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Hydroxy group
Hydroxy group
Hydroxy group
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Representation of an organic compound 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.

In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula −OH and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy groups. Both the negatively charged anion HO−, called hydroxide, and the neutral radical HO·, known as the hydroxyl radical, consist of an unbonded hydroxy group.

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

Properties

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Sulfuric acid contains two hydroxyl groups.

Water, alcohols, carboxylic acids, and many other hydroxy-containing compounds can be readily 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 bonding 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 sugars and amino acid.[2][3]

Occurrence

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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.[4]

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.[citation needed]

Hydroxyl radical

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Main article: 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.[5]

Planetary observations

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Airglow of the Earth

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The Earth's night sky is illuminated by diffuse light, called airglow, that is produced by radiative transitions of atoms and molecules.[6] 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.[7]

Surface of the Moon

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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, also known as "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."[8] NASA also reported in 2009 that the LCROSS probe revealed an ultraviolet emission spectrum consistent with hydroxyl presence.[9]

On 26 October 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).[10] 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.[11]

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.[12]

Atmosphere of Venus

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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.[13] This was the first evidence for OH in the atmosphere of any planet other than Earth.[13]

Atmosphere of Mars

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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).[14]

Exoplanets

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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.[15] Evidence for OH in the atmosphere of exoplanet WASP-76b was subsequently found.[16] 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

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Wikimedia Commons has media related to Hydroxyl group.

References

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Further

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  • Reece J, Urry L, Cain M, Wasserman S, Minorsky P, Jackson R (2011). "Chapter 4&5". In Berge S, Golden B, Triglia L (eds.). Campbell Biology. Vol. Unit 1 (9th ed.). San Francisco: Pearson Benjamin Cummings. ISBN 978-0-321-55823-7.
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Hydroxy group

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The hydroxy group, also known as the hydroxyl group and denoted as -OH, is a fundamental in consisting of an oxygen atom covalently bonded to a and attached to a carbon atom within an organic . It serves as the defining characteristic of alcohols, where it is represented by the general formula ROH, with R denoting an alkyl or . This group imparts distinct chemical and physical properties to the molecules it is part of, distinguishing them from hydrocarbons of similar size. The hydroxy group's polarity arises from the significant difference between oxygen and , making the O-H bond highly polar and capable of forming bonds as both a donor (via the ) and an acceptor (via the oxygen's lone pairs). These hydrogen-bonding capabilities lead to elevated boiling and melting points for hydroxy-containing compounds compared to nonpolar hydrocarbons of equivalent molecular weight, as well as enhanced in . In terms of reactivity, the hydroxy group can undergo to form alkoxides under basic conditions, participate in oxidation reactions to yield carbonyl compounds, and serve as a site for esterification or formation, influencing the synthesis and behavior of organic materials. Hydroxy groups are prevalent in biological systems, appearing in carbohydrates (such as glucose with multiple -OH groups enabling glycosidic linkages), amino acids like serine and threonine (where the side-chain hydroxy facilitates phosphorylation), and lipids including steroids. Their hydrogen-bonding prowess is vital for maintaining protein secondary structures, stabilizing DNA base pairs indirectly through water interactions, and enabling enzymatic reactions. In medicinal chemistry, hydroxy groups contribute to drug efficacy by enhancing solubility, binding affinity to targets, and metabolic stability; notably, they are present in approximately 40% of approved drugs and 65% of known natural products. Nomenclature for compounds bearing this group prioritizes it highly, using the suffix -ol for alcohols (e.g., ethanol) or the prefix hydroxy- when another higher-priority functional group is present.

