Hydroxyl is a reactive molecule composed of one hydrogen and one oxygen atom that can exist as part of larger molecules or as an ionic form. It is essential for life and plays a crucial role in inorganic and organic chemistry. Hydroxyl can form weak hydrogen bonds and is important for the structure of water and DNA. Alkali metal ions attached to hydroxyl form strong chemical bases, which can neutralize acids to form salts and water.
Hydroxyl is the union of two atoms, one hydrogen and one oxygen, which act as a single unit through covalent bonding. It can exist as the neutral part of a larger molecule or it can be held more freely 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 which 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 when dissolved in water, oxygen carries the aforementioned additional electron.
Hydroxides neutralize acids to form compounds called salts. A water molecule 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 with inorganic acid-base reactions, hydroxyl imparts a measure of reactivity to organic compounds, allowing for chemical change. Two molecules of ethanol, the alcohol found in wine and beer, combine by dehydration to produce one molecule of ether, used in surgical procedures in the 19th century. 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 fabrics.
A very special ability of the hydroxyl group is its ability to form weak hydrogen bonds in addition to its covalent bonding with oxygen and its bonding to other atoms through oxygen. In water, these hydrogen bonds form between the hydrogen atom and other nearby atoms. The oxygen atom also participates. A soluble solid crystal, such as silver nitrate, when placed in pure water, will dissolve rapidly in part because the hydrogen atoms pull on the outermost nitrate ions and the oxygen atoms pull on the silver ions.
In no area is hydroxyl more important than that of living organisms. Hydrogen bonds affect the spacing and configuration of surrounding molecules. When it turns into 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 were denser than its liquid form, as is the case with most substances, it would freeze and sink and never again receive the sun’s rays to thaw. The liquid left on top of the ice would repeat the process. Eventually, many pools of water would become solid ice.
Furthermore, the inorganic molecule of water is life-sustaining. Water contains the largest percentage of hydroxyl of any compound. Another chemical that is absolutely essential to life is DNA, sometimes called “the thread of life.” DNA has as the backbone of its structure, long chains derived from hydroxyl-containing sugars and hydroxyl-containing phosphate groups.
These are sugars and the phosphate groups are connected by ester bonds, which also come from hydroxyl groups. DNA determines and contains most of the hereditary traits of plants and animals, including humans. Therefore, a hydroxyl is one of the most important structures found in the laboratory and in nature.
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