What is a functional group? List some examples.

The hydroxyl group is a fundamental functional group in organic chemistry and is denoted by the chemical symbol (-OH). It consists of an oxygen atom bonded to a hydrogen atom, and this pairing is attached to a carbon skeleton of the organic molecule.

The presence of a hydroxyl group within a molecule typically makes the molecule an alcohol. Alcohols are known for their solubility in water due to the hydroxyl's ability to form hydrogen bonds with water molecules. This property is significant in the metabolism and transport of substances within living organisms. Examples of hydroxyl-containing compounds include ethanol, found in alcoholic beverages, and glycerol, a component of lipids.

The carboxyl group is written as (-COOH) and is composed of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group. This arrangement forms part of larger molecules known as carboxylic acids.

Carboxylic acids, such as acetic acid found in vinegar, are well known for their sharp, sour taste and are widely used in food products as preservatives. The acidic nature of the carboxyl group is due to the tendency of the -OH part to donate a proton (H+) in solution, making it a site of chemical reactivity.

The amino group, with the chemical structure (-NH2), consists of a nitrogen atom attached to two hydrogen atoms and to the carbon skeleton. It is an essential part of amino acids, which are the building blocks of proteins.

Amino groups act as bases, meaning they can pick up a proton from the surrounding environment, which makes them important in maintaining the pH balance within biological systems. An example of a molecule containing an amino group is the amino acid lysine; it plays a pivotal role in protein synthesis.

A carbonyl group (>C=O) consists of a carbon atom double-bonded to an oxygen atom. The simplicity of this group belies its importance; it forms the core of several organic compounds such as aldehydes and ketones.

The reactivity of the carbonyl group lies in the partial positive charge of the carbon atom, which makes it susceptible to attack by nucleophiles. This property is central to many reactions in both laboratory and biological settings. A familiar substance that contains a carbonyl group is acetone, commonly used as a solvent and nail polish remover.

The phosphate group, shown as (-PO4), is a phosphorus atom bonded to four oxygen atoms. It is a key component in many biochemical molecules such as adenosine triphosphate (ATP), which is the primary energy carrier in cells.

The high-energy bonds between the phosphorus and oxygen atoms are the source of energy for many biochemical reactions. The group's ability to link with other molecules through these bonds, forming longer chains or rings, is fundamental to the structure and function of DNA and RNA, as well as cellulose in plants and energy storage in animals.