Definition and Fundamentals

Chemical Definition

The hydroxy group, denoted as −OH, is a composed of an oxygen atom covalently bonded to a . In , it is typically attached to a carbon atom, forming alcohols with the general structure R−OH, where R represents an alkyl or . This attachment imparts specific reactivity to the due to the polar of the O−H bond. In , the hydroxy group can bond to metals or nonmetals, as in compounds like aluminum hydroxide (Al(OH)3) or (HOCl), where it functions as a or part of an oxoacid structure. The hydroxy group differs fundamentally from related species such as the hydroxide ion (OH−) and the hydroxyl radical (OH•). The hydroxide ion is an anionic species in which the oxygen atom is bonded to hydrogen and carries a formal negative charge, with eight valence electrons around oxygen including three lone pairs; it typically exists in ionic compounds or aqueous solutions. In contrast, the hydroxyl radical is a neutral, highly reactive species featuring an unpaired electron on the oxygen atom, resulting in seven valence electrons around oxygen and only two lone pairs, making it a free radical rather than a stable functional group. The hydroxy group itself involves covalent bonding where oxygen shares electrons with both the attached atom (e.g., carbon) and hydrogen, achieving a stable octet without charge or unpaired electrons. The recognition of the hydroxy group in organic compounds traces back to the late , when conducted combustion analyses that revealed the elemental composition of alcohols like (spirit of wine), identifying it as containing , , and in a specific ratio consistent with the R−OH structure. These experiments laid the groundwork for understanding functional groups in by shifting focus from qualitative observations to quantitative .

Nomenclature

In organic chemistry, the hydroxy group (-OH) in alcohols is named using substitutive nomenclature according to IUPAC recommendations, where the parent hydride chain is selected and modified by replacing the ending "e" with the suffix "-ol" to indicate the principal characteristic group. The position of the hydroxy group is specified by a locant, with the chain numbered to give the lowest possible number to the carbon atom bearing the -OH. For example, CH₃CH₂OH is named ethanol, reflecting its two-carbon parent chain with the hydroxy group at position 1. In cases of multiple hydroxy groups, the suffix becomes "-diol," "-triol," etc., with locants for each, such as 1,2-ethanediol for HOCH₂CH₂OH. When the hydroxy group is not the principal function, it is expressed as the prefix "hydroxy-." In inorganic chemistry, nomenclature distinguishes between ionic compounds containing hydroxide ions (OH−) and coordination compounds with covalent hydroxy ligands. Simple ionic metal hydroxides, such as NaOH, are named by combining the cation name with "hydroxide," yielding sodium hydroxide. For coordination compounds, the covalent hydroxy ligand is named "hydroxido," listed alphabetically before the central atom, which includes its oxidation state in parentheses; for instance, [Fe(OH)(H2O)5]²⁺ is pentaaquahydroxidoiron(3+). This distinguishes ionic hydroxides from covalent hydroxy ligands in complexes. Special cases arise for phenols and enols. Phenols, where the hydroxy group is directly attached to an aromatic ring (Ar-OH), are named as derivatives of the retained name "phenol," such as 2-bromophenol for the ortho-substituted compound, with the parent structure being numbered from the carbon attached to -OH. Enols, featuring a hydroxy group on a carbon-carbon and often existing as tautomers of carbonyl compounds, are named substitutively with the "-ol" suffix and an "en" for the , like prop-1-en-2-ol for the form of acetone. The of hydroxy compounds has undergone significant evolution, particularly in the , transitioning from archaic descriptive systems to standardized approaches under IUPAC. Early names like "carbinol" for derivatives (e.g., dimethyl carbinol for propan-2-ol) were common in the 19th and early 20th centuries but were largely replaced by the systematic "-ol" suffix following the establishment of IUPAC rules in the and refinements through the . This shift, formalized in documents like the 1957 and 1971 IUPAC recommendations, emphasized unambiguous, generative naming based on parent structures and functional priorities.

