Peroxy acids that are also known as peracids are generally strong oxidizers and possess an ーOOH group. Peroxide acids are formed when the ーOH group of the oxy group is replaced by an acids ーOOH group. This replacement of the group happens because of the fact that in the ーOH group, the oxygen is covalently bonded with hydrogen and can be broken easily to form a cation and a hydrogen anion. Now the new bond that is formed with alkyl or the benzyl group is called a peroxy bond.
For example, when sulphuric acid which is an oxo acid (HOSO2 or H2 SO4) undergoes a chemical reaction with hydrogen peroxide, it forms peroxysulphuric acid (HOSO2 ーOOH). During the formation of peroxy bonds, a little amount of sulphuric acid or other subsequent strong acids are added to the solution in order to accelerate the reaction.
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Peroxy bonds (ーOOH) are generally weak in nature as the O─O bond (due to the repulsion between the electrons present in the electron-rich clouds of both the atoms of oxygen) in the peroxide group can be easily broken. It is quite evident from its dissociation energy which scales from 45 - 50 Kcal/mol. The strength of the O─O bond is less than half the strength of the C─C or H─H or C─O bond. Though peroxide bonds are weak in nature, the peracids are very good oxidizers and are mostly derived from either conventional acids like sulphuric acids or the peroxy derivatives of organic carboxylic acids. There are mainly two categories of peroxy acids, inorganic peracids and organic peracids.
Inorganic Peroxy Acid
Inorganic peroxy acids are derived from conventional mineral acid among which peroxysulphuric acid which is commonly known as Caro’s acid is most important in terms of its production scale followed by Peroxyphosphoric acid (H3PO5). Both these acids are produced by treating their respective oxy acids, that is, sulphuric acid and phosphoric acid with hydrogen peroxide. The properties of a few of the important peroxy acids are discussed below.
Peroxy Phosphoric Acid / Peroxymonophosporic Acid: peroxy phosphoric acid (H3PO5) is a peroxy acid of phosphorus and its salt is known as peroxy phosphate. It is one of the two known peroxy acids of phosphorus. Another one is called peroxy diphosphoric acid. The molecular structure of peroxy phosphoric acid is as given below:
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The scientific name which is popularly known as the IUPAC name of the acid is hydroxy dihydrogen phosphate with a molecular formula H3PO5. The molecular weight of this mineral peroxy acid is 113.99 g/mol. Initially, peroxy phosphoric acid was synthesized by reacting phosphorus pentoxide with an aqueous solution of hydrogen peroxide at a higher concentration. Since the reaction proceeds vigorously, it gets difficult to control. Hence, this method of synthesis was omitted.
Today there are several methods that are accepted worldwide for the manufacturing or synthesis of this acid. Today one of the widely accepted methods to synthesise peroxy phosphoric acid is by hydrolysis of the potassium present in lithium peroxy diphosphate in a strong acid like perchloric acid. By the electrolysis process, peroxy phosphate salts can be obtained from their respective phosphate salts. The reaction is as follows:
P2O84- + H2O → H3PO5 + H3PO4 (reaction in presence of HClO4)
The second method to obtain peroxy phosphoric acid is by reacting phosphorus pentoxide with a highly-concentrated aqueous solution of hydrogen peroxide in presence of an inert solvent like carbon tetrachloride or acetonitrile. The reaction of the process is as follows:-
P4O10 +4 H2O2 + 2H2O → 4H3PO5
Peroxy phosphoric acid is one of the best electrophilic reagents and thus is used in organic syntheses, such as oxidation of alkynes, alkenes, aromatic compounds and amines. Some of the physical and chemical properties of this acid are:-
Peroxysulfuric Acid: Peroxy acid is commonly known as Caro’s acid and has a molecular formula. The other common names of this acid are persulfuric acid and peroxymonosulfuric acid. The scientific name or IUPAC name of the compound is known as (dioxidanido)hydroxidodioxidosulfur with a molecular structure as follows:-
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In this structural formula, sulfur carries the tetra geometry and the connectivity of other atoms with sulfur is indicated by a linear formula, HOㄧOㄧS(O2)ㄧOH. It is considered to be one of the strongest oxidants with E0 = +2.51V and therefore is highly explosive in nature. In the laboratory, Caro’s acid is prepared by reacting chlorosulfuric acid with hydrogen peroxide. The reaction is as follows:-
H2O2 + CISO2OH ⇋ H2SO5 + HCl
Another method that helps in obtaining Caro’s acid is by reacting hydrogen sulphate with highly-concentrated hydrogen peroxide as an aqueous solution. Though this reaction is carried out for the synthesis of a strong oxidant and bleaching agent, potassium monopersulphate (PMPS), the intermediate compound formed is Caro’s acid. The reaction is as follows:-
H2O2 + H2O4 ⇋ H2SO5 + H2O
Few Physical and Chemical Properties of Caro’s Acid are Listed Below-
Many organic peracids have large commercial applications and are often synthesised by reacting their corresponding carboxylic acid with hydrogen peroxide. The reaction is stated below.
RCO2H + H2O2 ⇋ RCO3H + H2O
One of the most common methods used in the laboratory to synthesise organic peracid is by treating carboxylic anhydride with hydrogen peroxide. In this method, the cyclic anhydrides are converted into their corresponding mono peroxyacid. The reaction is as follows:-
(RCO2) O + H2O2 ⇋ RCO3H +RCO2H
The third method of preparing organic peracid is by treating acid chlorides with hydrogen peroxide. The reaction is stated below.
RC(O)Cl + H2O2 ⇋ RCO3H + HCl
Percarboxylic acids are very less acidic as compared to their corresponding carboxylic acid. It is because, in these acids, resonance stabilization of the anion is not possible. Hence their pKa value for its substitutes is relatively insensitive. Although they are weak acids, they have huge application in chemical synthesis. These organic peracids are used for the conversion of alkenes into epoxides and the reaction mechanism for the reaction is known as the Prilezhaev reaction. Another such reaction takes place in presence of peracid for converting cyclic ketones into their corresponding ring expanded esters. This mechanism is known as Baeyer-Villiger oxidation. They are also used for the conversion of thioethers and amines into amine oxides and sulfoxides by oxidation reactions.