FEATURE Peroxides and peroxide- forming compounds By Donald E. Clark Bretherick5 included a discussion of nated. However, concentrated hydro- organic peroxide5 in a chapter on gen peroxide (Ͼ30%), in contact with norganic and organic peroxides, highly reactive and unstable com- ordinary combustible materials (e.g., because of their exceptional reac- pounds and used “oxygen balance” to fabric, oil, wood, or some resins) Itivity and oxidative potential are predict the stability of individual com- poses significant fire or explosion haz- widely used in research laboratories. pounds and to assess the hazard po- ards. Peroxides of alkali metals are not This review is intended to serve as a tential of an oxidative reaction. Jack- particularly shock sensitive, but can 6 guide to the hazards and safety issues son et al. addressed the use of decompose slowly in the presence of associated with the laboratory use, peroxidizable chemicals in the re- moisture and may react violently with handling, and storage of inorganic and search laboratory and published rec- a variety of substances, including wa- organic peroxy-compounds and per- ommendations for maximum storage ter. Thus, the standard iodide test for oxide-forming compounds. time for common peroxide-forming peroxides must not be used with these The relatively weak oxygen-oxygen laboratory solvents. Several solvents, water-reactive compounds.1 linkage (bond-dissociation energy of (e.g., diethyl ether) commonly used in Inorganic peroxides are used as ox- 20 to 50 kcal moleϪ1) is the character- the laboratory can form explosive re- idizing agents for digestion of organic istic structure of organic and inor- action products through a relatively samples and in the synthesis of or- ganic peroxide molecules, and is the slow oxidation process in the pres- ganic peroxides. They react violently basis for their reactivity and tendency ence of atmospheric oxygen. The risk with reducing agents and with several for spontaneous decomposition. The of explosion can be greatly reduced by classes of organic compounds to gen- unusual weakness of the -O-O- bond following storage and handling prac- erate organic peroxide and hydroper- is probably a consequence of the mo- tices that are compatible with the oxide products.4 Dry Caro’s reagent 1,6 lecular and electronic structure of per- properties of these materials. More (monopersulfuric acid — K2O2 ϩ con. 7 oxide molecules and of the relatively recently, Kelly reviewed the chemis- H2SO4) reacts readily with carbonyl high electronegative character of the try and safe handling of peroxide- compounds (in the synthesis of or- oxygen atoms. As a class, peroxides forming chemicals and included pro- ganic peroxides) and can react explo- are exceptionally prone to violent de- cedures on detection and removal of sively with aldehydes and alco- composition that can be initiated by peroxides from laboratory solvents. hols.3,4,5 Similar characteristics are heat, mechanical shock, or friction, Safety awareness, prudent handling associated with other inorganic perox- especially in the presence of certain and proper storage are essential when ides and persalts. Peroxides may form 1,2 catalysts and promoters. working with these compounds. on the surface of finely divided alkali The hazards of inorganic and or- metals and their amides and readily ganic peroxides and peroxide-forming form superoxides, and ozonides such chemicals have been long recognized INORGANIC PEROXIDES AND as KO3.8 so that most relevant information is PEROXYACIDS Inorganic per-compounds (e.g., so- now found in text books on organic The O-O bond of hydrogen peroxide dium peroxide, sodium perborate, and chemistry and laboratory safety. A is covalent. In solution, persalts of al- sodium persulfate) may react with in- comprehensive three-volume series kali metals (M2O2) are ionized to the organic cobalt and copper com- on the chemistry of organic peroxides monopositive alkali metal ion (Mϩ), pounds, iron or iron compounds, includes a chapter on safety issues as- and the dinegative peroxide ion metal oxide salts, and acids or bases 1,2,3,4 Ϫ2 sociated with these materials. (O2 ). Metallic peroxides are consid- resulting in rapid, uncontrolled de- ered to be salts of hydrogen peroxide composition reactions. Persulfates and react with water to produce (e.g., peroxy sulfate) are highly reac- 1 Donald E. Clark, Ph.D., FAIC, RBP H2O2. tive and may ignite when in contact is the Chemical and Biological Safety Hydrogen peroxide alone is not ex- with metals.2,4,8 Officer at Texas at A&M University, plosive and has a long shelf life if it is Perhalogen compounds (e.g., peroxy College Station, TX, USA. handled properly and is not contami- chloride) are extremely shock sensitive 12 © Division of Chemical Health and Safety of the American Chemical Society 1074-9098/01/$20.00 Published by Elsevier Science Inc. PII S1074-9098(01)00247-7 and should be avoided unless abso- of the properties, hazards and use of agents. Each peroxide compound is lutely