Peroxides diacyl

Diacyl Peroxides. Table 3 Hsts several commercial diacyl peroxides and their corresponding 10-h half-hfe temperatures, deterrnined in benzene and other solvents (32). Although diacyl peroxides cleave at the oxygen—oxygen bond, decarboxylation can occur, either simultaneously or subsequentiy (eq. 22) ... 

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

Diacyl peroxides are used in a broad spectmm of apphcations, including curing of unsaturated polyester resin compositions, cross-linking of elastomers, production of poly(vinyl chloride), polystyrene, and polyacrjlates, and in many nonpolymeric addition reactions. 

Aromatic diacyl peroxides such as dibenzoyl peroxide (BPO) [94-36-0] may be used with promoters to lower the usehil decomposition temperatures of the peroxides, although usually with some sacrifice to radical generation efficiency. The most widely used promoter is dimethylaniline (DMA). The BPO—DMA combination is used for hardening (curing) of unsaturated polyester resin compositions, eg, body putty in auto repair kits. Here, the aromatic amine promoter attacks the BPO to initially form W-benzoyloxydimethylanilinium benzoate (ion pair) which subsequentiy decomposes at room temperature to form a benzoate ion, a dimethylaniline radical cation, and a benzoyloxy radical that, in turn, initiates the curing reaction (33) ... 

Peroxyesters, particularly those with a-hydrogens or conjugated double bonds, are susceptible to iaduced decomposition under certain conditions, but they are generally less susceptible than diacyl peroxides. Lower molecular weight peroxyesters that have some water solubiUty can be hydroly2ed. 

This is basically the same type of induced decomposition that occurs with other peroxide classes, eg, the dialkyl peioxydicaibonates and diacyl peroxides. Table 8. Commercial rerf-Alkyl Hydroperoxides ... 

Organic peroxides can be classified according to peroxide stmcture. There are seven principal classes hydroperoxides dialkyl peroxides a-oxygen substitued alkyl hydroperoxides and dialkyl peroxides primary and secondary ozonides peroxyacids diacyl peroxides (acyl and organosulfonyl peroxides) and alkyl peroxyesters (peroxycarboxylates, peroxysulfonates, and peroxyphosphates). 

Petoxycatboxyhc acids also have been prepared by the reaction of acid chlorides, anhydrides, or boric-catboxyhc anhydrides with hydrogen or sodium peroxide. These reactions ate carried out at low temperature and with excess peroxide to avoid the formation of diacyl peroxides (44,168,181,184). 

Also, basic hydrolysis or pethydtolysis of diacyl peroxides has been used to produce petoxycatboxyhc acids (44,181). Pethydtolysis produces two moles of the petoxycatboxyhc acid salt ... 

Acyl peroxides of structure (20) are known as diacyl peroxides. In this structure and are the same or different and can be alkyl, aryl, heterocychc, imino, amino, or fiuoro. Acyl peroxides of stmctures (21), (22), (23), and (24) are known as dialkyl peroxydicarbonates, 00-acyl O-alkyl monoperoxycarbonates, acyl organosulfonyl peroxides, and di(organosulfonyl) peroxides, respectively. and R2 ia these stmctures are the same or different and generally are alkyl and aryl (4—6,44,166,187,188). Many diacyl peroxides (20) and dialkyl peroxydicarbonates (21) ate produced commercially and used ia large volumes. 

Physical Properties. Almost all Hquid diacyl peroxides (20) and concentrated solutions of the soHd compounds are unstable to normal ambient temperature storage many must be stored well below 0°C. Most of the soHd compounds are stable at ca 20°C but many are shock-sensitive (187). Other physical constants and properties have been reviewed (187,188). The melting poiats and refractive iadexes of some acyl peroxides are Hsted ia Tables 10-12. 

Chemical Properties. Diacyl peroxides (20) decompose when heated or photoly2ed (<300 mm). Although photolytic decompositions generally produce free radicals (198), thermal decompositions can produce nonradical and radical iatermediates, depending on diacyl peroxide stmcture. Symmetrical aUphatic diacyl peroxides of certain stmctures, ie, diacyl peroxides (20, = alkyl) without a-branches or with a mono-cx-methyl... 

