Degradation epoxidation

Some bacteria can degrade epoxides, converting them into p-keto acids. For example, in Xanthobacter Py2, poly-p-hydroxyalkanoates are produced [70,71]. The epoxide degradation seems to be regulated by the presence or absence... 

Other materials that are often referred to as secondary plasticizers iaclude materials such as epoxidized soybean oil (ESBO) and epoxidized linseed oil (ELO) and similar materials. These can act as lubricants but also as secondary stabilizers to PVC due to thein epoxy content which can remove HCl from the degrading polymer. 

Some cleavage takes place even if the phenoHc hydroxyl is blocked as an ether link to another phenylpropane unit and quinonemethide formation is prevented. If the a- or y-carbon hydroxyl is free, alkaH-catalyzed neighboring-group attack can take place with epoxide formation and P-aryloxide elimination. In other reactions, blocked phenoHc units are degraded if an a-carbonyl group is present. 

Methylene chloride is one of the more stable of the chlorinated hydrocarbon solvents. Its initial thermal degradation temperature is 120°C in dry air (1). This temperature decreases as the moisture content increases. The reaction produces mainly HCl with trace amounts of phosgene. Decomposition under these conditions can be inhibited by the addition of small quantities (0.0001—1.0%) of phenoHc compounds, eg, phenol, hydroquinone, -cresol, resorcinol, thymol, and 1-naphthol (2). Stabilization may also be effected by the addition of small amounts of amines (3) or a mixture of nitromethane and 1,4-dioxane. The latter diminishes attack on aluminum and inhibits kon-catalyzed reactions of methylene chloride (4). The addition of small amounts of epoxides can also inhibit aluminum reactions catalyzed by iron (5). On prolonged contact with water, methylene chloride hydrolyzes very slowly, forming HCl as the primary product. On prolonged heating with water in a sealed vessel at 140—170°C, methylene chloride yields formaldehyde and hydrochloric acid as shown by the following equation (6). 

The selective epoxidation of ethylene by hydrogen peroxide ia a 1,4-dioxane solvent ia the presence of an arsenic catalyst is claimed. No solvent degradation is observed. Ethylene oxide is the only significant product detected. The catalyst used may be either elemental arsenic, an arsenic compound, or both. 

When cured with room temperature curing system these resins have similar thermal stability to ordinary bis-phenol A type epoxides. However, when they are cured with high-temperature hardeners such as methyl nadic anhydride both thermal degradation stability and heat deflection temperatures are considerably improved. Chemical resistance is also markedly improved. Perhaps the most serious limitation of these materials is their high viscosity. 

In this oxidative degradation, MTO decomposes into catalytically inert perrhenate and methanol. The decomposition reaction is accelerated at higher pH, presumably through the reaction between the more potent nucleophile H02- and MTO. The decomposition of MTO under basic conditions is rather problematic, since the selectivity for epoxide formation certainly profits from the use of nonacidic conditions. 

The speed of autoxidation was compared for different carotenoids in an aqueous model system in which the carotenoids were adsorbed onto a C-18 solid phase and exposed to a continnons flow of water saturated with oxygen at 30°C. Major products of P-carotene were identified as (Z)-isomers, 13-(Z), 9-(Z), and a di-(Z) isomer cleavage prodncts were P-apo-13-carotenone and p-apo-14 -carotenal, and also P-carotene 5,8-epoxide and P-carotene 5,8-endoperoxide. The degradation of all the carotenoids followed zero-order reaction kinetics with the following relative rates lycopene > P-cryptoxanthin > (E)-P-carotene > 9-(Z)-p-carotene. 

The products formed after heating dried P-carotene at 180°C for 2 hr in a sealed ampoule (SI) with air circulation (S2) stirring with starch and water (S3) and during extrusion process (S4) were isolated. - In all systems, 5,6-epoxy-P-carotene (trans and two cis isomers), 5,8-epoxy-P-carotene (trans and four cis isomers), and 5,6,5,6-diepoxy-P-carotene were identified, along with 5,6,5,8-diepoxy-P-carotene in systems S3 and S4. Later on, along with the epoxides previously found, 5 P-apocarotenals with 20 to 30 carbons, P-caroten-4-one, and 6 different P-carotene cis isomers were isolated in systems S3 and S4, whereas lower numbers of degradation products were found in the other systems. ... 

Tetrachloroethene may be degraded by bacteria via the epoxide, and chemical hydrolysis of this produces CO and CO2 from oxalyl chloride as major products, whereas only low amounts of trichloroacetate were produced (Yoshioka et al. 2002). 

The first step in the aerobic degradation of alkenes is epoxidation. Epoxidation is then followed by several alternatives. In one of them, the epoxides may nndergo carboxylation the enzyme... 

Xanthobacter sp. strain Py2 was isolated by enrichment on propene that is metabolized by initial metabolism to the epoxide. The monooxygenase that is closely related to aromatic monooxygenases is able to hydroxylate benzene to phenol before degradation, and toluene to a mixture of 2-, 3-, and 4-methylphenols that are not further metabolized (Zhou et al. 1999). 

The degradation of trichloroethene by methylotrophic bacteria involves the epoxide as intermediate (Little et al. 1988). Further transformation of this may produce CO that can toxify the bacterium, both by competition for reductant and by enzyme inhibition (Henry and Grbic-Galic 1991). The inhibitory effect of CO may, however, be effectively overcome by adding a reductant such as formate. 

Carter SF, DJ Leak (1995) The isolation and characterisation of a carbocyclic epoxide-degrading. Corynebac-terium sp. Biocatalysis Biotrans 13 111-129. 

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