Physical and Chemical Properties

Bonding and Structure

The hydroxy group (-OH) consists of an oxygen atom to a and typically to a carbon atom in organic compounds such as alcohols. The O-H is approximately 0.96 , as determined from experimental data on simple alcohols like and . In alcohols, the C-O is about 1.43 , reflecting a single between the carbon and oxygen atoms. The bond angle around the carbon atom attached to the -OH group approaches 109°, consistent with tetrahedral geometry in sp³-hybridized carbon centers. The oxygen atom in the hydroxy group of alcohols is sp³ hybridized, forming four equivalent hybrid orbitals: two used for sigma bonds to carbon and , and the remaining two occupied by lone pairs of electrons. This hybridization results in a bent structure at the oxygen, with the C-O-H bond angle slightly less than 109° due to repulsion from the lone pairs. In , where the -OH group is directly attached to an aromatic ring, one of the oxygen's lone pairs participates in delocalization into the pi system of the benzene ring, leading to partial double-bond character in the C-O bond and altered bond orders throughout the ring. Spectroscopically, the hydroxy group exhibits characteristic signatures that reveal its bonding. In infrared (IR) spectroscopy, the O-H stretching appears as a broad absorption band between 3200 and 3600 cm⁻¹, arising from the polar O-H bond and influenced by molecular environment. In nuclear magnetic resonance (¹H NMR) spectroscopy, the -OH proton signal is variable, typically ranging from 1 to 5 ppm, due to rapid proton exchange that averages the and often broadens the peak.

Polarity and Intermolecular Interactions

The hydroxy group (-OH) exhibits significant polarity due to the electronegativity difference between oxygen (3.44) and hydrogen (2.20) on the Pauling scale, resulting in a partial negative charge (δ⁻) on the oxygen atom and a partial positive charge (δ⁺) on the hydrogen atom. This charge separation creates a polar covalent O-H bond, with the dipole moment enhancing the group's ability to participate in intermolecular interactions. The oxygen atom's two lone pairs of electrons, arising from its bonding structure, further facilitate these interactions by allowing the hydroxy group to act both as a hydrogen bond donor (via the δ⁺ hydrogen) and acceptor (via the δ⁻ oxygen). Hydrogen bonding is the primary intermolecular interaction involving the hydroxy group, where the partially positive of one -OH forms an attractive force with the partially negative oxygen of another -OH or a similar group. These bonds typically have energies around 20 kJ/mol, as seen in dimers (approximately 20.8 kJ/mol) and alcohol systems like . In , each can form up to four hydrogen bonds, creating an extensive network that influences molecular arrangement. Similarly, in alcohols such as and , the -OH groups enable both intra- and intermolecular hydrogen bonding, leading to associated structures in the liquid state. This polarity and hydrogen bonding significantly elevate the boiling points of compounds containing hydroxy groups compared to non-polar analogs of similar molecular weight. For instance, (C₂H₅OH) has a of 78°C, much higher than (C₃H₈) at -42°C, due to the required to disrupt hydrogen bonds during vaporization. Hydrogen bonding also enhances in ; small alcohols like and are fully miscible, as their -OH groups form hydrogen bonds with molecules, overcoming hydrophobic effects from alkyl chains. In comparison to other functional groups, the hydroxy group's hydrogen bonding is stronger than that in amines or ethers. Amines form weaker N-H···N bonds (due to nitrogen's lower of 3.04), resulting in lower boiling points for similar-sized compounds, such as (16°C) versus (78°C)./03:_An_Introduction_to_Organic_Compounds_Structure_and_Nomenclature/3.07:_The_Physical_Properties_of_Alkanes_Alkyl_Halides_Alcohols_Ethers_and_Amines) Ethers, lacking a hydrogen donor, rely only on dipole-dipole interactions and exhibit even lower boiling points, like (35°C), underscoring the hydroxy group's superior intermolecular attraction./03:_An_Introduction_to_Organic_Compounds_Structure_and_Nomenclature/3.07:_The_Physical_Properties_of_Alkanes_Alkyl_Halides_Alcohols_Ethers_and_Amines)

