In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom Chemical bond to one hydrogen atom. In organic chemistry, alcohols and contain one or more hydroxy groups. Both the negatively charged anion , called hydroxide, and the neutral radical , known as the hydroxyl radical, consist of an unbonded hydroxy group.
According to IUPAC definitions, the term hydroxyl refers to the hydroxyl radical () only, while the functional group is called a hydroxy group.
Properties
Water, alcohols,
carboxylic acids, and many other hydroxy-containing compounds can be readily
Deprotonation due to a large difference between the electronegativity of oxygen (3.5) and that of hydrogen (2.1). Hydroxy-containing compounds engage in intermolecular
increasing the electrostatic attraction between molecules and thus to higher boiling and melting points 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 hydroxyl groups, as illustrated by
and
amino acid.
Occurrence
The hydroxy group is pervasive in chemistry and biochemistry. Many inorganic compounds contain hydroxyl 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 Aldose 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 to make a 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,
, and
.
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.
[
] 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.
Surface of the Moon
In 2009, India's Chandrayaan-1 satellite and the
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 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."
also reported in 2009 that the
LCROSS probe revealed an
ultraviolet emission spectrum consistent with hydroxyl presence.
On 26 October 2020, NASA reported definitive evidence of water on the sunlit surface of the Moon, in the vicinity of the crater Clavius, obtained by the Stratospheric Observatory for Infrared Astronomy (SOFIA).
The Chang'e 5 probe, which landed on the Moon on 1 December 2020, carried a mineralogical spectrometer that could measure infrared reflectance spectra of lunar rock and regolith. The reflectance spectrum of a rock sample at a wavelength of 2.85 micrometers indicated localized water/hydroxyl concentrations as high as 180 parts per million.
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.
This was the first evidence for OH in the atmosphere of any planet other than Earth.
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).
[
]
Exoplanets
In 2021, evidence for OH in the dayside atmosphere of the exoplanet WASP-33b was found in its emission spectrum at wavelengths between 1 and 2 micrometers.
Evidence for OH in the atmosphere of exoplanet WASP-76b was subsequently found.
Both WASP-33b and WASP-76b are ultra-hot Jupiters and it is likely that any water in their atmospheres is present as dissociated ions.
See also
Further
External links