necessary. They can react with perchlorates, including perchloric characterized by a specific, condition- acids (especially organic acids) to pro- acid in the CRC Handbook of Labo- dependent rate of decomposition. A duce near-anhydrous perchloric acid. ratory Safety. He stated that, “The change in critical parameters (e.g., in- Perhalogen compounds of alkali metal most detailed available account of the creased temperature) can promote and alkali earth elements are explosive, chemistry of perchloric acid and a ref- rapid increase in the decomposition but are less sensitive than heavy metal erence highly recommended to every- rate, culminating in a violent explo- perchlorates and organic perchlorates. one who will be working with per- sion.1 The potential energy of organic Ammonium periodate is especially sen- chlorates is given by J. S. Schumacher peroxides is low compared to conven- sitive to friction. Perchlorates (e.g. mag- in the American Chemical Society tional explosives, but high enough to nesium perchlorate [Mg(ClO4)2] mar- Monograph Series 146, Perchlorates, be very hazardous. keted as “Anhydrone”) should not be Their Properties, Manufacture, and The acute toxicity of organic perox- used as a drying agent if in contact with Uses.” Procedures for dismantling an ides is relatively low (i.e., peracetic organic compounds or with a strong de- exhaust ventilation system suspected acid: oral LD50, rat; 1540 mg/kg; der- hydrating acid (such as in a drying train of perchlorate contamination are in- mal LD50, rabbit 1410 mg/kg). How- 9 that includes a sulfuric acid bubble cluded in the CRC Handbook. Pub- ever, most are highly irritating to skin, 9,10 counter) is possible. Perchlorate lication of an updated version of eyes, and mucous membranes and hazards are multiplied by increased Schilt’s Perchloric Acid and Per- peracetic acid may be a weak carcin- 11 temperature, dryness and/or perchlor- chlorates is pending. ogen in mice. There are no data to ate content. Abuse of any one or more Dilute hydrogen peroxide solutions suggest that exposure to peracetic acid of these parameters accounts for nearly (i.e., 3%) are contact irritants and produces carcinogenic, reproductive all perchlorate related incidents. higher concentrations can cause se- or developmental toxicity in humans. Perhalogen acids (e.g., hot perchlo- vere chemical burns. Peroxide chemi- Other organic peracids (e.g., perben- ric acid) are widely used as potent ox- cal burns should be washed gently but zoic acid and m-chloroperbenzoic idizing agents. Cold 70% perchloric thoroughly and may require medical acid) are less toxic, less volatile and 5,10 acid is a strong acid but is not a strong attention. The acute toxicity of per- less hazardous to handle than perace- oxidizing agent. The oxidizing power chloric acid is moderate [oral LD50 tic acid.12 Exposure to peroxides and of perchloric acid increases with tem- (rat) 1100 mg/kg; oral LD50 (dog) 400 associated materials, including vapors perature and hot, concentrated solu- mg/kg]. It is a potent irritant at low and aerosols, should be minimized. tions can be very dangerous. The max- concentrations and is very corrosive Aliphatic peroxyacids have a sharp, imum concentration of perchloric and can cause severe burns to skin, unpleasant odor, the intensity of acid commercially available is an eyes and mucous membranes at higher which decreases with increasing mo- aqueous solution of 70% HClO4. concentrations (especially when hot). lecular weight.1,4 Liquid peroxides However, perchloric acid solutions Perchloric acid has not been found to may penetrate rubber gloves upon ex- can become highly concentrated from be carcinogenic or to cause reproduc- tended exposure.5 evaporation (e.g., spill or heated di- tive, or developmental toxicity in gestion procedure) and anhydrous humans.12 perchloric acid is capable of sponta- neous explosion. Highly concentrated ORGANIC PEROXIDES: SYNTHESIS perchloric should be disposed of ORGANIC PEROXIDES AND OTHER AND USE within 10 days or if any discoloration PER-COMPOUNDS The synthesis of organic peroxides fre- develops. Anhydrous perchloric is ex- Organic peroxides are characterized quently begins with hydrogen perox- plosive in contact with wood, paper, by the bivalent -O-O- structure and ide. Hydroperoxides can be prepared carbon, and organic solvents. Diges- are considered to be structural deriv- by reaction of alkyl halides, esters of tion of organic material in boiling per- atives of hydrogen peroxide with one sulfuric or sulfonic acids, or alcohols chloric acid must be conducted is a or both of the hydrogen atoms re- with hydrogen peroxide in alkaline so- chemical fume hood that is specifi- placed by an organic moiety. Because lution. Peroxy groups may be intro- cally designed for that purpose. Per- of the weak peroxide bond, organic duced into susceptible organic mole- chloric acid fume hoods include a spe- peroxides are predisposed to sponta- cules by treatment with a hydro- cial wash-down feature to prevent neous decomposition.