Representation of an organic compound organic hydroxy group, where R represents a hydrocarbon or other organic moiety, the red and grey spheres represent oxygen and hydrogen atoms respectively, and the rod-like connections between these, covalent chemical bonds.

A hydroxy or hydroxyl group is the entity with the formula OH. It contains oxygen bonded to hydrogen. In organic chemistry, alcohols and carboxylic acids contain hydroxy groups. Both the negatively charged anion OH^-, called hydroxide, and the neutral radical ·OH, known as the hydroxyl radical, consist of an unbounded hydroxyl group.

According to IUPAC rules, the term hydroxyl refers to the hydroxyl radical (·OH) only, while the functional group -OH is called hydroxy group.^[1]

Properties

Sulfuric acid contains two hydroxyl groups.

Water, alcohols, carboxylic acids, and many other hydroxy-containing compounds can be deprotonated readily. This behavior is rationalized by the disparate electronegativities of oxygen and hydrogen. Hydroxyl-containing compounds engage in hydrogen bonding, which causes them to stick together, leading to higher boiling and melting than found for compounds that lack this functional group. Organic compounds, which are often poorly soluble in water, become water-soluble when they contain two or more hydroxy groups, as illustrated by sugars and amino acid.

Occurrence

The hydroxy group is pervasive in chemistry and biochemistry. Many inorganic compounds contain hydroxy groups, including 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 to glycerol to form a triacylglycerol removes the -OH from the carboxy end of the fatty acid. The joining of two aldehyde sugars to form a disaccharide removes the -OH from the carboxy group at the aldehyde end of one sugar. The creation of a peptide bond to link two amino acids to make a protein removes the -OH from the carboxy group of one amino acid. used for.

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, lipids, and proteins.

Planetary observations

Airglow of the Earth

The Earth's night sky is illuminated by diffuse light, called airglow, that is produced by radiative transitions of atoms and molecules.^[2] 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 showed that these were transitions of the hydroxyl molecule, OH.^[3]

Surface of the Moon

In 2009, India's Chandrayaan-1 satellite and the National Aeronautics and Space Administration (NASA) Cassini spacecraft and Deep Impact probe each detected evidence of water by evidence of hydroxyl fragments on the Moon. As reported by Richard Kerr, "A spectrometer [the 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."^[4]NASA also reported in 2009 that the LCROSS probe revealed an ultraviolet emission spectrum consistent with hydroxyl presence.^[5]

On October 26, 2020, NASA reported definitive evidence of water on the sunlit surface of the Moon, in the vicinity of the crater Clavius (crater), obtained by the Stratospheric Observatory for Infrared Astronomy (SOFIA).^[6] 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. ^[7]

Atmosphere of Venus

The Venus Express orbiter collected 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.^[8] 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 (CRISM).^[9]

See also

• Hydronium
• Ion
• Oxide
• Hydroxylation

References

External links

• Reece, Jane; Urry, Lisa; Cain, Michael; Wasserman, Steven; Minorsky, Peter; Jackson, Robert (2011). "Unit 1, Chapter 4 &5." In Campbell Biology (9th ed.). Berge, Susan; Golden, Brandy; Triglia, Logan (eds.). San Francisco: Pearson Benjamin Cummings. ISBN 978-0-321-55823-7