Products Formed from Hydroperoxides

As with oleate and linoleate, some volatile decomposition compounds are formed from linolenate hydroperoxides that cannot be explained by the classical A and B cleavage mechanisms, including acetaldehyde, butanal, 2-butyl furan, methyl heptanoate, 4,5-epoxyhepta-2-enal, methyl nonanoate, methyl 8-oxooctanoate, and methyl lO-oxo-8-decenoate. Some of these minor volatile oxidation products can be attributed to further oxidation of unsaturated aldehydes. Other factors contribute to the complexity of volatile products formed from hydroperoxides, including temperature of oxidation, metal catalysts, stability of volatile products and competing secondary reactions including dimerization, cyclization, epoxidation and dihydroperoxidation (Section E). 

The study of the interaction of hydroperoxide with other products of hydrocarbon oxidation showed the intensive initiation by reactions of hydroperoxide with formed alcohols, ketones, and acids [6,134]. Consequently, with the developing of the oxidation process the variety of reactions of initiation increases. In addition to reactions of hydroperoxide with the hydrocarbon and the bimolecular reaction of ROOH, reactions of hydroperoxide with alcohol and ketone formed from hydroperoxide appear. The values of rate constants (in L mol 1 s 1) of these reactions for three oxidized hydrocarbons are given below. 

Stereospecific Epoxidation of 2-Butene. The hydroperoxide epoxidation reaction is stereospecific. Pure cis- and trans-2-butene were epoxi-dized separately by cumene hydroperoxide. The cis olefin gave exclusively cis epoxide, and the trans olefin gave exclusively trans epoxide. In both cases, the epoxide was the sole product formed from the olefin. They can be distinguished easily by their different retention times on a gas chromatography column of 20% diisodecyl phthalate on Chromosorb W(60-80 mesh). They were also identified by comparing their infrared spectra with authentic samples. 

Sodium tetrachloroaurate cat yzes the photo-oxidation of hydrocarbons in solution. Typical of this is the irradiation with wavelengths >310 nm of aerated solutions of cyclohexane in acetonitrile or methylene chloride. The principal isolable products formed from this treatment are cyclohexanol and cyclohexanone . A mechanism that might involve the formation of a metal peroxo or metal oxo complex has been suggested. Such complexes are known to react with alkanes to yield hydroxy derivatives . The principal organic intermediate is the unstable cyclohexylhydroperoxide that readily decomposes to afford cyclohexanone and cyclohexanol. Further study has shown that the hydroperoxide accumulates during the oxidation . The influence of wavelength (A. 300, 365 or 436 nm) on the reaction has also been studied . Hexane and ethylbenzene can be similarly oxidized to yield analogous products . 

Because the 12,15-diene has a 1,4-diene system, it oxidizes like linoleate to form two conjugated dienoic 12- and 16-hydroperoxides. However, in contrast to linoleate, the external 16-hydroperoxide is formed at a higher concentration than the internal 12-hydroperoxide, (with normalized concentrations of 58% and 42%) (Figure 2.12). Therefore, both the 9,15- and 12,15-dienes produce allylic radicals in which the terminal carbons 16 and 17, closest to the end of the fatty acid chain, are the most reactive with oxygen. The same preference of oxygen attack at the terminal double bond position is also observed in other polyunsaturated fatty acids with n-3 double bonds (linolenate, eicosapentaenoic and docosahexaenoic acids), and n-6 double bonds (arachidonic acid). Volatile decomposition products derived from hydroperoxides containing an n-3 double bond are particularly significant for their impact on flavor (Chapter 4). 

The main volatile decomposition products formed from oleate, linoleate and linolenate are those expected from the cleavage of the alkoxyl radicals formed from the hydroperoxides of autoxidized and photosensitized oxidized fatty... 

Polyalkoxy and polyperoxy radicals formed from hydroperoxide decomposition are unstable intermediates, owing to the presence of an unpaired electron which weakens the dissodation energy for bonds in the yS-position by some 100 kj mol . Monomolecular yS-scission is the main mechanism for a decrease in MW and accounts for the variety of oxygenated products observed during oxidative degradation (see Section 15.4.4.6). 

Another important group of reactions are those in which the hydrocarbon chain of the alkoxyl radicals cleaves to form low molecular weight products, mainly volatile and sensory active compounds. The cleavage takes place on both sides of the alkoxyl radicals (Figure 3.45). The composition of reaction products formed from alkoxyl radicals derived from unsaturated fatty acids depends on which carbon, next to the hydroperoxide group, the double bond is located. In addition to non-volatile oxoacids and hydroxy acids. 

During each photodegradation (in vacuum or air) small amounts of several gaseous and liquid low molecular weight compounds are formed and they can only be detected by chromatographic or mass spectrometry methods (cf. section 10.6). Table 2.1 shows, for example, products formed from polypropylene hydroperoxide photolysis. 

The initial product formed from the migration of an alkyl group has the formula R2BOR. It continues to react with hydroperoxide ion to give RB(OR)2 and eventually the trialkyl borate, (RO)3B. Subsequent hydrolysis of the borate in basic solution gives the alcohol and sodium borate. 

