Peroxy acid
Peroxy acids, also known as peracids, are a class of chemical compounds defined by the presence of a perhydroxyl group (-OOH), where the acidic hydrogen is attached to one oxygen atom in a peroxide linkage.[1] They encompass both inorganic variants, derived from mineral acids such as peroxymonosulfuric acid (H₂SO₅, or Caro's acid), and organic peracids, typically of the form R-C(=O)-OOH, exemplified by peracetic acid (CH₃CO₃H) and meta-chloroperoxybenzoic acid (mCPBA).[2] These compounds exhibit strong oxidizing properties due to their high redox potentials (e.g., 1.06–1.96 V for peracetic acid), making them versatile reagents in chemical transformations.[2] Peroxy acids were first synthesized in the late 19th century. The inorganic Caro's acid was discovered by Heinrich Caro in 1898 through the reaction of hydrogen peroxide with sulfuric acid. Their utility in organic synthesis was demonstrated shortly thereafter, with the Baeyer-Villiger oxidation reported by Adolf von Baeyer and Victor Villiger in 1899, and the Prilezhaev epoxidation by Nikolai Prilezhaev in 1909.[3][4] Organic peroxy acids are commonly prepared via the equilibrium reaction of hydrogen peroxide (H₂O₂) with the corresponding carboxylic acid or anhydride, often catalyzed by sulfuric acid. Inorganic peroxy acids, such as peroxymonosulfate (HSO₅⁻), are synthesized by reacting concentrated H₂SO₄ with H₂O₂ or through electrolytic oxidation of sulfuric acid.[2] These preparation methods highlight the peracids' instability, as they tend to decompose back to the parent acid and oxygen, necessitating careful handling to avoid explosive risks from concentrated forms.[1] In organic synthesis, peroxy acids play a pivotal role in selective oxidations, most notably in the Prilezhaev reaction, where they epoxidize alkenes to form oxiranes in a stereospecific, concerted manner using reagents like mCPBA.[3] They are also central to the Baeyer-Villiger oxidation, converting ketones to esters or cyclic ketones to lactones via migratory aptitude-determined insertion of oxygen adjacent to the more substituted carbon.[4] Beyond synthesis, peroxy acids find industrial applications in disinfection (e.g., peracetic acid in food processing and healthcare), pulp bleaching, and advanced oxidation processes for degrading micropollutants in wastewater, leveraging their ability to generate reactive species like hydroxyl radicals (•OH) upon activation.[2]Introduction
Definition and Structure
Peroxy acids, also known as peracids, are a class of chemical compounds defined as acids in which an acidic −OH group has been replaced by an −OOH group.[5] This structural modification imparts strong oxidizing properties to these compounds, distinguishing them from conventional acids.[6] They are broadly classified into two main types: inorganic peroxy acids, derived from mineral acids such as sulfuric acid, and organic peroxy acids, derived from carboxylic acids.[7] The general structure of organic peroxy acids is R-C(O)OOH, where R represents an alkyl or aryl group attached to the carbonyl carbon.[5] In contrast, inorganic peroxy acids feature structures like HOSO_2OOH for peroxymonosulfuric acid, where the peroxy group is integrated into the framework of the parent mineral acid.[8] Nomenclature for peroxy acids follows IUPAC conventions, employing the prefix "peroxy-" to denote the −OOH group in the name of the corresponding parent acid, such as peroxyacetic acid for CH_3C(O)OOH.[9] This prefix clearly differentiates peroxy acids from hydroperoxides, which possess the simpler R-OOH structure without an adjacent carbonyl or sulfo group.[5] Peroxy acids exhibit greater oxidizing power than hydrogen peroxide (H_2O_2) primarily because the terminal oxygen in the −OOH group is electrophilic, facilitating nucleophilic attack by substrates during oxidation reactions.[6] This electrophilicity arises from the electron-withdrawing effect of the adjacent carbonyl or sulfo moiety, enhancing the reactivity of the peroxy linkage compared to the neutral O-O bond in H_2O_2.[6]Historical Background
