pose explosively when exposed to a ground glass joint or other sharp surfaces (Organic Syntheses, 1973, 1961).

4.D.3.2 Azos, Peroxides, and Peroxidizables

Organic azo compounds and peroxides are among the most hazardous substances handled in the chemical laboratory but are also common reagents that often are used as free radical sources and oxidants. They are generally low-power explosives that are sensitive to shock, sparks, or other accidental ignition. They are far more shock sensitive than most primary explosives such as TNT. Inventories of these chemicals should be limited and subject to routine inspection. Many require refrigerated storage. Liquids or solutions of these compounds should not be cooled to the point at which the material freezes or crystallizes from solution, however, because this significantly increases the risk of explosion. Refrigerators and freezers storing such compounds should have a backup power supply in the event of electricity loss. Users should be familiar with the hazards of these materials and trained in their proper handling.

Certain common laboratory chemicals form peroxides on exposure to oxygen in air (see Tables 4.8 and 4.9). Over time, some chemicals continue to build peroxides to potentially dangerous levels, whereas others accumulate a relatively low equilibrium concentration of peroxide, which becomes dangerous only after being concentrated by evaporation or distillation. (See Chapter 6, section 6.G.3.) The peroxide becomes concentrated because it is less volatile than the parent chemical. A related class of compounds includes inhibitor-free monomers prone to free radical polymerization that on exposure to air can form peroxides or other free radical sources capable of initiating violent polymerization. Note that care must be taken when storing and using these monomers—most of the inhibitors used to stabilize these compounds require the presence of oxygen to function properly, as described below. Always refer to the MSDS and supplier instructions for proper use and storage of polymerizable monomers.

Essentially all compounds containing C—H bonds pose the risk of peroxide formation if contaminated with various radical initiators, photosensitizers, or catalysts. For instance, secondary alcohols such as isopropanol form peroxides when exposed to normal fluorescent lighting and contaminated with photosensitizers, such as benzophenone. Acetaldehyde, under normal conditions, autoxidizes to form acetic acid. Although this autoxidation proceeds through a peroxy acid intermediate, the steady-state concentrations of that intermediate are extremely low and pose no hazard. However, in the presence of catalysts (Co2+) and under the proper conditions of ultraviolet light, temperature, and oxygen concentration, high concentrations of an explosive peroxide can be formed. The chemicals described in Table 4.9 represent only those materials that form peroxides in the absence of such contaminants or otherwise atypical circumstances.

TABLE 4.8 Classes of Chemicals That Can Form Peroxides


Class A: Chemicals that form explosive levels of peroxides without concentration

Isopropyl ether

Sodium amide (sodamide)

Butadiene

Tetrafluoroethylene

Chlorobutadiene (chloroprene)

Divinyl acetylene

Potassium amide

Vinylidene chloride

Potassium metal

Class B: These chemicals are a peroxide hazard on concentration (distillation/evaporation). A test for peroxide should be performed if concentration is intended or suspected.* (See Chapter 6, section 6.C.3)

Acetal

Dioxane (p-dioxane)

Cumene

Ethylene glycol dimethyl

Cyclohexene

ether (glyme)

Cyclooctene

Furan

Cyclopentene

Methyl acetylene

Diaacetylene

Methyl cyclopentane

Dicyclopentadiene

Methyl-isobutyl ketone

Diethylene glycol dimethyl

Tetrahydrofuran

ether (diglyme)

Tetrahydronaphthalene

Diethyl ether

Vinyl ethers

Class C: Unsaturated monomers that may autopolymerize as a result of peroxide accumulation if inhibitors have been removed or are depleteda

Acrylic acid

Styrene

Butadiene

Vinyl acetate

Chlorotrifluoroethylene

Vinyl chloride

Ethyl acrylate

Vinyl pyridine

Methyl methacrylate


*These lists are illustrative, not comprehensive.
SOURCES: Jackson et al. (1970) and Kelly (1996).

Although not a requirement, it is prudent to discard old samples of organic compounds of unknown origin or history, or those prone to peroxidation if contaminated; secondary alcohols are a specific example.

Class A compounds are especially dangerous when

TABLE 4.9 Types of Compounds Known to Autoxidize to Form Peroxides


Ethers containing primary and secondary alkyl groups (never distill an ether before it has been shown to be free of peroxide)

Compounds containing benzylic hydrogens

Compounds containing allylic hydrogens (C=C—CH)

Compounds containing a tertiary C—H group (e.g., decalin and 2,5-dimethylhexane

Compounds containing conjugated, polyunsaturated alkenes and alkynes (e.g., 1,3-butadiene, vinyl acetylene)

Compounds containing secondary or tertiary C—H groups adjacent to an amide (e.g., 1-methyl-2-pyrrolidinone)




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