Acidity and Reactivity

The hydroxy group in alcohols exhibits weak acidity, with pK_a values typically ranging from 15 to 18, making challenging under standard conditions. For example, has a pK_a of 15.9, and has a pK_a of 15.5. In contrast, the hydroxy group attached to an aromatic ring, as in , is more acidic, with a pK_a around 10, due to stabilization of the resulting phenoxide , where the negative charge is delocalized into the ring. This delocalization enhances the stability of the conjugate base compared to the ions from aliphatic alcohols. The basicity of the hydroxy group is quite weak, as protonation occurs on the oxygen atom to form the oxonium ion R-OH₂⁺, which has a pK_a of approximately -2, indicating that such species are strong acids and poor bases in neutral environments. This low basicity arises from the high electronegativity of oxygen, which holds electron density tightly, limiting its ability to accept protons effectively. Substituents adjacent to the hydroxy group can significantly influence its acidity through electronic effects. Electron-withdrawing groups stabilize the conjugate base by dispersing the negative charge, thereby lowering the pK_a; for instance, has a pK_a of 12.4 due to the inductive withdrawal by the trifluoromethyl group. Electron-donating substituents, conversely, increase the pK_a by destabilizing the anion. In terms of reactivity, the oxygen atom in the hydroxy group serves as a in bimolecular (S_N2) reactions, where it attacks centers such as alkyl halides, though its nucleophilicity is moderated by and the need for in some cases. The attached to oxygen, meanwhile, acts as an in processes, facilitating acid-base reactions with stronger bases. Hydrogen bonding in protic solvents can modestly enhance the effective acidity of the hydroxy group by stabilizing the for proton transfer.

Occurrence and Applications

In Organic Compounds

In organic compounds, the hydroxy group (-OH) is a key that imparts distinct chemical properties when attached to carbon atoms, primarily manifesting in classes such as alcohols, polyols, , and enols. Alcohols, characterized by the -OH group bonded to a saturated carbon atom, are foundational examples and are classified based on the substitution at the carbon bearing the hydroxy group. Primary alcohols have the -OH attached to a carbon bonded to only one other carbon atom, such as methanol (CH3OHCH_3OH) and ethanol (CH3CH2OHCH_3CH_2OH), which exhibit relatively higher reactivity in oxidation processes. Secondary alcohols feature the -OH on a carbon bonded to two other carbons, exemplified by isopropanol ((CH3)2CHOH(CH_3)_2CHOH), while tertiary alcohols have the -OH on a carbon bonded to three other carbons, like tert-butanol ((CH3)3COH(CH_3)_3COH). This classification influences their physical properties and reactivity patterns, with tertiary alcohols generally showing greater steric hindrance. Polyols are organic compounds containing multiple hydroxy groups attached to carbon atoms, enhancing their polarity and hydrogen-bonding capabilities. Diols possess two -OH groups, such as (HOCH2CH2OHHOCH_2CH_2OH), and triols have three, as seen in (HOCH2CH(OH)CH2OHHOCH_2CH(OH)CH_2OH), which serves as a structural component in various lipid-related molecules. These multiple hydroxy groups allow polyols to form extensive intermolecular interactions, contributing to their roles in molecular assembly. Phenols feature the hydroxy group directly attached to an aromatic ring, such as in phenol (C6H5OHC_6H_5OH), where resonance delocalization with the benzene ring increases acidity compared to aliphatic alcohols. In contrast, enols contain the -OH group bonded to a vinylic carbon in a C=C−OHC=C-OH arrangement, often existing in equilibrium with keto forms through tautomerism in carbonyl compounds like acetone enol (CH2=C(OH)CH3CH_2=C(OH)CH_3). This tautomerism stabilizes the enol form in certain systems, particularly those conjugated with aromatic rings. The hydroxy group in alcohols undergoes interconversions, such as oxidation, where primary alcohols are converted to aldehydes and then carboxylic acids, and secondary alcohols to ketones, using oxidizing agents like . Tertiary alcohols resist oxidation under mild conditions due to the absence of a on the hydroxy-bearing carbon. of alcohols, typically under acidic conditions, eliminates to form s, with secondary and tertiary alcohols yielding more stable alkene products via intermediates. The hydroxy group's ability to form bonds significantly enhances the of lower-molecular-weight alcohols and polyols.

