hydroxyl group

hydroxyl group (―OH), in chemistry, a functional group with one hydrogen and one oxygen atom. An oxygen atom normally forms two σ bonds with other atoms; the water molecule, H₂O, is the simplest and most common example. If one hydrogen atom is removed from a water molecule, a hydroxyl functional group (―OH) is generated.

Hydroxyl groups play a key role in the structure of many molecules. For example, alcohols are a class of organic compounds characterized by one or more hydroxyl groups attached to a carbon atom of an alkyl group (hydrocarbon chain). Water and alcohols have similar properties because water molecules contain hydroxyl groups that can form hydrogen bonds with other water molecules and with alcohol molecules, and likewise alcohol molecules can form hydrogen bonds with other alcohol molecules as well as with water. Because alcohols form hydrogen bonds with water, they tend to be relatively soluble in water. The hydroxyl group is referred to as a hydrophilic (“water-loving”) group, because it forms hydrogen bonds with water and enhances the solubility of an alcohol in water. Methanol, ethanol, n-propyl alcohol, isopropyl alcohol, and t-butyl alcohol are all miscible with water. Alcohols with higher molecular weights tend to be less water-soluble, because the hydrocarbon part of the molecule, which is hydrophobic (“water-hating”), is larger with increased molecular weight. Because they are strongly polar, alcohols are better solvents than hydrocarbons for ionic compounds and other polar substances.

When the hydroxyl group is joined to an aryl ring, a phenol results. Both alcohols and phenols are widespread in nature, with alcohols being especially ubiquitous. The hydroxyl group of alcohols and phenols is responsible for an interesting variety of physical and chemical properties. The biochemical action of vitamin E, for example, depends largely on the reactivity of the phenol functional group.

An oxygen atom is much more electronegative than carbon or hydrogen atoms are, so both carbon-oxygen and hydrogen-oxygen bonds are polar. The oxygen atom is slightly negatively charged, and the carbon and hydrogen atoms are slightly positively charged. The polar bonds of the hydroxyl group are responsible for the major reaction characteristics of alcohols and phenols. In general, these reactions are initiated by reaction of electron-deficient groups with the negatively charged oxygen atom or by reaction of electron-rich groups with the positively charged atoms—namely, carbon or hydrogen—bonded to oxygen.