Hydroxy group

In chemistry, a hydroxy or hydroxyl group is a special part of molecules with the chemical formula −OH. It is made of one oxygen atom connected to one hydrogen atom. This group is very important in many kinds of chemicals.

Many common compounds, like alcohols and carboxylic acids, have one or more hydroxy groups in their structure. These groups help give the molecules their special properties and uses.

There are also other forms related to the hydroxy group. The negatively charged piece called hydroxide has the formula HO⁻. There is also a neutral piece called the hydroxyl radical, which is very reactive and important in some chemical reactions.

The way scientists talk about these can be a little different. According to IUPAC rules, the word hydroxyl really only means the hydroxyl radical (·OH), while the −OH group in molecules is called a hydroxy group.

Properties

Water, alcohols, carboxylic acids, and many other compounds with hydroxy groups can easily lose a hydrogen ion because oxygen is much more electronegative than hydrogen. These compounds also form special bonds called hydrogen bonds with each other, which makes them stick together more strongly. This results in higher boiling and melting points compared to compounds without hydroxy groups. When organic compounds have two or more hydroxy groups, they often become soluble in water, like sugars and amino acid.

Occurrence

The hydroxy group is very common in chemistry and biology. Many important inorganic compounds, like sulfuric acid, contain hydroxy groups.

Hydroxy groups are also important in building larger molecules from smaller ones. For example, when a fatty acid joins with glycerol to form a triacylglycerol, a hydroxy group is removed. Similarly, when two aldehyde sugars combine to make a disaccharide, a hydroxy group is also removed. The same happens when two amino acids link together to create a protein through a peptide bond.

Hydroxyl radical

Main article: Hydroxyl radical

Hydroxyl radicals are very reactive and don’t stay around for long because they quickly react with other substances. In living things, these radicals can sometimes harm cells. They may interact with important parts of cells like DNA, lipids, and proteins, which can cause problems for the cells.

Planetary observations

The Earth's night sky glows with a soft light called airglow, created by atoms and molecules. In 1950, scientist Aden Meinel discovered that this glow includes light from the hydroxyl molecule, OH.

Space probes have found signs of water and hydroxyl on the Moon. In 2009, India's Chandrayaan-1 and NASA's Cassini spacecraft and Deep Impact probe detected hydroxyl fragments there. In 2020, NASA confirmed water on the Moon's surface using the Stratospheric Observatory for Infrared Astronomy (SOFIA). China's Chang'e 5 probe also found traces of water in lunar soil.

The Venus Express spacecraft discovered hydroxyl in Venus's atmosphere in 2008. In 2013, hydroxyl was observed in the atmosphere of Mars. In 2021, scientists found hydroxyl in the atmospheres of distant planets called WASP-33b and WASP-76b, which are very hot planets known as ultra-hot Jupiters.