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A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
atom
covalently bonded A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the Coulomb's law, electrostatic force of attraction bet ...
to one
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
atom. In
organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and th ...
,
alcohols In chemistry, alcohol is an organic compound that carries at least one hydroxyl functional group (−OH) bound to a Saturated and unsaturated compounds, saturated carbon atom. The term alcohol originally referred to the primary alcohol ethan ...

alcohols
and
carboxylic acid A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
s contain one or more hydroxy groups. Both the negatively charged
anion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
HO, called
hydroxide Hydroxide is a diatomic anion with chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other sym ...
, and the neutral
radical Radical may refer to: Arts and entertainment Music *Radical (mixtape), ''Radical'' (mixtape), by Odd Future, 2010 *Radical (Smack album), ''Radical'' (Smack album), 1988 *"Radicals", a song by Tyler, The Creator from the 2011 album ''Goblin (album ...
HO•, known as the
hydroxyl radical The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals is produced from the d ...

hydroxyl radical
, consist of an unbonded hydroxyl group. According to
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the authoritative power over chemist ...
definitions, the term hydroxyl refers to the hydroxyl radical (·OH) only, while the functional group −OH is called hydroxy group.


Properties

Water, alcohols, carboxylic acids, and many other hydroxy-containing compounds can be readily
deprotonated
deprotonated
due to a large difference between the electronegativity of oxygen (3.5) and that of hydrogen (2.1). Hydroxy-containing compounds engage in intermolecular
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
ing increasing the electrostatic attraction between molecules and thus to higher boiling and melting points than found for compounds that lack this
functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions re ...
. Organic compounds, which are often poorly soluble in water, become water-soluble when they contain two or more hydroxy groups, as illustrated by
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosacc ...

sugar
s and
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
.


Occurrence

The hydroxy group is pervasive in chemistry and biochemistry. Many inorganic compounds contain hydroxy groups, including
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
, the chemical compound produced on the largest scale industrially. Hydroxy groups participate in the dehydration reactions that link simple biological molecules into long chains. The joining of a
fatty acid In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
to
glycerol Glycerol (; also called glycerine in British English and glycerin in American English) is a simple polyol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
to form a
triacylglycerol 300px, Example of an unsaturated fat triglyceride (C55H98O6). Left part: glycerol; right part, from top to bottom: palmitic acid, oleic acid">palmitic_acid.html" ;"title="glycerol; right part, from top to bottom: palmitic acid">glycerol; right par ...
removes the −OH from the carboxy end of the fatty acid. The joining of two
aldehyde sugars
aldehyde sugars
to form a
disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed wh ...
removes the −OH from the carboxy group at the
aldehyde Chemically, an aldehyde is a compound containing a functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same ...

aldehyde
end of one
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosacc ...

sugar
. The creation of a
peptide bond In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, pro ...

peptide bond
to link two
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s to make a
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
removes the −OH from the carboxy group of one amino acid.


Hydroxyl radical

Hydroxyl radicals are highly reactive and undergo chemical reactions that make them short-lived. When biological systems are exposed to hydroxyl radicals, they can cause damage to cells, including those in humans, where they can react with
DNA Deoxyribonucleic acid (; DNA) is a molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical ...

DNA
,
lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
s, and
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s.


Planetary observations


Airglow of the Earth

The Earth's night sky is illuminated by diffuse light, called
airglow Airglow (also called nightglow) is a faint emission of light by a planetary atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer ...

airglow
, that is produced by radiative transitions of atoms and molecules. Among the most intense such features observed in the Earth's night sky is a group of infrared transitions at wavelengths between 700 nanometers and 900 nanometers. In 1950,
Aden Meinel Aden B. Meinel (November 25, 1922 – October 3, 2011) was an American astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomi ...
showed that these were transitions of the hydroxyl molecule, OH.


Surface of the Moon

In 2009, India's
Chandrayaan-1 Chandrayaan-1 (, ) was the first Indian lunar Lunar most commonly means "of or relating to the Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continen ...

Chandrayaan-1
satellite and the
National Aeronautics and Space Administration (NASA)
National Aeronautics and Space Administration (NASA)
Cassini spacecraftCassini may refer to: People * Cassini (surname) * Oleg Cassini (1913-2006), American fashion designer :Cassini family: * Giovanni Domenico Cassini Giovanni Domenico Cassini, also known as Jean-Dominique Cassini (8 June 1625 – 14 Septem ...

Cassini spacecraft
and Deep Impact probe each detected evidence of water by evidence of hydroxyl fragments on the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
. As reported by Richard Kerr, "A
spectrometer A spectrometer () is a scientific instrument used to separate and measure Spectrum, spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon ...

spectrometer
he Moon Mineralogy Mapper, a.k.a. "M3"detected an infrared absorption at a wavelength of 3.0 micrometers that only water or hydroxyl—a hydrogen and an oxygen bound together—could have created."
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

NASA
also reported in 2009 that the
LCROSS The Lunar Crater Observation and Sensing Satellite (LCROSS) was a robotic spacecraft 250px, An artist's interpretation of the '' MESSENGER'' spacecraft at Mercury A robotic spacecraft is an uncrewed spacecraft, usually under telerobotic cont ...
probe revealed an
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

ultraviolet
emission spectrum consistent with hydroxyl presence. On October 26, 2020,
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

NASA
reported definitive evidence of water on the sunlit surface of the Moon, in the vicinity of the crater
Clavius (crater) Image:Clavius crater 4136 h1.jpg, 240px, Lunar Orbiter 4 view of Clavius Clavius is one of the largest Impact crater, crater formations on the Moon and the second largest crater on the visible near side (very close in size to Deslandres (crater), ...
, obtained by the Stratospheric Observatory for Infrared Astronomy (SOFIA). The SOFIA Faint Object infrared Camera for the SOFIA Telescope (FORCAST) detected emission bands at a wavelength of 6.1 micrometers that are present in water but not in hydroxyl. The abundance of water on the Moon's surface was inferred to be equivalent to the contents of a 12-ounce bottle of water per cubic meter of lunar soil.


Atmosphere of Venus

The
Venus Express ''Venus Express'' (VEX) was the first Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object i ...
orbiter collected
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
science data from April 2006 until December 2014. In 2008, Piccioni, ''et al.'' reported measurements of night-side airglow emission in the atmosphere of Venus made with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on Venus Express. They attributed emission bands in wavelength ranges of 1.40 - 1.49 micrometers and 2.6 - 3.14  micrometers to vibrational transitions of OH. This was the first evidence for OH in the atmosphere of any planet other than Earth's.


Atmosphere of Mars

In 2013, OH near-infrared spectra were observed in the night glow in the polar winter atmosphere of Mars by use of the
Compact Reconnaissance Imaging Spectrometer for Mars The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) is a visible-infrared spectrometer aboard the Mars Reconnaissance Orbiter searching for mineralogic indications of past and present water on Mars Mars is the fourth planet fro ...
(CRISM).


See also

*
Hydronium In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...

Hydronium
*
Ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
*
Oxide An oxide () is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by che ...
*
Hydroxylation In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...


References


Further

*


External links

{{Authority control Alcohols Functional groups Hydroxides