The discovery of hydrogen peroxide in 1818 by French chemist Louis Jacques Thénard, through the reaction of barium peroxide with acids, provided the foundational compound for subsequent peroxy acid developments.[10] Thénard's isolation of this unstable oxidant marked a key advancement in peroxide chemistry, enabling later explorations into more complex peroxy derivatives.[11] The first peroxy acid, peroxymonosulfuric acid (also known as Caro's acid), was isolated in 1898 by German chemist Heinrich Caro through the reaction of concentrated sulfuric acid with hydrogen peroxide.[12] Caro's work demonstrated the formation of this powerful oxidant, which became a precursor for industrial applications due to its enhanced reactivity compared to hydrogen peroxide alone.[13] In the early 20th century, organic peroxy acids emerged, with peracetic acid first synthesized around 1900–1910 initially for bleaching purposes.[14] This period also saw the formalization of the Prilezhaev reaction in 1909 by Russian chemist Nikolai Prilezhaev, who described the epoxidation of alkenes using peroxy acids, laying groundwork for their role in organic synthesis.[15] Peroxy acids, including peracetic acid, found early industrial use in bleaching applications, such as for textiles in the 1940s, and later in the pulp and paper sector starting in the 1950s, offering a chlorine-free alternative that improved fiber brightness.[16][17] In the mid-20th century, synthetic organic peroxy acids like meta-chloroperoxybenzoic acid (mCPBA) were developed, enhancing their utility in precise organic syntheses.[18] By the late 20th century, peroxy acids evolved toward broader applications, particularly as disinfectants following U.S. Environmental Protection Agency (EPA) registrations: hydrogen peroxide in 1977 and peracetic acid in 1985.[19] These approvals facilitated their adoption in antimicrobial uses across food processing, healthcare, and water treatment, reflecting a shift from primarily oxidative roles to sanitation-focused implementations.Inorganic Peroxy Acids
Preparation and Examples
Inorganic peroxy acids are typically prepared by reacting concentrated solutions of mineral acids with hydrogen peroxide, leading to the insertion of a peroxide group into the acid structure. This general method exploits the nucleophilic attack of hydrogen peroxide on the electrophilic central atom of the mineral acid, forming the peroxy acid along with water.[20] A key example is peroxymonosulfuric acid (H₂SO₅), also known as Caro's acid, which is synthesized via the equilibrium reaction of concentrated sulfuric acid (85–98% H₂SO₄) with concentrated hydrogen peroxide (50–90% H₂O₂):H₂SO₄ + H₂O₂ ⇌ H₂SO₅ + H₂O.
This process is exothermic and requires cooling to control the reaction temperature, typically around 0–10°C, to maximize yield and minimize decomposition. The equilibrium favors the peroxy acid under these conditions, but due to its reactivity and tendency to hydrolyze back to sulfuric acid and hydrogen peroxide, Caro's acid is rarely isolated in pure form and is instead used directly in solution.[21] For laboratory-scale isolation of a purer product, hydrogen peroxide is added dropwise to cooled chlorosulfonic acid (ClSO₃H) at low temperatures (e.g., -40°C), yielding H₂SO₅ and HCl:
H₂O₂ + ClSO₃H → H₂SO₅ + HCl.
This method produces a viscous, oily liquid that can be distilled under reduced pressure, though yields are limited by the compound's instability.[20] Another representative compound is peroxymonophosphoric acid (H₃PO₅), prepared analogously by reacting phosphorus pentoxide (P₂O₅) with concentrated hydrogen peroxide in a controlled biphasic system to manage the vigorous, exothermic reaction.[22] Typical conditions involve a P₂O₅:H₂O₂ molar ratio of 0.5:1 at 2°C for 120–180 minutes, achieving approximately 70% conversion of H₂O₂ to the peroxy acid.[22] Like Caro's acid, H₃PO₅ exists in equilibrium with phosphoric acid (H₃PO₄) and hydrogen peroxide and decomposes readily, necessitating in situ generation. Other inorganic peroxy acids include those derived from carbonate and borate systems, such as the peroxymonocarbonate anion [HCO₄]⁻, formed from bicarbonate and hydrogen peroxide, and perborate anions like [B(O₂)(OH)₂]⁻ in sodium perborate, which is produced by reacting borax with hydrogen peroxide under alkaline conditions. Peroxydicarbonate anions, such as [O₃CO₂]²⁻, have been synthesized electrochemically from carbonate solutions.[23] Owing to their thermal and hydrolytic instability, inorganic peroxy acids are not stored commercially but generated on-site at industrial scales immediately prior to use, often in continuous-flow reactors to ensure safety and efficiency.[20]