In Inorganic and Biochemical Contexts

In inorganic chemistry, the hydroxy group commonly appears in metal hydroxides, where it serves as a ligand bound to metal cations, often forming ionic or layered structures. For instance, calcium hydroxide, Ca(OH)₂, consists of Ca²⁺ ions coordinated to two OH⁻ anions, resulting in a compound with strong basic properties due to the dissociation of the hydroxide ions in aqueous solution, yielding a pH of approximately 12.5–12.8. This structure exemplifies the role of hydroxy groups in stabilizing metal centers through coordinate bonds, contributing to the material's use in applications like pH adjustment, though here the focus is on its fundamental coordination. Similarly, in coordination chemistry, hydroxy groups participate in aquo-hydroxy complexes, such as the tetrahedral aluminate ion [Al(OH)₄]⁻, which predominates in alkaline solutions where aluminum exists as a tetrahedrally coordinated species with four hydroxy ligands. These complexes illustrate how hydroxy groups can bridge or terminal-ligate metal ions, influencing speciation and stability in aqueous environments, as seen in the transition from octahedral aquo species to hydroxy-dominant forms under basic conditions. In biochemical contexts, hydroxy groups are integral to the structure and function of biomolecules beyond carbon-based frameworks. In , the side-chain hydroxy groups of serine and enable key interactions; for example, the -CH₂OH in serine and -CH(OH)CH₃ in facilitate hydrogen bonding and serve as sites for post-translational modifications. These hydroxy moieties are critical in active sites, where they participate in proton transfer mechanisms, such as in serine proteases like , in which the serine hydroxyl acts as a after facilitated by a , enabling of peptide bonds. In carbohydrates, multiple hydroxy groups on glucose—such as those at C-1, C-2, C-3, C-4, and C-6—contribute to the molecule's solubility and reactivity through hydrogen bonding, allowing glucose to form structural like or serve as an energy source in . Additionally, in cofactors like ascorbic acid (), the enediol hydroxy groups at C-2 and C-3 enable its role as a and cofactor, donating electrons in reactions, such as collagen synthesis, due to the proximity of these groups to carbonyl functionalities. The biological functions of hydroxy groups extend to dynamic processes like and signaling. In , glycosyltransferases attach sugar moieties to the hydroxy groups of or residues on proteins, modulating , stability, and interactions in cellular pathways. This modification plays a pivotal role in , as glycosylated proteins on the cell surface influence receptor-ligand interactions and , such as in pathways where altered glycosylation affects receptor and downstream effects like proliferation. Regarding pH-dependent speciation, hydroxy groups in these biomolecules, with pKa values typically ranging from 15 to 18 for aliphatic alcohols, remain predominantly protonated (as R-OH) under physiological conditions at 7.4, limiting but allowing participation in networks without . This protonated state ensures stability while enabling reversible proton transfer in enzymatic reactions, contrasting with more acidic phenolic hydroxy groups like in (pKa ≈ 10), which can partially deprotonate near neutral .

Industrial and Synthetic Uses

The hydroxy group plays a central role in industrial synthesis of alcohols through methods like the hydration of alkenes and the reduction of carbonyl compounds, leveraging its ability to form stable bonds in reactions. One prominent industrial process is the indirect hydration of to produce , where reacts with concentrated to form ethyl hydrogen sulfate, followed by with to yield and regenerate the acid; this method, though largely superseded by direct hydration using catalysts at high temperatures (around 300°C) and pressures (60-70 atm), remains relevant for understanding early routes. Direct hydration, employed by major producers, achieves yields up to 95% and is economically viable due to the abundance of from cracking. Another key synthetic route involves the reduction of carbonyl groups in aldehydes and ketones to primary and secondary alcohols, respectively; industrially, this is predominantly achieved via catalytic using gas and metal catalysts like or under moderate pressures (10-50 atm) and temperatures (100-200°C), as seen in the production of fatty alcohols from aldehydes derived from natural fats or synthetic sources, offering high selectivity and scalability over lab-scale reagents like NaBH₄. Ethanol, a bearing a , exemplifies the economic significance of these syntheses, serving as a versatile , , and chemical intermediate with global production reaching approximately 110 billion liters (about 87 million metric tons) annually in the early , driven largely by demand in the United States and for blended into . Glycerol, a with three , is another critical industrial product obtained as a byproduct of from vegetable oils or animal fats via , and it finds applications in the manufacture of for explosives—where its react with to form the —and in pharmaceuticals as a , , and stabilizer in formulations like cough syrups and suppositories. In broader applications, hydroxy-containing compounds like fatty alcohols (e.g., lauryl and cetyl alcohols) are essential raw materials for non-ionic surfactants in detergents, shampoos, and industrial cleaners, where the hydroxy group facilitates to produce emulsifiers that reduce and enhance properties, with global demand exceeding 3 million tons per year due to their role in household and . In the pharmaceutical sector, the phenolic hydroxy group in compounds like (acetaminophen) contributes to its and activity by enabling hydrogen bonding interactions with biological targets; is synthesized industrially via of p-aminophenol, a process yielding over 100,000 tons annually worldwide for pain relief medications. The reactivity of the hydroxy group in these syntheses and applications stems from its nucleophilicity and ability to participate in hydrogen bonding, which underpins their utility in forming esters, ethers, and other derivatives. From an environmental perspective, aliphatic alcohols such as and exhibit high biodegradability, with over 90% degradation in aerobic within days via microbial oxidation of the hydroxy group to carboxylic acids, facilitating their use in eco-friendly formulations; in contrast, phenolic hydroxy compounds like those in substituted can persist longer in the environment due to their aromatic stability and potential formation of toxic intermediates during , necessitating advanced treatment in industrial effluents to mitigate ecotoxicity.

