Deactivation reactions

Disulfides. As shown in Figure 4, the and h-chains of insulin are connected by two disulfide bridges and there is an intrachain cycHc disulfide link on the -chain (see Insulin and other antidiabetic drugs). Vasopressin [9034-50-8] and oxytocin [50-56-6] also contain disulfide links (48). Oxidation of thiols to disulfides and reduction of the latter back to thiols are quite common and important in biological systems, eg, cysteine to cystine or reduced Hpoic acid to oxidized Hpoic acid. Many enzymes depend on free SH groups for activation—deactivation reactions. The oxidation—reduction of glutathione (Glu-Cys-Gly) depends on the sulfhydryl group from cysteine. 

Reaction progress kinetic analysis offers a reliable alternative method to assess the stability of the active catalyst concentration, again based on our concept of excess [e]. In contrast to our different excess experiments described above, now we carry out a set of experiments at the same value of excess [ej. We consider again the proline-mediated aldol reaction shown in Scheme 50.1. Under reaction conditions, the proline catalyst can undergo side reactions with aldehydes to form inactive cyclic species called oxazolidinones, effectively decreasing the active catalyst concentration. It has recently been shown that addition of small amounts of water to the reaction mixture can eliminate this catalyst deactivation. Reaction progress kinetic analysis of experiments carried out at the same excess [e] can be used to confirm the deactivation of proline in the absence of added water as well to demonstrate that the proline concentration remains constant when water is present. 

Volume I). The most rapid metabolic deactivating reaction is oxidation to the bioinert C15 oxo prostaglandins. Converting the latter to a tertiary methyl carbinol led to the desired orally active gastric antisecretory agents. 

One of the main concerns for resid HDT catalysts is the life cycle, since it is shorten by the deactivating reactions (formation of ammonia, from HDN coke from poor hydrogenation, and metals accumulation from demetallization, etc.) and the complexity of the... 

Synthesis of well defined functionalized (- telechellc or multifunctional-) macromolecules Is an Important task for polymer chemists. The polymers with P0(0R)2, - Si(0R)3, -OH, - . .. functional groupslrS. are produced In limited quantities. The need for polymeric materials possessing specific properties has led to a renewed Interest Is functional polymers, especially if the initial material Is a common hydrocarbon polymer. One of the techniques that we use in our laboratory to prepare these new molecules Is based on anionic processes. This anionic technique is best suited to control the length of the chains prepared and to obtain samples with low polydlsperslty. Although the functionalization of carbanionic sites with various deactivating reagents Is easier than with other methods because of the long lived species, It Is still necessary to carefully control the deactivation reaction to prevent secondary reactions. 

The deactivation reaction transfers an active catalyst into the inert (non-reactive) polymer. This phenomenon, when cyclic sulfonium zwitterions act as anionic initiators, can be utilized for the control of the cyclotrimerization of difunctional isocyanates. Therefore the degree of oligomerization of difunctional isocyanates can be controlled by the concentration of the initiator, rate of addition of the initiator, as well as by the temperature of the reaction system. 

Irradiation of a PMMA film containing ITX results in rapid photoreduction of ITX as is shown in Figure 5. Similar experiments in a deaerated benzene solution containing PMMA and ITX also result in photoreduction of ITX, albeit at much lower rate. The most likely interpretation of these observations is that triplet excited ITX abstracts hydrogen from the polymer to form a ketyl radical and a macro alkyl radical. The higher photoreduction rate in the film may result from deactivation reactions of triplet excited ITX which are faster in solution than in a glassy film. The triplet lifetime for ITX in a PMMA film is 1 3 sec compared with 10-4 sec in a benzene solution (20). In solution triplet-triplet annihilation... 

When dehydration occurs as a consecutive reaction, its effect on polarographic curves can be observed only, if the electrode process is reversible. In such cases, the consecutive reaction affects neither the wave-height nor the wave-shape, but causes a shift in the half-wave potentials. Such systems, apart from the oxidation of -aminophenol mentioned above, probably play a role in the oxidation of enediols, e.g. of ascorbic acid. It is assumed that the oxidation of ascorbic acid gives in a reversible step an unstable electroactive product, which is then transformed to electroinactive dehydroascorbic acid in a fast chemical reaction. Theoretical treatment predicted a dependence of the half-wave potential on drop-time, and this was confirmed, but the rate constant of the deactivation reaction cannot be determined from the shift of the half-wave potential, because the value of the true standard potential (at t — 0) is not accessible to measurement. 

Ito S, Mitarai A, Hikino K, Hirama M, Sasaki K. Deactivation reaction in the hydroxylation of benzene with Fenton s reagent. J Org Chem 1992 57 6937-6941. 

Since the conversion is constant, the rate constant of the deactivation reaction can be written as... 

Several consecutive values of c are examined to identify significant analytical errors for its eiiminationThis way,an estimated state of the system was calculated and used in a simplified auxiliary model of reaction and reactor (first order,CSTR behavior),This model supplied with the actual feed composition and an initial value of the reaction parameter (n.b.not of the deactivation reaction), enable to calculate an estimated value of a better reaction parameter k which than is used for adjusting the feed composition. 

Catalyst deactivation is assumed to take place by a poisoning mechanism only. The deactivation of catalyst by thiophene will serve as a model deactivation reaction. Weng et al. (40 analyzed the deactivation data and proposed a rate equation, linear in concentration of poison, x, and activity 0 ... 

---