Peroxy acids, also known as peracids, are a distinct class of acids that contain an extra oxygen atom compared to ordinary carboxylic acids. They exist in both organic and inorganic forms, with the additional oxygen atom making them powerful oxidizing agents. This property makes peroxy acids widely useful in laboratory and industrial chemistry, especially for bleaching, disinfection, and environmental cleanup. ^[1-4]

Structure and Formula

In organic chemistry, the structure of a peroxy acid closely resembles that of a carboxylic acid (R–C(O)–OH). The difference lies in the hydroxyl group (–OH) of the –COOH group, which is replaced by a peroxy group (–OOH). The general formula of an organic peroxy acid is therefore: R–C(O)–OOH ^[1-4]

Chemical Properties ^[4]

1. Strong Oxidizing Ability

The most significant feature of peroxy acids is the highly polarized and weak O–O bond. This bond is electron-rich and readily breaks, releasing an oxygen atom that can be transferred to other molecules. This property underlies many important oxidation reactions, including epoxidation of alkenes and the Baeyer–Villiger oxidation of ketones.

2. Weaker Acidity

Peroxy acids are generally weaker acids than their corresponding carboxylic acids, because the additional oxygen atom in the –OOH group destabilizes the conjugate base and reduces its ability to delocalize negative charge.

3. Variable Stability

The stability of peroxy acids depends largely on the nature of the substituents attached to the alkyl or aryl group (R):

• Electron-withdrawing groups (e.g., –Cl, –NO₂) stabilize the peroxy group by lowering its electron density, reducing the risk of uncontrolled decomposition.
• Electron-donating groups (e.g., –CH₃, –OCH₃) destabilize the O–O bond, making the compound more prone to explosive breakdown.

Although many peroxy acids are unstable and potentially hazardous due to their strong oxidizing nature, they can be handled more safely. Strict safety precautions are necessary to minimize risks of decomposition or explosive reactions.

Preparation ^[4]

1. From Carboxylic Acid

Peroxy acids can be prepared by the direct oxidation of carboxylic acids with hydrogen peroxide (H₂O₂), often catalyzed by sulfuric acid (H₂SO₄):

R–COOH + H_2​O₂​ → ​R–C(O)–OOH + H₂​O

Example: Acetic acid (CH₃COOH) reacts with hydrogen peroxide to form peracetic acid (CH₃–C(O)–OOH).

CH₃–COOH + H_2​O₂​ → ​CH₃–C(O)–OOH + H₂​O

2. From Acyl Chloride

Another common method is the reaction of acyl chlorides (RCOCl) with hydrogen peroxide. The chlorine atom is replaced by a peroxy group, producing the corresponding peroxy acid under relatively mild conditions:

R–C(O)Cl + H₂​O₂ ​⟶ R–C(O)–OOH + HCl

Example: Benzoyl chloride (C₆H₅COCl) reacts with hydrogen peroxide to form peroxybenzoic acid (C₆H₅COOOH).

C₆H₅–C(O)Cl + H₂​O₂ ​⟶ C₆H₅–C(O)–OOH + HCl

Chemical Reactions ^[4,5]

1. Epoxidation of Alkene (Prilezhaev Reaction)

Peroxy acid adds oxygen across a carbon–carbon double bond to form an epoxide (a three-membered cyclic ether):

RCH=CHR′ + R′′–C(O)–OOH ⟶ RCH–O–CHR′ + R′′–C(O)–OH

Example: Cyclohexene reacts with m-chloroperoxybenzoic acid (mCPBA) to form cyclohexene oxide.

2. Baeyer–Villiger Oxidation

In this transformation, the peroxy acid oxidizes a ketone (RCOR′) into ester (RCOOR′) by inserting an oxygen atom adjacent to the carbonyl group:

R–C(O)–R′ + R′′–C(O)–OOH ⟶ R–C(O)–O–R′ + R′′–C(O)–OH

Example: Cyclohexanone reacts with peracetic acid to produce ε-caprolactone (oxepan-2-one).

List of Common Peroxy Acids ^[5,6]

Peroxy acids are strong oxidizing agents with various applications in both laboratory and industry. Their weak O–O bond enables key reactions such as epoxidation and Baeyer–Villiger oxidation, making them valuable tools in organic synthesis as well as in disinfection and environmental processes.