Hydroxyl Radical

Structure and Formation

The (OH•) is a diatomic free radical composed of a covalently bonded to an oxygen atom, featuring an primarily localized on the oxygen, resulting in a ground electronic state of 2Π^2\Pi. This structure imparts high reactivity, distinguishing it from the stable, closed-shell (-OH) found in organic molecules like alcohols. The O-H in the is 0.97 Å, while the bond dissociation energy, which corresponds to the energy required to break the bond into ground-state H and O atoms, is 428 kJ/mol at 298 K. OH• forms through several key mechanisms, including the direct photodissociation of water molecules under ultraviolet radiation: H2O+hν→OH∙+H∙\mathrm{H_2O + h\nu \rightarrow OH^\bullet + H^\bullet} This process occurs prominently in laboratory settings and the upper atmosphere with vacuum UV light (below 185 nm). In combustion environments, such as flames, OH• is generated via radical chain reactions, for example, through the interaction of hydrogen atoms with oxygen: H+O2→OH∙+O\mathrm{H + O_2 \rightarrow OH^\bullet + O}. Atmospheric lightning also produces OH• by subjecting air to extreme temperatures (up to 30,000 K), causing thermal dissociation of water vapor and molecular oxygen into reactive species including the radical. Detection of OH• relies on its spectroscopic signatures, with ultraviolet absorption being a primary method due to the strong A 2Σ+←^2\Sigma^+ \leftarrow X 2Π^2\Pi transition, featuring a prominent absorption band at 281 nm. Electron spin resonance (ESR) further confirms its presence by detecting the signal, often enhanced through spin-trapping agents like DMPO for sensitivity in complex matrices. The radical exhibits a short lifetime in air—on the order of 1 second in the —owing to its rapid reactions with trace gases such as CO and CH₄, limiting its persistence and necessitating continuous production for sustained atmospheric roles.

