What Are Peroxides? Structure, Properties, and Common Uses in Chemistry
Peroxides are defined as groups of compounds in which two oxygen atoms are joined together by a single covalent bond. They have the typical structure of R-O-O-R where R denotes any kind of atom. The O-O the bond present is called the peroxide group. Usually, the oxygen ion has a 2- oxidation number, but the oxygen atoms in the O-O bond has an oxidation number of 1-. Peroxides are unstable compounds and release oxygen when heated to decomposition. Thus, peroxides are strong oxidising agents. Peroxides can be formed by the direct reaction of an element with oxygen.
Hydrogen peroxide (H2O2) is the most common peroxide found. It is almost colourless and its solutions are colourless as well. It is very dangerous when it comes into contact with organic compounds. However, it is biochemically produced and synthesized inside our bodies as a result of the oxidase enzyme range.
Categories
There are a few major classes of peroxides:
Peroxy Acids: Peroxy derivatives of familiar acids, for example, peracetic acid,
Primary Group Peroxides: Compounds with the structure E-O-O-E where E is the main group element,
Metal Peroxides: The main element is a metal, for example, zinc peroxide (ZnO2),
Organic Peroxides: The main element is carbon and the main structure is C-O-O-C or C-O-O-H, for example, tertiary butyl hydroperoxide.
Examples
The most common peroxide is hydrogen peroxide (H2O2) which acts as a bleaching agent. Metallic class oxides which contain the divalent -O-O- bond are also considered peroxide. Na2O2 is one such example. It is also used as a bleaching agent. Organic compounds that contain the -O-O- bond or the peroxide anion are also considered peroxides. These kinds of compounds are explosive in nature. Ozone, ozonides and superoxides are also peroxides but tend to be ignored as peroxides due to their specific characteristics.
There are some compounds that resemble the peroxide formula but do not contain the -O-O- bond such as Manganese peroxide (MnO2).
Uses
Peroxides have a wide range of purposes in everyday life as well as in our bodies. Inside our bodies, hydrogen peroxide is formed during some kind of biochemical process. Peroxides formed inside our bodies are called peroxisomes. Although it is formed momentarily, it is toxic to our cells, especially the DNA. This characteristic feature of hydrogen peroxide is useful for killing bacteria and pathogens inside our bodies. Peroxisomes are used in the synthesis of compounds that are important for the normal functioning of the brain and the lungs. They are also useful for the synthesis of fatty acids and polyamines.
Plants also use peroxides for signalling defence against pathogens.
Peroxides like hydrogen peroxide are used as bleaching agents and in hair products to lighten hair colour. Peroxides are also used to synthesize drugs and some other chemicals.
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FAQs on Peroxide: Meaning, Types & Applications
1. What is a peroxide in chemistry?
A peroxide is a chemical compound containing an oxygen-oxygen single bond (–O–O–). In these compounds, each oxygen atom has an oxidation state of -1, which is an exception to the more common -2 oxidation state for oxygen. This peroxide linkage makes the compounds highly reactive and strong oxidizing agents.
2. What is the general formula and structure of a peroxide?
The general formula for a peroxide is R–O–O–R', where R and R' can be hydrogen, a metal, or an organic group. The defining feature is the peroxide group (–O–O–). For instance, in hydrogen peroxide (H–O–O–H), the simplest peroxide, the molecule has a unique, non-planar, open-book structure.
3. Can you give some common examples of different types of peroxides?
Peroxides are classified based on the groups attached to the oxygen-oxygen bond. Common examples include:
- Inorganic Peroxides: Hydrogen peroxide (H₂O₂), sodium peroxide (Na₂O₂), and barium peroxide (BaO₂).
- Organic Peroxides: Dibenzoyl peroxide, which is used as a catalyst in polymerisation, and tert-butyl hydroperoxide.
- Peroxy Acids: Peracetic acid (CH₃CO₃H) and peroxy-monosulfuric acid (H₂SO₅), also known as Caro's acid.
