Hydroxyl is the joining of two atoms, one of hydrogen and one of oxygen, acting as a single unit through covalent bonding. It can exist either as a neutral part of a larger molecule or can be more loosely held in ionic form, carrying an extra electron located on its oxygen. Both varieties of hydroxyl are quite reactive and useful to the chemist. Hydroxyl is absolutely essential for life.
Alkali metal ions that include sodium or potassium form strong chemical bases when attached to hydroxyl ions. Some examples of these are sodium hydroxide and potassium hydroxide. The name change to hydroxide reflects the fact that upon dissolving in water, the oxygen carries the additional electron mentioned above.
Hydroxides neutralize acids to form compounds called salts. A molecule of water is also formed. Thus, sodium hydroxide reacts with hydrochloric acid to form ordinary table salt plus water.
In organic chemistry, the hydroxyl group is part of the structures of alcohols, sugars and phenols. As in the case of inorganic acid-base reactions, hydroxyl imparts a measure of reactivity to organic compounds, enabling chemical change. Two molecules of ethanol, the alcohol found in wine and beer, combine by dehydration to produce one molecule of ether, which was used in 19th-century surgical procedures. Organic compounds containing two hydroxyl groups per molecule react with those containing two acid groups to form polyester residues that are used in bottles, tires, clothing and textiles.
One very special capacity of the hydroxyl group is its ability to form weak hydrogen bonds in addition to its covalent bond with oxygen and its bond to other atoms through oxygen. In water, these hydrogen bonds form between the hydrogen atom and other nearby atoms. The oxygen atom also takes part. A crystal of soluble solid, such as silver nitrate, if placed in pure water, will dissolve quickly in part because the hydrogen atoms tug at the outermost nitrate ions, and the oxygen atoms tug at the silver ions.
In no area is hydroxyl more important than that of living organisms. Hydrogen bonds influence the distance and configuration of surrounding molecules. Upon turning to ice, water adopts a less-dense structure than that of liquid water. This means that ice is lighter than water, so it floats on water.
If frozen water was more dense than its liquid form, as is the case for most substances, it would freeze and sink and never receive the sun’s rays to thaw again. The liquid that remained above the ice would repeat the process. Eventually, many pools of water would become solid ice.
Additionally, the inorganic molecule water is life sustaining. Water contains the greatest percentage of hydroxyl of any compound. Another chemical absolutely essential to life is DNA, sometimes called “the thread of life.” DNA has as the backbone of its structure, lengthy chains derived from hydroxyl-containing sugars and hydroxyl-containing phosphate groups.
These are sugars, and phosphate groups are connected by ester linkages, which also come from hydroxyl groups. DNA determines and contains most of the hereditary traits of plants and animals, including humans. Thus, a hydroxyl is one of the most important structures found in the laboratory and in nature.