Chemical Behavior and Reactions

The hydroxyl radical (OH•) exhibits exceptionally high reactivity due to its , making it one of the most potent oxidizing agents in both atmospheric and biological environments. In the gas phase, OH• primarily undergoes reactions with saturated , where it abstracts a to form and an alkyl radical; typical rate constants for these abstractions from alkanes range from approximately 10^{-14} to 10^{-12} cm³ molecule⁻¹ s⁻¹ at 298 K, depending on the specific hydrocarbon and site of . For example, the reaction with proceeds with k = (6.4 ± 0.7) × 10^{-15} cm³ molecule⁻¹ s⁻¹ at 298 K, illustrating the radical's efficiency in initiating oxidation chains. Additionally, OH• adds rapidly to carbon-carbon double bonds in alkenes, forming β-hydroxyalkyl radicals; this addition is diffusion-controlled with rate constants typically (2–8) × 10^{-11} cm³ molecule⁻¹ s⁻¹ at 298 K for simple alkenes, significantly faster than abstraction pathways for unsaturated compounds. These reactions enable OH• to propagate chain mechanisms critical to environmental processes. In tropospheric chemistry, the reaction of OH• with (CO) plays a key role in formation under NOx-rich conditions: \ceOH∙+CO−>[M]HOCO∙−>CO2+H∙\ce{OH^\bullet + CO ->[M] HOCO^\bullet -> CO2 + H^\bullet} followed by H• + O₂ → HO₂• and subsequent cycling that net produces O₃; this pathway accounts for a substantial fraction of global OH• consumption and influences urban air quality. In combustion environments, OH• sustains propagation by abstracting from molecules and participating in radical branching cycles, with concentrations peaking during ignition to accelerate oxidation rates. OH• also serves as a primary atmospheric scavenger, oxidizing volatile organic compounds (VOCs) and other pollutants to prevent their accumulation; it reacts with over 90% of emitted VOCs, converting them into less harmful products like CO₂ and , thus maintaining the troposphere's self-cleansing capacity. In biological contexts, OH• generated via Fenton chemistry induces by attacking cellular components, notably causing DNA strand breaks and base modifications such as formation, which can lead to mutations if unrepaired. This reactivity implicates OH• in aging and diseases like cancer, where elevated levels overwhelm defenses.

Astronomical Observations

In Earth's Atmosphere and Airglow

The hydroxyl radical (OH) plays a significant role in the chemistry of Earth's , where its concentrations exhibit a peak of approximately 10⁶ molecules cm⁻³ at altitudes between 40 and 50 km. This layer corresponds to the upper , where OH acts as a key oxidant influencing the budgets of and other trace gases. The radical's abundance in this region is governed by photochemical production and loss processes, with concentrations decreasing toward lower altitudes due to reduced solar radiation and increasing air density. Observations indicate that these peak values are typical during daytime conditions, reflecting the radical's sensitivity to flux. In the context of , the contributes to the nightside through the excitation mechanism involving the reaction H + O₃ → OH* + O₂, which produces vibrationally excited OH* molecules. These excited states subsequently emit light in the near-infrared as they relax, forming the characteristic Meinel bands spanning wavelengths from 700 to 900 nm. The Meinel bands arise from transitions between vibrational levels in the ground electronic state of OH, with the emission layer peaking around 85-90 km altitude in the , slightly above the stratospheric OH maximum. This serves as a diagnostic tool for upper atmospheric dynamics and temperatures, as the intensity and rotational structure of the bands correlate with local conditions. The diurnal cycle of stratospheric OH is pronounced, driven primarily by solar-driven . During daytime, the main production pathway is the reaction of electronically excited oxygen atoms with : O(¹D) + H₂O → 2 OH, initiated by photolysis under ultraviolet radiation. At night, OH concentrations decline sharply due to the absence of this photolytic source, with residual levels sustained by minor recombination processes involving HO₂ and other radicals. This cycle results in OH levels varying by orders of magnitude between day and night, underscoring its role as a transient atmospheric cleanser. Satellite-based monitoring has provided critical insights into stratospheric OH distributions and their links to . The Microwave Limb Sounder (MLS) instrument on the satellite, launched in 2004, measures OH along with and other species using microwave emissions, enabling global profiles from the upper to the . Data from MLS reveal correlations between elevated OH and regions of ozone loss, particularly in the presence of radicals from human-emitted chlorofluorocarbons, highlighting OH's catalytic role in ozone destruction cycles. These observations, spanning over two decades, support assessments of stratospheric recovery and the impacts of atmospheric variability.

