chemistry to describe substances that either contain a single hydroxyl group or can donate a single proton in an acid–base reaction. The term is often encountered in discussions of alcohols and acids, and it can refer to two related concepts depending on the context.

In the context of alcohols, monohydric (or monosubstituted) alcohols are those that have one hydroxyl (OH) group per molecule. Classic examples include methanol, ethanol, and propanol. These compounds are contrasted with dihydric or polyhydric alcohols, which contain two or more hydroxyl groups, such as ethylene glycol (two OH groups) and glycerol (three OH groups). The classification helps describe properties related to polarity, solubility, and reactivity.

In acid–base chemistry, monohydric (more precisely monoprotic) acids are capable of donating only one proton per molecule. Examples include hydrogen chloride (HCl), nitric acid (HNO3), and acetic acid (CH3COOH). By contrast, diprotic or polyprotic acids—such as sulfuric acid (H2SO4) and phosphoric acid (H3PO4)—can donate two or more protons. In modern usage, the term monoprotic is more common for describing acids, while monohydric remains a historical or introductory descriptor.

Etymology-wise, monohydric derives from Greek monos meaning one and hydor meaning water, referencing the hydroxyl group. While still encountered in some textbooks and older literature, the term is largely supplanted by more specific terminology such as monohydric versus polyhydric for alcohols, and monoprotic versus polyprotic for acids.