Organic peroxides
Organic peroxides are organic compounds containing the peroxide functional group (ROOR'). If the R' is hydrogen, the compound is called an organic hydroperoxide. Peresters have general structure RC(O)OOR. The O-O bond easily breaks and forms free radicals of the form RO·. Thus, organic peroxides are useful as initiators for some types of polymerisation, such as the epoxy resins used in glass-reinforced plastics. MEKP and benzoyl peroxide are commonly used for this purpose. However, the same property also means that organic peroxides can either intentionally or unintentionally initiate explosive polymerisation in materials with unsaturated chemical bonds, and this process has been used in explosives. Organic peroxides, like their inorganic counterparts, are powerful bleaching agents.[1]
Properties
The O-O bond distance in peroxides is about 1.45 A 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 a bond dissociation energy of 45 - 50 kcal/mol, less than half the strengths of C-C, C-H, and C-O bonds.[2]
The oxidizing tendency of peroxides is related to the electronegativity of the substituents. Electrophilic peroxides are stronger O-atom transfer agents. For hydroperoxides, the O-atom donor tendency correlates with the acidity of the O-H bond. Thus, the order of oxidizing power is CF3CO3H > CH3CO3H > H2O2.
Industrial uses
In polymer chemistry
Organic peroxides find numerous applications, often involving similar chemistry. Thus, peroxides serve as as accelerators, activators, cross-linking agents, curing and vulcanization agents, hardeners, polymerisation initiators, and promoters. Drying oils, as found in many paints and varnishes function via the formation of hydroperoxides.
Methyl ethyl ketone peroxide, benzoyl peroxide and to a smaller degree acetone peroxide are used as initiators for radical polymerization of some resins, e.g. polyester and silicone, often encountered when making fiberglass.
Bleaching and disinfecting agents
Benzoyl peroxide and hydrogen peroxide are used as bleaching and "maturing" agents for treating flour to make its grain release gluten easier; 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 of acne.
In the synthesis of organic compounds
Many organic compounds are prepared using peroxides, most famously epoxides from alkenes. Tert-butyl hydroperoxide (TBHP) is an organic-soluble oxidant employed in a variety of metal-catalyzed oxidations, such as the Halcon process (to give propylene oxide) and the Sharpless epoxidation.
Explosives
Acetone peroxide is an ingredient in explosive for paramilitaries because of its ease of manufacture, despite its instability. It is notorious for its susceptibility to heat, friction, and shock.[3]
Preparation
By autoxidation
Most peroxides are generated either by the addition of O2 to hydrocarbons. Compounds with allylic and benzylic C-H bonds are amenable to this method.[4] Cumene hydroperoxide is an intermediate in the cumene process of industrial synthesis of phenol. Ethers susceptible to this reaction, a typical example being the formation diethyl ether peroxide. The photooxidation of dienes affords dialkyl peroxides.
Many organometallic compounds insert O2 into the M-C bond. Organolithium and Grignard reagents react with O2 to give hydroperoxides upon hydrolysis. Oxymercuration of alkenes followed by reaction with a hydroperoxide proceeds similarly.
From H2O2
Carboxylic acids react with hydrogen peroxide to give peroxy acids.
Reactions
Some peroxide reactions are:
- organic reduction to alcohols with lithium aluminium hydride or phosphite esters
- cleavage to ketones and alcohols in the base catalyzed Kornblum–DeLaMare rearrangement
Safety
As they combine unstably bound oxygen together with hydrogen and carbon in the same molecule, organic peroxides ignite easily and burn rapidly and intensely. The same applies to organic materials contaminated with organic peroxides. Since peroxides can form spontaneously in some materials, some caution must be exercised with such "peroxide forming materials", such as the common ethers diethyl ether, tetrahydrofuran or ethylene glycol dimethyl ether.
Acetone peroxide, a powerful explosive, is an unwanted and dangerous byproduct of several chemical reactions, ranging from synthesis of MDMA (where it is a by-product of isosafrole oxidation in acetone) to industrial production of phenol (where the second product of the cumene process, acetone, is partially oxidized to peroxide on the second reaction step).
Accidental preparation of organic peroxides can occur by mixing ketone solvents (most commonly acetone) with waste materials containing hydrogen peroxide or other oxidizers and leaving the mixture standing for several hours.
Organic peroxides tend to react, explosively, with metals.
Organic peroxides are sensitive to light and have to be stored in darkness. Some decompose at room temperature and release gaseous products; gas ejectors on the lids of the containers are required. Based on the Self Accelerating Decomposition Temperature, some peroxides have to be stored refrigerated.
Disposal of peroxides
Small amounts of organic peroxides can be disposed of by careful burning of a substance diluted to under 10% or hydrolyzed. For hydrolysis, using a solution consisting of 80 parts water, 20 parts sodium hydroxide, and 0.3 parts surfactant to allow easier wetting of the crystals of the peroxide. The recommended amount of the solution is 10 times the weight of the peroxide. The peroxide has to be slowly poured into the solution, with constant stirring to avoid local overheating. The reaction is slightly exothermic, but additional cooling is not required. The reaction is slow, and requires stirring for some 12 to 24 hours; the peroxide can then be considered decomposed.
See also
External links
References
- ^ Herbert Klenk, Peter H. Götz, Rainer Siegmeier, Wilfried Mayr "Peroxy Compounds, Organic" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a19_199
- ^ "A Reassessment of the Bond Dissociation Energies of Peroxides. An ab Initio Study" Robert D. Bach, Philippe Y. Ayala, H. B. Schlegel J. Am. Chem. Soc., 1996, volume 118, pp 12758–12765. doi:10.1021/ja961838i
- ^ See for example the 2006 transatlantic aircraft plot.
- ^ H. B. Knight and Daniel Swern (1963). "Tetralin hydroperoxide". Organic Syntheses; Collected Volumes, vol. 4, p. 895.