Group degradative reaction

Certain nitroaromatic groups are susceptible to photochemical reactivity (43). A well-known nitro group degradation reaction occurs for the API nifedipine (Fig. 60). Under ultraviolet as well as visible radiation, the nitro group of nifedipine is rapidly converted to a nitroso compound along with aroma-tization to a substituted pyridine ring (99). In the presence of molecular oxygen, the nitroso functionality is re-oxidized to the nitro derivative (100). 

Thermal, Thermooxidative, and Photooxidative Degradation. Polymers of a-olefins have at least one tertiary C-H bond in each monomer unit of polymer chains. As a result, these polymers are susceptible to both thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups. For example, thermal decomposition of... 

Polyisobutylene has the chemical properties of a saturated hydrocarbon. The unsaturated end groups undergo reactions typical of a hindered olefin and are used, particularly in the case of low mol wt materials, as a route to modification eg, the introduction of amine groups to produce dispersants for lubricating oils. The in-chain unsaturation in butyl mbber is attacked by atmospheric ozone, and unless protected can lead to cracking of strained vulcanizates. Oxidative degradation, which leads to chain cleavage, is slow, and the polymers are protected by antioxidants (75). 

As the data in Table XIV indicate, over platinum demethylation of a ring is slow compared to C—C bond rupture within a ring. On the other hand, it is well established [e.g., Kochloefl and Bazant (161) that if one uses a supported nickel catalyst which is known to favor stepwise alkane degradation, reaction with an alkylcycloalkane is largely confined to the alkyl group (s) which are degraded in a stepwise fashion and are finally removed entirely from the ring. 

A phosphite degradation reaction that occurs during hydroformylation using an aryl-phosphite-modified rhodium catalyst involves replacement of one of the aryl groups with an alkyl group corresponding to the alkyl group of the hydroformylation prod-uct[.[27] This is illustrated in Equation 2.7... 

Year and reference Reactor/reaction Esterification, free EG Esterification, tlXi Polycondensation DEG formation AA formation 0 Diester group degradation 0 Polycondensation of vinyl groups ... 

Fig. 6.25. Simplified mechanism of two degradation reactions between peptides and reducing sugars occurring in solids, a) Maillard reaction between a side-chain amino (or amido) group showing the formation of an imine (Reaction a), followed by tautomerization to an enol (Reaction b) and ultimately to a ketone (Reaction c). Reaction c is known as the Amadori rearrangement (modified from [8]). b) Postulated mechanism of the reaction between a reducing sugar and a C-terminal serine. The postulated nucleophilic addition yields an hemiacetal (Reaction a) and is followed by cyclization (intramolecular condensation Reaction b). Two subsequent hydrolytic steps (Reactions c and d) yield a serine-sugar conjugate and the des-Ser-peptide...

Hoffmann developed a method for preparation of primary amines by treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide. In this degradation reaction, migration of an alkyl or aryl group takes place from carbonyl carbon of the amide to the nitrogen atom. The amine so formed contains one carbon less than that present in the amide. 

Examples of Beckmann degradation reactions with assistance by mesomeric donation of electron lone pairs (example Y = N, O, S) or n electrons (example R = Ph or aromatic group) are well documented in the literature ... 

All the membranes considered were based on quaternary ammonium groups as anion-exchange groups. However, these conductive groups may decompose in concentrated alkaline solution following the Hofmaim degradation reaction [43]. 

Interestingly, it should not be assumed that a polymer will be useless above its ceiling temperature. A dead polymer that has been removed from the reaction media will be stable and will not depolymerize unless an active end is produced by bond cleavage of an end group or at some point along the polymer chain. When such an active site is produced by thermal, chemical, photolytic, or other means, depolymerization will follow until the monomer concentration becomes equal to [M]c for the particular temperature. The thermal behavior of many polymers, however, is much more complex. Degradative reactions other than depolymerization will often occur at temperatures below the ceiling temperature. 

In his later papers on degradation reactions, Zempl4n employed sodium methoxide and used chloroform as the solvent for the sugar derivative. When the acyl groups are removed, the cyanohydrin can be considered an intermediate product which loses hydrocyanic acid to yield an aldose. It is possible that the acetyl and nitrile groups are eliminated simultaneously by concurrent reactions. 

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