Organic peroxides

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Organic peroxide
)
The general structure of an organic peroxide

In

bleaching agents.[1]

Types of organic peroxides

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]

  • Examples of organic peroxides
  • tert-Butyl hydroperoxide, a hydroperoxide (formula: ROOH), which is used to epoxide alkenes.
    tert-Butyl hydroperoxide, a hydroperoxide (formula: ROOH), which is used to epoxide alkenes.
  • Dicumyl peroxide, a dialkyl peroxide (formula: ROOR), which is used to initiate polymerizations.
    Dicumyl peroxide, a dialkyl peroxide (formula: ROOR), which is used to initiate polymerizations.
  • tert-butylperoxybenzoate, a peroxy ester (formula: RCO3R') that used as a radical initiator.
    tert-butylperoxybenzoate, a peroxy ester (formula: RCO3R') that used as a radical initiator.
  • dibenzoyl peroxide, a diacyl peroxide (formula: (RCO2)2)) is also used as an initiator for polymerizations.
    dibenzoyl peroxide
    , a diacyl peroxide (formula: (RCO2)2)) is also used as an initiator for polymerizations.
  • Peroxyacetic acid, a peroxycarboxylic acid (formula: (RCO3H), is a reagent in organic synthesis.
    peroxycarboxylic acid (formula: (RCO3H), is a reagent in organic synthesis
    .
  • Prostaglandin G2, an endo peroxide, the precursor to other prostaglandins
    Prostaglandin G2, an endo peroxide, the precursor to other prostaglandins

One gap in the classes of organic peroxides is diphenyl peroxide. Quantum chemical calculations predict that it undergoes a nearly barrierless reaction akin to the

benzidine rearrangement.[3]

Properties

The O−O

bond 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]

Biology

The peroxide ascaridole, derived from terpinene.

Peroxides play important roles in biology. Hundreds of peroxides and hydroperoxides are known, being derived from fatty acids, steroids, and terpenes.[6] The prostaglandins are biosynthesized by initial formation of a bicyclic peroxide ("endoperoxide") derived from arachidonic 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 artificial antioxidants destroy peroxides.

In

1,2-dioxetane. The dioxetane is unstable and decays spontaneously to carbon dioxide and excited ketones, which release excess energy by emitting light (bioluminescence).[8]

Loss of CO2 of a dioxetane, giving rise to an excited ketone, which relaxes by emitting light.

Industrial uses

In polymer chemistry

Many peroxides are used as a radical initiators, e.g., to enable polymerization of acrylates. Industrial resins based on acrylic and/or methacrylic 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]

carbon fiber composites
(CFRP), with examples including boats, RV units, bath tubs, pools, sporting equipment, wind turbine blades, and a variety of industrial applications.

High 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, and

Polycarbonates
.

Bleaching and disinfecting agents

acne
.

Preparation

From hydrogen peroxide

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]

H2O2 + 2 RCOCl → (RCO2)2 + 2 HCl
H2O2 + (RCO)2O → (RCO2)2 + H2O

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 being tert-Butyl peroxybenzoate
, are produced by treating acid anhydrides or acid chlorides with hydroperoxides.

From O2

Cyclic peroxides can be obtained by

dioxetanes can be obtained by 2+2 cycloaddition of oxygen to alkenes.[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 with

C-H bonds
.

Reactions

Homolysis

Organic peroxides are widely used to initiate

olefins, e.g. the formation of polyethylene. A key step is homolysis
:

ROOR ⇌ 2 RO.

The tendency to homolyze is also exploited to modify polymers by

exothermic decomposition is safely absorbed by the surrounding medium (e.g. polymer compound or emulsion
).

Self-oxidation

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 the rate 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 the self-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 from Hurricane Harvey, which destroyed main and back-up power generators at the site.[14]

Cumene process

Hydroperoxides are intermediates or reagents in major commercial processes. In the cumene process, acetone and phenol are produced by decomposition of cumene hydroperoxide (Me = methyl):

C6H5CMe2(O2H) → C6H5OH + O=CMe2

Reduction

Organoperoxides can be reduced to alcohols with lithium aluminium hydride, as described in this idealized equation:

4 ROOH + LiAlH4 → LiAlO2 + 2 H2O + 4 ROH

The phosphite esters and tertiary phosphines also effect reduction:

ROOH + PR3 → P(OR)3 + ROH

Cleavage to ketones and alcohols occurs in the base-catalyzed Kornblum–DeLaMare rearrangement, which involves the breaking of bonds within peroxides to form these products.

Some peroxides are drugs, whose action is based on the formation of radicals at desired locations in the organism. For example, artemisinin and its derivatives, such as artesunate, possess the most rapid action of all current drugs against falciparum malaria.[15] Artesunate is also efficient in reducing egg production in Schistosoma haematobium infection.[16]

Organic synthesis

tert-Butyl hydroperoxide is used for epoxidation and hydroxylation reagents in conjunction with metal catalysts.[17]

Analysis of peroxides

Iodine-starch test
. Note the blackening (left) of initially yellowish (right) starch.

Several analytical methods are used for qualitative and quantitative determination of peroxides.

iodine-starch reaction.[19] Here peroxides, hydroperoxides or peracids oxidize the added potassium iodide into iodine, which reacts with starch 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 various indigo 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 potentiometric titration with lithium aluminium hydride.[22] Another way to evaluate the content of peracids and peroxides is the volumetric titration with alkoxides such as sodium ethoxide.[23]

Active oxygen in peroxides

Each peroxy group is considered to contain one active oxygen atom. The concept of active oxygen content is useful for comparing the relative

molecular weight
of the organic groups, the lower the energy content and, usually, the lower the hazard.

The term active 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]

where p is the number of peroxide groups in the molecule, and m is the molecular mass of the pure peroxide.

Organic peroxides are often sold as formulations that include one or more

phlegmatizing 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.

Safety

The GHS transport pictogram for organic peroxides.

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 their chemical reactions. Small amounts of peroxides, which emerge from storage or reaction vessels are neutralized using reducing agents such as iron(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 the self accelerating decomposition temperature of the compound.[27]

The shipping of organic peroxides is restricted. The

US 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 mass
active 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

See also

External links

  • Organic Peroxide Producers Safety Division
  • OSH Answers – organic peroxides
  • "The Perils of Peroxides". carolina.com. Burlington, NC: Carolina Biological Supply Company. Archived from the original on 2007-12-18.
  • European Organic Peroxide Safety Group

References