Of these diacyl peroxides the ones that generate the most stable radicals (R ) are the most unstable diacyl peroxides. Most other diacyl peroxides decompose by competing free-radical and polar decomposition, ie, carboxy iaversion (188). Carboxy iaversion occurs to a much greater extent with certain diacyl peroxides having unsymmetrical diacyl peroxide stmctures (52,187,188,199) ... 

Cychc diacyl peroxides decompose thermally and photolyticaHy to yield products derived from diradical intermediates (188,198,205) (eq. 31). 

Diacyl peroxides (20, = alkyl oi aiyl) also undergo three types of radical induced decomposition (187) all of which produce the radical... 

As a consequence of their susceptibility to radical-induced decomposition, neat and concentrated solutions of diacyl peroxides undergo self-accelerating... 

Diacyl peroxide decompositions also are cataly2ed by the metal ions of copper, iron, cobalt, and manganese ... 

This ladical-geneiating reaction has been used in synthetic apphcations, eg, aioyloxylation of olefins and aromatics, oxidation of alcohols to aldehydes, etc (52,187). Only alkyl radicals, R-, are produced from aliphatic diacyl peroxides, since decarboxylation occurs during or very shortiy after oxygen—oxygen bond scission in the transition state (187,188,199). For example, diacetyl peroxide is well known as a source of methyl radicals (206). 

Hydrolysis and perhydrolysis of diacyl peroxides yields peroxycarboxyhc acids. Carbanions react by displacement on oxygen ... 

Diacyl peroxides have been reduced with a variety of reduciag agents, eg, lithium aluminum hydride, sulfides, phosphites, phosphines, and haUde ions (187). Hahdes yield carboxyUc acid salts (RO) gives acid anhydrides. With iodide ion and certain trivalent phosphoms compounds, the reductions are sufftcientiy quantitative for analytical purposes. 

Amines also react with diacyl peroxides by nucleophilic displacement on the oxygen—oxygen bond forming an ion pair iatermediate (187) ... 

The fate of the ion pair iatermediate depends on the stmcture of the amine and the reaction conditions. Certain tertiary amines, eg, dimethylaruline (DMA), react with specific diacyl peroxides such as diben2oyl peroxide (BPO) to generate free radicals at ca 20°C. Some reactions, eg, DMA—BPO, are explosive when neat reactants are mixed. Primary and secondary amines do not yield free radicals. 

Phenols with bulky ortho- and para-substituents, eg, phenoHc antioxidants, do not undergo this reaction however, they scavenge radicals generated by thermolysis of diacyl peroxides and other peroxides. Diacyl peroxides react with potassium superoxide, KO2, forming singlet oxygen (207). 

Synthesis. Symmetrical diacyl peroxides (20, R = R = alkyl or aryl) are prepared by the reaction of an acyl chloride or anhydride with sodium peroxide or hydrogen peroxide and a base ... 

Use of diacid chlorides for acyl chlorides in the latter reaction results in generation of di(diacyl peroxides) (25). 

Other unsymmetrical peroxides can be prepared by this reaction by employing other acylating agents, eg, alkyl chloroformates, organosulfonyl chlorides, and carbamoyl chlorides (210). Unsymmetrical and symmetrical di(diacyl peroxides) also are obtained by the reaction of dibasic acid chlorides directiy with peroxycarboxyhc acids or monoacid chlorides directiy with diperoxycarboxyhc acids in the presence of a base (44,187,203). 

Polymeric diacyl peroxides (26) can be prepared from the reaction of dibasic acid chlorides, eg, succinoyl, fumaryl, sebacoyl, and terephthaloyl chlorides, with sodium or hydrogen peroxide (187). 

Cychc diacyl peroxides can be generated from suitable dibasic acid chlorides and sodium or hydrogen peroxide, especially in dilute solutions (187,205), eg, 2,3-ben2odioxin-l,4-dione [4733-52-2] from phthaloyl chloride ... 

Symmetrical or unsymmetrical diacyl peroxides (20, R, R = alkyl or aryl) can be synthesi2ed directiy from carboxyhc acids and hydrogen peroxide or from peroxycarboxyhc acids with dicyclohexylcarbodiimide or A/,A/-dicarbonyldiimida2ole as condensing agents (187) ... 

Alkyl peroxyesters are much less sensitive to radical-induced decompositions than diacyl peroxides. Induced decomposition is only significant in peroxyesters containingnonhindered a-hydrogens or a, P-unsaturation (213,242). 

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