On Lunar and Planetary Surfaces

The presence of hydroxy groups (OH) on the lunar surface has been detected through and impact experiments, revealing their incorporation into the regolith as a result of interactions with particles. The Mineralogy Mapper (M³) instrument aboard India's spacecraft, launched in 2008, identified widespread absorptions at approximately 3 μm indicative of OH and H₂O in the lunar , particularly at high latitudes and in permanently shadowed regions. This detection was complemented by NASA's Lunar Crater Observation and Sensing Satellite (LCROSS) mission in 2009, which impacted the Cabeus crater and analyzed the ejecta plume, confirming the presence of water molecules (including OH components) at concentrations up to 5.6% by weight in the . These findings attribute the OH formation primarily to the of oxygen atoms in the regolith by hydrogen ions. On Mars, spectroscopic observations from the orbiter, operational since 2003, have revealed seasonal variations in OH signatures within the polar caps, associated with hydration processes in the underlying . The Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) instrument detected broad 3 μm absorptions consistent with OH-bearing phases in the near the south polar cap, with intensities varying seasonally due to temperature-driven exchange between surface ice and subsurface hydrated minerals. These observations link the transient OH enrichment to the sublimation and recondensation of from layers during polar summer, influencing the hydration state of phyllosilicates and other minerals in the cap margins. In the Venusian atmosphere, transient OH has been observed in the upper cloud layers (around 65–70 km altitude) through , arising from photochemical reactions involving (SO₂) and (H₂O). The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the mission (2005–2014) detected OH nightglow emissions at 2.8 μm and 1.46 μm on the planet's nightside, indicating short-lived OH radicals formed via oxidation processes in the . These emissions are linked to reactions such as the interaction of SO₂ with H₂O under irradiation, contributing to the dynamic sulfur-water chemistry that sustains the upper cloud haze. The formation of OH on airless planetary bodies like the primarily occurs through and driven by exposure. involves the embedding of protons into the top few nanometers of grains, where they react with lattice oxygen to form OH bonds, as demonstrated in simulations using H⁺ and D⁺ beams on lunar analogs. complements this by ejecting surface atoms and molecules, including nascent OH, through momentum transfer from incident , though it also leads to volatile loss; this process is particularly relevant on bodies lacking atmospheres, where maturity influences OH retention.

In Exoplanetary Atmospheres

The presence of the (OH) in exoplanetary atmospheres has been detected primarily in ultra-hot s, where intense stellar irradiation dissociates into OH and other species. These detections provide insights into high-temperature and dynamics. The first confirmed observation occurred in the dayside atmosphere of WASP-33b, an ultra-hot Jupiter with equilibrium temperatures exceeding 2700 K, using high-dispersion with the Subaru/IRD instrument. This revealed OH emission lines at wavelengths around 1-2 μm, indicating thermal dissociation of H₂O in the upper atmosphere. Subsequent observations have identified OH absorption features during planetary transits, employing transmission to probe the atmospheric composition along the line of sight. For instance, in , ground-based high-resolution during transit detected OH absorption with a of 6.1, particularly in the near-infrared, confirming its role as a tracer of in hot Jupiter atmospheres. The (HST) has contributed to related studies by detecting in hot Jupiters like HD 189733b since the 2010s, where models suggest OH formation via H₂O dissociation under UV irradiation, though direct OH lines remain elusive at HST's resolution. The (JWST), operational since 2021, enhances these capabilities with superior near-infrared sensitivity and resolution for transmission spectroscopy, targeting absorption features at 1-2 μm during transits of hot s. While JWST has yet to report direct OH detections as of 2025, its observations of -bearing atmospheres in similar systems enable refined modeling of OH production. Recent ground-based advances include the 2025 detection of OH emission in the dayside of , the hottest known ultra-hot (T_eq ≈ 4600 K), using the SPIRou spectrograph on the Canada-France-Hawaii Telescope, with a of 5.3, further validating OH as a ubiquitous feature in dissociated layers. These OH signatures imply active driven by stellar EUV radiation, where breaks down into OH radicals, which can further photolyze to release and oxygen, facilitating hydrodynamic escape and atmospheric mass loss. In ultra-hot Jupiters, such processes sculpt the upper atmosphere, potentially enriching it with oxygen from water dissociation. Although not a direct , OH presence in cooler, habitable-zone exoplanets could indirectly signal water photolysis and oxygen accumulation, aiding assessments of atmospheric and potential , akin to processes observed in solar system giant planets.

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