Properties and Specific Uses
Inorganic peroxy acids, such as peroxymonosulfuric acid (H₂SO₅, also known as Caro's acid), exhibit strong acidity with pKₐ values of approximately 1 for the first proton and 9.3 for the second, making them comparable in acidity to sulfuric acid's second dissociation but with enhanced reactivity due to the peroxy group.[24][25] These compounds possess extreme oxidizing power, characterized by a standard electrode potential of +2.51 V, positioning them among the strongest known oxidants and enabling selective oxidation of recalcitrant species under acidic conditions.[24][25] Their instability is a defining trait, leading to rapid thermal and hydrolytic decomposition into sulfuric acid and oxygen gas, often requiring on-site generation to mitigate explosive risks during storage or transport.[26][27] These peroxy acids are typically handled as concentrated solutions in sulfuric acid, where peroxymonosulfuric acid forms an equilibrium mixture with hydrogen peroxide and water, enhancing solubility and practical utility in industrial settings.[24] Crystalline salts, such as potassium peroxymonosulfate (commercially known as Oxone, a triple salt with the formula 2KHSO₅·KHSO₄·K₂SO₄), offer greater stability and are highly soluble in water (>250 g/L at 20°C), allowing for easier formulation into powders or granules for controlled release applications.[28][29] In the pulp and paper industry, Caro's acid serves as an effective bleaching agent, particularly in totally chlorine-free (TCF) sequences, where it delignifies and brightens chemical pulps like kraft pulp by oxidizing residual lignin and chromophores without introducing harmful chlorinated byproducts, achieving brightness gains of up to 10-15% ISO in extraction stages.[30][31] For cyanide detoxification in gold mining operations, Caro's acid rapidly oxidizes free cyanide (CN⁻) to less toxic cyanate (CNO⁻) at near-neutral pH, enabling compliance with environmental discharge limits while minimizing reagent consumption compared to alternatives like sodium hypochlorite.[32][33] Potassium peroxymonosulfate plays a limited but targeted role in water treatment, oxidizing heavy metals such as chromium(VI) to less mobile forms or facilitating their precipitation, often in advanced oxidation processes for industrial wastewater remediation.[34][35]Organic Peroxy Acids
Synthesis Methods
Organic peroxy acids are commonly synthesized in the laboratory and on an industrial scale through the acid-catalyzed equilibrium reaction of the corresponding carboxylic acid with hydrogen peroxide, which establishes a dynamic balance between the reactants and products. This method is particularly prevalent for peracetic acid, where glacial acetic acid reacts with hydrogen peroxide in the presence of a mineral acid catalyst such as sulfuric acid (0.1–1.5 wt%) to form an equilibrium mixture typically containing 5–40 wt% peroxy acid, along with unreacted acetic acid, excess hydrogen peroxide, and water. The reaction proceeds as follows: \ce{RCO2H + H2O2 ⇌ RCO3H + H2O} Equilibrium is generally reached within 24–72 hours at temperatures of 20–50°C, with the peroxy acid concentration influenced by the molar ratio of acetic acid to hydrogen peroxide (often 1:1 to 1:1.5) and the catalyst amount; higher acid concentrations shift the equilibrium toward peroxy acid formation.[36][37] An alternative route involves the reaction of carboxylic acid anhydrides with hydrogen peroxide to produce symmetrical peroxy acids, avoiding water formation and allowing for higher concentrations. In this process, the anhydride undergoes nucleophilic attack by hydrogen peroxide, yielding two equivalents of the peroxy acid: \ce{(RCO)2O + H2O2 -> 2 RCO3H} This method is effective for preparing peroxy acids like peracetic acid from acetic anhydride, often conducted at 0–20°C to control exothermicity, and is favored when anhydrous conditions are desired. For instance, monoperphthalic acid is synthesized from phthalic anhydride and alkaline hydrogen peroxide, followed by acidification.