Carboxylation alkane degradation

The degradation of squalene by Marinobacter sp. strain 2sq31 (Rontani et al. 2002) is initiated by oxidative fission, although the subsequent steps are carried out by ()-oxidation and carboxylation that are comparable to those used for branched alkanes. 

The degradation of alkynes has been the subject of sporadic interest during many years, and the pathway has been clearly delineated. It is quite distinct from those used for alkanes and alkenes, and is a reflection of the enhanced nucleophilic character of the alkyne C C bond. The initial step is hydration of the triple bond followed by ketonization of the initially formed enol. This reaction operates during the degradation of acetylene itself (de Bont and Peck 1980), acetylene carboxylic acids (Yamada and Jakoby 1959), and more complex alkynes (Figure 7.18) (Van den Tweel and de Bont 1985). It is also appropriate to note that the degradation of acetylene by anaerobic bacteria proceeds by the same pathway (Schink 1985b). 

The first term, representing acid-"catalyzed" hydrolysis, is important in reactions of carboxylic acid esters but is relatively unimportant in loss of phosphate triesters and is totally absent for the halogenated alkanes and alkenes. Alkaline hydrolysis, the mechanism indicated by the third term in Equation (2), dominates degradation of pentachloroethane and 1,1,2,2-tetrachloroethane, even at pH 7. Carbon tetrachloride, TCA, 2,2-dichloropropane, and other "gem" haloalkanes hydrolyze only by the neutral mechanism (Fells and Molewyn-Hughes, 1958 Molewyn-Hughes, 1953). Monohaloalkanes show alkaline hydrolysis only in basic solutions as concentrated as 0.01-1.0 molar OH- (Mabey and Mill, 1978). In fact, the terms in Equation(2) can be even more complex both elimination and substitution pathways can operate, leading to different products, and a true unimolecular process can result from initial bond breaking in the reactant molecule. 

Determination of the residual antioxidant content in polymers by HPLC and MAE is one way to determine the amoimt needed for reasonable stabilization of a material, and also to compare different antioxidants and their individual efficiencies. During ageing and oxidation of PE, carboxyhc acids, dicarboxylic acids, alcohols, ketones, aldehydes, n-alkanes and 1-alkenes are formed [86-89]. The carboxyhc acids are formed as a result of various reactions of alkoxy or peroxy radicals [90]. The oxidation of polyolefins is generally monitored by various analytical techniques. GC-MS analysis in combination with a selective extraction method is used to determine degradation products in plastics. ETIR enables the increase in carbonyls on a polymer chain, from carboxylic acids, dicarboxyhc acids, aldehydes, and ketones, to be monitored. It is regarded as one of the most definite spectroscopic methods for the quantification and identification of oxidation in materials, and it is used to quantify the oxidation of polymers [91-95]. Mechanical testing is a way to determine properties such as strength, stiffness and strain at break of polymeric materials. 

An unusual pathway has been proposed for the degradation of n-alkanes to the carboxylic acids by a Pseudomonas sp. under anaerobic conditions this involves initial dehydrogenation and hydrox-ylation followed by successive oxidations (Figure 6.7) (Parekh et al. 1977). 

The degradation of alkanoic acids by P-oxidation has been noted parenthetically above, but alternative pathways may occur. For example, the metabolism of hexanoic acid by strains of Pseudomonas sp. may take place by co-oxidation with subsequent formation of succinate and 2-tetrahydrofurany-lacetate as a terminal metabolite (Kunz and Weimer 1983). In a strain of Cory neb acterium sp., the specificities of the relevant catabolic enzymes are consistent with the production of dodecanedioic acid by co-oxidation of dode-cane but not of hexadecanedioic acid from hexadecane (Broadway et al. 1993). Hydroxylation at subterminal (co-1, co-2, and co-3) positions of carboxylic acids with chain lengths of 12 to 18—and less readily of the corresponding alcohols, but not the carboxylic acids or the alkanes—has been observed (Miura and Fulco 1975) for a soluble enzyme system from a strain of Bacillus megaterium. Whereas in this organism co-2 hydroxylation is carried out by a soluble cytochrome P-450 BM 3 (Narhi and Fulco 1987), co-hydroxylation in P. oleovorans that carries the OCT plasmid is mediated by a three-component hydroxylase that behaves like a cytoplasmic membrane protein (Ruettinger et al. 1974 Kok et al. 1989). 

Decarboxylation.—Selenoesters (RCOSePh), derived from carboxylic acids by way of the acid chloride, are transformed to the alkane (RH) on heating with tri-n-butyltin hydride in the presence of a radical initiator, whereas under photolytic conditions the corresponding aldehyde (RCHO) is the major product. Yields of decarboxylated product from various steroidal acids are high, but the conditions must be carefully controlled. The same degradation can be... 

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