Inorganic chemistry,organic peroxides areorganic compounds containing theperoxidefunctional group (R−O−O−R′). If the R′ ishydrogen, the compounds are calledhydroperoxides, which are discussed in that article. The O−O bond of peroxides easily breaks, producingfree radicals of the formRO• (the dot represents anunpaired electron). Thus, organic peroxides are useful asinitiators for some types ofpolymerization, such as theacrylic, unsaturated polyester, andvinyl esterresins used inglass-reinforced plastics.MEKP andbenzoyl peroxide are commonly used for this purpose. However, the same property also means that organic peroxides can explosively combust. Organic peroxides, like their inorganic counterparts, are often powerfulbleaching agents.[1]
Organic peroxides are classified (i) by the presence or absence of a hydroxyl (−OH) terminus and (ii) by the presence of alkyl vs acyl substituents.[2]
One gap in the classes of organic peroxides is diphenyl peroxide. Quantum chemical calculations predict that it undergoes a nearly barrierless reaction akin to thebenzidine rearrangement.[3]
The O−Obond length in peroxides is about 1.45 Å, and the R−O−O angles (R = H, C) are about 110° (water-like). Characteristically, the C−O−O−R (R = H, C) dihedral angles are about 120°. The O−O bond is relatively weak, with abond dissociation energy of 45–50 kcal/mol (190–210 kJ/mol), less than half the strengths of C−C, C−H, and C−O bonds.[4][5]
Peroxides play important roles in biology. Hundreds of peroxides and hydroperoxides are known, being derived from fatty acids, steroids, and terpenes.[6] Theprostaglandins are biosynthesized by initial formation of a bicyclic peroxide ("endoperoxide") derived fromarachidonic acid.[7]
Many aspects of biodegradation or aging are attributed to the formation and decay of peroxides formed from oxygen in air. Countering these effects, an array of biological and artificialantioxidants destroy peroxides.
Infireflies, oxidation ofluciferins, which is catalyzed byluciferases, yields a peroxy compound1,2-dioxetane. The dioxetane is unstable and decays spontaneously tocarbon dioxide and excitedketones, which release excess energy by emitting light (bioluminescence).[8]
Many peroxides are used as aradical initiators, e.g., to enablepolymerization of acrylates. Industrial resins based on acrylic and/ormethacrylic acid esters are invariably produced by radical polymerization with organic peroxides at elevated temperatures.[9] The polymerization rate is adjusted by suitable choice of temperature and type of peroxide.[10]
Methyl ethyl ketone peroxide,benzoyl peroxide and to a smaller degreeacetone peroxide are used as initiators forradical polymerization of somethermosets, e.g.unsaturated polyester andvinyl ester resins, often encountered when making fiberglass orcarbon fiber composites (CFRP), with examples including boats, RV units, bath tubs, pools, sporting equipment, wind turbine blades, and a variety of industrial applications.
Benzoyl peroxide,peroxyesters/peroxyketals, andalkylperoxy monocarbonates are used in production ofpolystyrene,expanded polystyrene, andHigh Impact Polystyrene, and benzoyl peroxide is utilized for many acrylate based adhesive applications.
Thermoplastic production techniques for many industrial polymerization applications include processes which are carried out in bulk, solution, or suspension type batches. Relevant polymers include:polyvinyl chloride (PVC),low-density polyethylene (LDPE),high-density polyethylene (HDPE),polymethyl methacrylate (PMMA),Polystyrene, andPolycarbonates.
Benzoyl peroxide andhydrogen peroxide are used asbleaching and "maturing" agents for treatingflour to make its grain releasegluten more easily; the alternative is letting the flour slowly oxidize by air, which is too slow for the industrialized era.Benzoyl peroxide is an effective topical medication for treating most forms ofacne.
Dialkyl peroxides, e.g.,dicumyl peroxide, are synthesized by addition of hydrogen peroxide to alkenes or by O-alkylation of hydroperoxides.
Diacyl peroxides are typically prepared by treating hydrogen peroxide with acid chlorides or acid anhydrides in the presence of base:[1]
The reaction competes with hydrolysis of the acylating agent but the hydroperoxide anion is a superior nucleophile relative to hydroxide. Unsymmetrical diacyl peroxides can be produced by treating acyl chlorides with the peroxy acid.
Peresters, an example beingtert-Butyl peroxybenzoate, are produced by treating acid anhydrides or acid chlorides with hydroperoxides.
Cyclic peroxides can be obtained bycycloaddition ofsinglet oxygen (generated by UV radiation) to dienes. An important example isrubrene. Six-membered cyclic peroxides are called endo peroxides.[11] The four-membereddioxetanes can be obtained by 2+2cycloaddition of oxygen toalkenes.[12][13]
The hazards associated with storage of ethers in air is attributed to the formation of hydroperoxides via the direct albeit slow reaction of triplet oxygen withC-H bonds.
Organic peroxides are widely used to initiatepolymerization ofolefins, e.g. the formation ofpolyethylene. A key step ishomolysis:
The tendency to homolyze is also exploited to modify polymers bygrafting orvisbreaking, orcross-link polymers to create athermoset. When used for these purposes, the peroxide is highly diluted, so the heat generated by theexothermic decomposition is safely absorbed by the surrounding medium (e.g. polymer compound oremulsion).