Finally, hydroperoxides are transformed into molecular products in heterolytic reactions with oxidation products (for example, under the action of acids) by the heterolytic decomposition on the reactor walls, under the action of the catalyst of heterolytic ROOH decomposition, which was specially introduced or randomly gotten into the system, and during the reaction in the cage of radicals formed from hydroperoxide. Let us consider the kinetics of RH oxidation at deep stages in this last case where ROOH decomposes in oxidized RH by the molecular reaction of a first order with the rate constant km. In this case (at k 23), the maximum with respect to ROOH is achieved under the condition v = a[RH](k23[ROOH]) = [ROOH], from which [ROOH], = and the kinetics of ROOH formation and... 

Products other than hydroperoxides are formed in oxidations by reactions such as those of equations 11 and 12. Hydroxyl radicals (from eq. 4) are very energetic hydrogen abstractors the product is water (eq. 11). 

Sales demand for acetophenone is largely satisfied through distikative by-product recovery from residues produced in the Hock process for phenol (qv) manufacture. Acetophenone is produced in the Hock process by decomposition of cumene hydroperoxide. A more selective synthesis of acetophenone, by cleavage of cumene hydroperoxide over a cupric catalyst, has been patented (341). Acetophenone can also be produced by oxidizing the methylphenylcarbinol intermediate which is formed in styrene (qv) production processes using ethylbenzene oxidation, such as the ARCO and Halcon process and older technologies (342,343). 

Low molecular weight ether hydroperoxides are similarly dangerous and therefore ethers should be tested for peroxides and any peroxidic products removed from them before ethers are distilled or evaporated to dryness. Many ethers autoxidize so readily that peroxidic compounds form at dangerous levels when stored in containers that are not airtight (133). Used ether containers should be handled cautiously and if they are found to contain hazardous soHd ether peroxides, bomb-squad assisted disposal may be required (134). ZeoHtes have been used for removal of peroxide impurities from ethers (135). 

In the initial period the oxidation of hydrocarbon RH proceeds as a chain reaction with one limiting step of chain propagation, namely reaction R02 + RH. The rate of the reaction is determined only by the activity and the concentration of peroxyl radicals. As soon as the oxidation products (hydroperoxide, alcohol, ketone, etc.) accumulate, the peroxyl radicals react with these products. As a result, the peroxyl radicals formed from RH (R02 ) are replaced by other free radicals. Thus, the oxidation of hydrocarbon in the presence of produced and oxidized intermediates is performed in co-oxidation with complex composition of free radicals propagating the chain [4], A few examples are given below. 

Another factor complicating the situation in composition of peroxyl radicals propagating chain oxidation of alcohol is the production of carbonyl compounds due to alcohol oxidation. As a result of alcohol oxidation, ketones are formed from the secondary alcohol oxidation and aldehydes from the primary alcohols [8,9], Hydroperoxide radicals are added to carbonyl compounds with the formation of alkylhydroxyperoxyl radical. This addition is reversible. 

The experimental data are in agreement with this equation. In the presence of dioxygen, the alkyl radicals formed from enol rapidly react with dioxygen and thus the formed peroxyl radicals react with Fe2+ with the formation of hydroperoxide. The formed hydroperoxide is decomposed catalytically to molecular products (AcOH and AcH) as well as to free radicals. The free radicals initiate the chain reaction resulting in the increase of the oxidation rate. 

Photolytic. Major products reported from the photooxidation of butane with nitrogen oxides under atmospheric conditions were acetaldehyde, formaldehyde, and 2-butanone. Minor products included peroxyacyl nitrates and methyl, ethyl and propyl nitrates, carbon monoxide, and carbon dioxide. Biacetyl, tert-butyl nitrate, ethanol, and acetone were reported as trace products (Altshuller, 1983 Bufalini et al, 1971). The amount of sec-butyl nitrate formed was about twice that of n-butyl nitrate. 2-Butanone was the major photooxidation product with a yield of 37% (Evmorfopoulos and Glavas, 1998). Irradiation of butane in the presence of chlorine yielded carbon monoxide, carbon dioxide, hydroperoxides, peroxyacid, and other carbonyl compounds (Hanst and Gay, 1983). Nitrous acid vapor and butane in a smog chamber were irradiated with UV light. Major oxidation products identified included 2-butanone, acetaldehyde, and butanal. Minor products included peroxyacetyl nitrate, methyl nitrate, and unidentified compounds (Cox et al., 1981). 

Next to TBHP also ferf-pentyl hydroperoxide, cumyl hydroperoxide and cyclohexyl hydroperoxide could be employed as oxidant and 2-hydroxycyclobutanone and 2-hydro xycyclododecanone were prepared by this method as well. In 1985, Vedejs and Larsen reported on a preparative method for the a-hydroxylation of camphor and a variety of other ketones utilizing overstoichiometric amounts of oxodiperoxomolybdenum(pyridine)(hexamethylphosphoric triamide) as source of oxygen (equation 67). Yields of products ranged from 34-81% and in some cases also the a-diketone is formed as by-product (0-26%). 

The proposed mechanism (Scheme 1) involves the mixed-valence compounds [Rh2" " ( Ji-cap)4(OH)] and [Rh2 (p.-cap)4(OOt-Bu)] formed from the homolytic cleavage of t-BuOOH. The t-BuOO radicals in the medium promote a selective hydrogen abstraction from the alkene to give the allylic alkenyl radical. This species traps the peroxide in [Rh2 (p.-cap)4 (OOt-Bu)] to produce the alkenyl hydroperoxide, which rapidly decomposes to the isolated products, thus regenerating the catalyst. 

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