Hydroxyl radical

Hydroxyl radical
	Stick model of the hydroxyl radical with molecular orbitals
Names
	IUPAC name Hydroxyl radical
	Systematic IUPAC name Oxidanyl (substitutive); Hydridooxygen(•) (additive);
	Other names Hydroxy; Hydroxyl; λ1-Oxidanyl;
Identifiers
CAS Number	3352-57-6;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:29191;
ChemSpider	138477 ;
Gmelin Reference	105
KEGG	C16844 ;
PubChem CID	157350;
CompTox Dashboard (EPA)	DTXSID901318744 ;
	InChI InChI=1S/HO/h1H Key: TUJKJAMUKRIRHC-UHFFFAOYSA-N ;
	SMILES [OH];
Properties
Chemical formula	HO
Molar mass	17.007 g·mol−1
Acidity (pKa)	11.8 to 11.9
Thermochemistry
Std molar; entropy (S⦵298)	183.71 J K−1 mol−1
Std enthalpy of; formation (ΔfH⦵298)	38.99 kJ mol−1
Related compounds
Related compounds	O2H+; OH−; O22−
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Skeletal formulae of 1-hydroxy-2(1H)-pyridinethione and its tautomer

The hydroxyl radical, ^•HO, is the neutral form of the hydroxide ion (HO^–). Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also an important radical formed in radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and stress corrosion cracking in coolant systems subjected to radioactive environments. Hydroxyl radicals are also produced during UV-light dissociation of H₂O₂ (suggested in 1879) and likely in Fenton chemistry, where trace amounts of reduced transition metals catalyze peroxide-mediated oxidations of organic compounds.

In organic synthesis hydroxyl radicals are most commonly generated by photolysis of 1-Hydroxy-2(1H)-pyridinethione.

The hydroxyl radical is often referred to as the "detergent" of the troposphere because it reacts with many pollutants, often acting as the first step to their removal. It also has an important role in eliminating some greenhouse gases like methane and ozone.^[3] The rate of reaction with the hydroxyl radical often determines how long many pollutants last in the atmosphere, if they do not undergo photolysis or are rained out. For instance, methane, which reacts relatively slowly with hydroxyl radicals, has an average lifetime of >5 years and many CFCs have lifetimes of 50+ years. Pollutants, such as larger hydrocarbons, can have very short average lifetimes of less than a few hours.

The first reaction with many volatile organic compounds (VOCs) is the removal of a hydrogen atom, forming water and an alkyl radical (R^•):

^•HO + RH → H₂O + R^•

The alkyl radical will typically react rapidly with oxygen forming a peroxy radical:

R^• + O₂ → RO₂

The fate of this radical in the troposphere is dependent on factors such as the amount of sunlight, pollution in the atmosphere and the nature of the alkyl radical that formed it (See chapters 12 & 13 in External Links "University Lecture notes on Atmospheric chemistry)

^ ^a ^b "Hydroxyl (CHEBI:29191)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute.
^ Perrin, D. D., ed. (1982) [1969]. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). Entry 32. ISBN 0-08-029214-3. LCCN 82-16524.
^ Forster, P.; V. Ramaswamy; P. Artaxo; T. Berntsen; R. Betts; D.W. Fahey; J. Haywood; J. Lean; D.C. Lowe; G. Myhre; J. Nganga; R. Prinn; G. Raga; M. Schulz; R. Van Dorland (2007). "Changes in Atmospheric Constituents and in Radiative Forcing" (PDF). In Solomon, S.; D. Qin; M. Manning; Z. Chen; M. Marquis; K.B. Averyt; M.Tignor; H.L. Miller (eds.). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. The hydroxyl free radical (OH) is the major oxidizing chemical in the atmosphere, destroying about 3.7 Gt of trace gases, including CH4 and all HFCs and HCFCs, each year (Ehhalt, 1999).

[Hydroxyl_(CHEBI:29191)-1] "Hydroxyl (CHEBI:29191)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute.

[P82db-2] Perrin, D. D., ed. (1982) [1969]. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). Entry 32. ISBN 0-08-029214-3. LCCN 82-16524.

[3] Forster, P.; V. Ramaswamy; P. Artaxo; T. Berntsen; R. Betts; D.W. Fahey; J. Haywood; J. Lean; D.C. Lowe; G. Myhre; J. Nganga; R. Prinn; G. Raga; M. Schulz; R. Van Dorland (2007). "Changes in Atmospheric Constituents and in Radiative Forcing" (PDF). In Solomon, S.; D. Qin; M. Manning; Z. Chen; M. Marquis; K.B. Averyt; M.Tignor; H.L. Miller (eds.). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. The hydroxyl free radical (OH) is the major oxidizing chemical in the atmosphere, destroying about 3.7 Gt of trace gases, including CH4 and all HFCs and HCFCs, each year (Ehhalt, 1999).

[1]

[2]

[3]