Especially when in concentrated form, organic peroxides can decompose by self-oxidation, since organic peroxides contain both an oxidizer (the O-O bond) and fuel (C-H and C-C bonds). A "self-accelerating decomposition" occurs when therate of peroxide decomposition generates heat at a faster rate than it can be dissipated to the environment. Temperature is the main factor in the rate of decomposition. The lowest temperature at which a packaged organic peroxide will undergo a self-accelerating decomposition within a week is defined as theself-accelerating decomposition temperature (SADT). A large fire at the Arkema Chemical Plant in Crosby, Texas (USA) in 2017 was caused by the decomposition of various organic peroxides following power failure and subsequent loss of cooling systems.[14] This occurred due to extreme flooding fromHurricane Harvey, which destroyed main and back-up power generators at the site.[14]
Hydroperoxides are intermediates or reagents in major commercial processes. In thecumene process, acetone and phenol are produced by decomposition ofcumene hydroperoxide (Me = methyl):
Anthrahydroquinone reacts spontaneously with oxygen to form anthraquinone and hydrogen peroxide, possibly through some organic peroxide intermediate. After extracting the hydrogen peroxide the anthraquinone is catalytically reduced to anthrahydroquinone and reused in the process. There are otherhydroquinones reacting in a similar fashion.
Organoperoxides can be reduced toalcohols withlithium aluminium hydride, as described in this idealized equation:
Thephosphite esters and tertiary phosphines also effect reduction:
Cleavage to ketones and alcohols occurs in the base-catalyzedKornblum–DeLaMare rearrangement, which involves the breaking of bonds within peroxides to form these products.
Some peroxides aredrugs, whose action is based on the formation of radicals at desired locations in the organism. For example,artemisinin and its derivatives, such asartesunate, possess the most rapid action of all current drugs againstfalciparummalaria.[15] Artesunate is also efficient in reducing egg production inSchistosoma haematobium infection.[16]
tert-Butyl hydroperoxide is used for epoxidation and hydroxylation reagents in conjunction with metal catalysts.[17]
Several analytical methods are used for qualitative and quantitative determination of peroxides.[18] A simple qualitative detection of peroxides is carried out with theiodine-starch reaction.[19] Here peroxides, hydroperoxides or peracids oxidize the addedpotassium iodide intoiodine, which reacts withstarch producing a deep-blue color. Commercial paper indicators using this reaction are available. This method is also suitable for quantitative evaluation, but it can not distinguish between different types of peroxide compounds. Discoloration of variousindigo dyes in presence of peroxides is used instead for this purpose.[20] For example, the loss of blue color in leuco-methylene blue is selective for hydrogen peroxide.[21]
Quantitative analysis of hydroperoxides can be performed using potentiometrictitration withlithium aluminium hydride.[22] Another way to evaluate the content of peracids and peroxides is the volumetric titration withalkoxides such assodium ethoxide.[23]
Each peroxy group is considered to contain one active oxygen atom. The concept of active oxygen content is useful for comparing the relativeconcentration of peroxy groups in formulations, which is related to the energy content. In general, energy content increases with active oxygen content, and thus the higher themolecular weight of the organic groups, the lower the energy content and, usually, the lower the hazard.
The termactive oxygen is used to specify the amount of peroxide present in any organic peroxide formulation. One of the oxygen atoms in each peroxide group is considered "active". The theoretical amount of active oxygen can be described by the following equation:[24]
wherep is the number of peroxide groups in the molecule, andm is themolecular mass of the pure peroxide.
Organic peroxides are often sold as formulations that include one or morephlegmatizing agents. That is, for safety sake or performance benefits the properties of an organic peroxide formulation are commonly modified by the use of additives to phlegmatize (desensitize), stabilize, or otherwise enhance the organic peroxide for commercial use. Commercial formulations occasionally consist of mixtures of organic peroxides, which may or may not be phlegmatized.
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Peroxides are also strong oxidizers and easily react with skin, cotton and wood pulp.[25] For safety reasons, peroxidic compounds are stored in a cool, opaque container, as heating and illumination accelerate theirchemical reactions. Small amounts of peroxides, which emerge from storage or reaction vessels are neutralized using reducing agents such asiron(II) sulfate. Safety measures in industrial plants producing large amounts of peroxides include the following:
1) The equipment is located within reinforced concrete structures with foil windows, which would relieve pressure and not shatter in case of explosion.
2) The products are bottled in small containers and are moved to a cold place promptly after the synthesis.
3) The containers are made of non-reactive materials such as stainless steel, some aluminium alloys or dark glass.[26]
For safe handling of concentrated organic peroxides, an important parameter is temperature of the sample, which should be maintained below theself accelerating decomposition temperature of the compound.[27]
The shipping of organic peroxides is restricted. TheUS Department of Transportation lists organic peroxide shipping restrictions and forbidden materials in 49 CFR 172.101 Hazardous Materials Table based on the concentration and physical state of the material:
| Chemical name | CAS Number | Prohibitions |
|---|---|---|
| Acetyl acetone peroxide | 37187-22-7 | > 9% by massactive oxygen |
| Acetyl benzoyl peroxide | 644-31-5 | solid, or > 40% in solution |
| Ascaridole | 512-85-6 | (organic peroxide) |
| tert-Butyl hydroperoxide | 75-91-2 | > 90% in solution (aqueous) |
| Di-(1-naphthoyl)peroxide | 29903-04-6 | |
| Diacetyl peroxide | 110-22-5 | solid, or > 25% in solution |
| Ethyl hydroperoxide | 3031-74-1 | |
| Methyl ethyl ketone peroxide | 1338-23-4 | > 9% by mass active oxygen in solution |
| Methyl isobutyl ketone peroxide | 37206-20-5 | > 9% by mass active oxygen in solution |