[6][38] The acyl chloride method provides a direct route for preparing specific peroxy acids, such as meta-chloroperoxybenzoic acid (mCPBA), by reacting the acyl chloride with hydrogen peroxide under basic conditions to neutralize the generated HCl. The procedure typically involves adding the acyl chloride (e.g., m-chlorobenzoyl chloride) to an aqueous solution of 30% hydrogen peroxide, sodium hydroxide, and magnesium sulfate in dioxane at 15–25°C, followed by extraction with dichloromethane and drying to isolate the peroxy acid as a white solid with 80–85% active oxygen content. The reaction is: \ce{RC(O)Cl + H2O2 -> RCO3H + HCl} This approach is advantageous for lab-scale synthesis of pure, isolable peroxy acids but requires careful temperature control to prevent decomposition.[39] Aromatic peroxy acids can also be obtained via the autoxidation of the corresponding aldehydes with molecular oxygen, a radical-initiated process that forms the peroxy acid as an intermediate before further oxidation to the carboxylic acid. For example, benzaldehyde autoxidizes in air at ambient conditions to generate perbenzoic acid (ArCOOOH), with the reaction proceeding through acylperoxy radical formation: \ce{ArCHO + O2 -> ArCOOOH} This method is less common for preparative purposes due to low yields and competing decomposition but is relevant for understanding peroxy acid formation in atmospheric or oxidative environments.[40][41] In industrial production, particularly for peracetic acid, the equilibrium method is scaled up using continuous reactors with sulfuric acid as both catalyst and stabilizer to inhibit decomposition, achieving annual outputs exceeding 30,000 tonnes in regions like Western Europe. Purification often involves vacuum distillation to concentrate the peroxy acid to 25–40 wt% while removing water and excess reactants, with the product stored below 0°C in stainless steel containers to enhance stability; additional stabilizers like diphosphonic acids may be incorporated to prevent peroxide breakdown during transport and storage.[36][42]Common Examples and Properties
Organic peroxy acids are commonly exemplified by peracetic acid (CH_3CO_3H), meta-chloroperoxybenzoic acid (mCPBA, m-ClC_6H_4CO_3H), and perbenzoic acid (C_6H_5CO_3H). These compounds serve as versatile oxidants in synthetic applications, with peracetic acid often employed in aqueous or alcoholic solutions due to its liquid state at room temperature, while mCPBA and perbenzoic acid are typically handled as solids.[43][44] Physically, peracetic acid is a colorless liquid with a boiling point of 105 °C and is miscible with water as well as organic solvents like ethanol and ether, facilitating its use in dilute solutions to mitigate instability. In contrast, mCPBA appears as a white solid with a melting point of 69–71 °C and good solubility in chlorinated hydrocarbons such as dichloromethane, though it has limited water solubility (approximately 0.15 g/100 mL). Perbenzoic acid is also a solid, melting at 41–43 °C, with a density of about 1.27 g/mL and partial decomposition upon heating to 100–110 °C under reduced pressure; it exhibits solubility in organic solvents but is less stable than its derivatives. These variations in form—equilibrium liquids for simple alkyl peroxy acids like peracetic or crystalline solids for aromatic ones—stem from their molecular structures and influence handling protocols.[45][43][46][47][44] Chemically, organic peroxy acids are weaker acids than their corresponding carboxylic acids, with pKa values roughly 1000 times higher (ΔpKa ≈ 3–4 units), reflecting the reduced acidity due to the electron-donating peroxy group. For instance, peracetic acid has a pKa of 8.2, compared to 4.76 for acetic acid, while mCPBA and perbenzoic acid exhibit pKa values around 7.6–7.8, similar to benzoic acid's pKa of 4.2, demonstrating minimal sensitivity to substituents like the meta-chloro group. This acidity profile contributes to their role as mild, selective oxidants. Their oxidizing strength follows the order trifluoroperacetic acid (CF_3CO_3H) > peracetic acid (CH_3CO_3H) > hydrogen peroxide (H_2O_2), driven by the electron-withdrawing ability of substituents that enhances the electrophilicity of the peroxy bond, as observed in Baeyer-Villiger oxidations.[43][48][47][44][49][50]| Compound | Formula | Physical Form | Key Physical Properties | pKa |
|---|---|---|---|---|
| Peracetic acid | CH_3CO_3H | Liquid | Boiling point 105 °C; miscible in water | 8.2[43] |
| mCPBA | m-ClC_6H_4CO_3H | White solid | Melting point 69–71 °C; soluble in CH_2Cl_2 | ~7.6 |
| Perbenzoic acid | C_6H_5CO_3H | Solid | Melting point 41–43 °C; decomposes ~105 °C | ~7.8 |