4. What are the main applications of peroxides in daily life and industry?
Peroxides have a wide range of applications due to their strong oxidizing properties. Key uses include:
- Bleaching Agent: Hydrogen peroxide is widely used for bleaching paper pulp, textiles, and hair.
- Disinfectant & Antiseptic: Dilute hydrogen peroxide solutions are used to clean wounds and sterilise surfaces.
- Polymerisation Initiator: Organic peroxides like benzoyl peroxide initiate free-radical reactions for producing polymers.
- Rocket Propellant: Highly concentrated hydrogen peroxide is used as a propellant in rocketry.
5. What is hydrogen peroxide (H₂O₂), and what makes it unique?
Hydrogen peroxide (H₂O₂) is the simplest peroxide, consisting of two hydrogen atoms and two oxygen atoms linked by a single covalent bond. It is a colourless liquid, slightly more viscous than water. Its uniqueness lies in its dual role; it can act as both an oxidising agent (e.g., in bleaching) and a reducing agent (e.g., with potassium permanganate), depending on the reaction conditions as per the CBSE 2025-26 syllabus.
6. How is the structure of hydrogen peroxide different from that of water, and why is it non-planar?
While water (H₂O) has a simple bent planar structure, hydrogen peroxide (H₂O₂) has a more complex non-planar, open-book structure. The two H–O bonds lie in different planes with a specific dihedral angle. This non-planar arrangement arises from the repulsion between the lone pairs of electrons on the two adjacent oxygen atoms. This structure minimises lone pair-lone pair repulsion, making it more stable than a flat, planar configuration.
7. What is the "peroxide effect" in organic chemistry, and how does it contradict Markovnikov's rule?
The peroxide effect, also known as the anti-Markovnikov addition or Kharasch effect, describes the addition of hydrogen bromide (HBr) to an unsymmetrical alkene in the presence of a peroxide. It contradicts Markovnikov's rule. In the peroxide effect, the reaction proceeds via a free-radical mechanism, causing the bromine atom to attach to the carbon atom of the double bond that has the greater number of hydrogen atoms.
8. What is the oxidation state of oxygen in a peroxide?
In a peroxide, the oxidation state of each oxygen atom is -1. This is an important exception in chemistry, as oxygen typically exhibits a -2 oxidation state in most of its compounds, such as oxides and hydroxides. The -1 state is due to the single covalent bond between the two oxygen atoms in the peroxide anion (O₂²⁻).
9. How do peroxides and superoxides differ in terms of their structure and the oxidation state of oxygen?
The primary difference between peroxides and superoxides lies in their anionic structure and the oxidation state of oxygen.
- Peroxides: Contain the peroxide anion (O₂²⁻), where the two oxygen atoms are joined by a single bond. The oxidation state of each oxygen atom is -1.
- Superoxides: Contain the superoxide anion (O₂⁻), which is a free radical. The average oxidation state of each oxygen atom is -1/2.
10. Why are peroxides, especially organic ones, often unstable and considered hazardous?
Peroxides are unstable and potentially hazardous primarily because of the weakness of the oxygen-oxygen (O–O) single bond. This bond has a low dissociation energy, meaning it can break easily when exposed to heat, light, or shock. This breakage forms highly reactive free radicals, which can initiate rapid, often explosive, decomposition or uncontrolled polymerization reactions, making them a significant safety risk.
11. In what biological processes are peroxides naturally found and what is their role?
Peroxides, especially hydrogen peroxide, are naturally produced in living organisms as byproducts of metabolic processes within organelles called peroxisomes. Their role is twofold:
- Cellular Damage: As a reactive oxygen species (ROS), they can cause oxidative stress if not controlled.
- Signalling and Defence: Organisms use enzymes like catalase to safely break down peroxides. This controlled process is vital for cellular signalling and as a defence mechanism in immune cells to kill pathogens.
