Inhibitor, p-benzoquinone

Sodium methylate acting on 2-chloroanthraquinone substitutes the methoxy group for chlorine and produces anion radicals of the substrate (Shtemshis et al. 1973). The study of kinetics has demonstrated that the amount of the substrate anion radical first increases and then sharply decreases. The inhibitor (p-benzoquinone) decelerates the formation of the anion radicals. The rate of formation of 2-methoxyanthraquinone also decreases. If the anion radicals were produced on the side pathway, the rate of formation of the end product upon the introduction of the inhibitor should not have decreased. Moreover, it should even rise, because oxidation of the anion radicals regenerates the uncharged molecules of the substrate. Hence, the anion radical mechanism controls this reaction. 

Neelsen et al. [140] investigated the effect of NaCl on the emulsion polymerization of vinyl chloride in the presence and absence of emulsifier (SDS) or inhibitor (p-benzoquinone (BQ) or chloranile (CA)). Under reaction conditions 2 [BQ] = [CA], the same induction periods and the conversion curves were obtained. The addition of NaCI (a coagulating agent) caused the decrease of the rate of polymerization and the increase of the particle size. The addition of NaCl led to an increase in the induction period. Here, the same induction period and the same shape of the conversion curves were obtained and [BQ] = [CA]. The different behavior of BQ in the presence of NaCl was explained by the reaction of O with BQ which leads to the chlorination of BQ. O radicals are generated by the reaction of peroxodisulfate with NaO (a labelled salt). The addition of NaO increased flocculation of particles which led to the formation of larger particles and lower rates of prflymerization. The dependence of the rate rtf polymerization on the NaO concentration was expressed by the following equation. 

Inhibitors are characterized by inhibition constants which are defined as the ratio of the rate constant for transfer to inhibitor to the propagation constant for the monomer in analogy with Eq. (6.87) for chain transfer constants. For styrene at 50°C the inhibition constant of p-benzoquinone is 518, and that for O2 is 1.5 X 10. The Polymer Handbook (Ref. 3) is an excellent source for these and most other rate constants discussed in this chapter. 

The absolute rate constants for attack of carbon-centered radicals on p-benzoquinone (38) and other quinones have been determined to be in the range I0M08 M 1 s 1.1 -04 This rate shows a strong dependence on the electrophilicity of the attacking radical and there is some correlation between the efficiency of various quinones as inhibitors of polymerization and the redox potential of the quinone. The complexity of the mechanism means that the stoichiometry of inhibition by these compounds is often not straightforward. Measurements of moles of inhibitor consumed for each chain terminated for common inhibitors of this class give values in the range 0.05-2.0.176... 

Acceptors of alkyl radicals are known to be very weak inhibitors of liquid-phase hydrocarbon oxidation because they compete with dioxygen, which reacts very rapidly with alkyl radicals. The situation dramatically changes in polymers where an alkyl radical acceptor effectively terminates the chains [3,49], The study of the inhibiting action of p-benzoquinone [50], nitroxyl radicals [51-53], and nitro compounds [54] in oxidizing PP showed that these alkyl radical acceptors effectively retard the oxidation of the solid polymer at concentrations ( 10-3 mol L 1) at which they have no retarding effect on liquid hydrocarbon oxidation. It was proved from experiments on initiated PP oxidation at different p02 that these inhibitors terminate chains by the reaction with alkyl macroradicals. The general scheme of such inhibitors action on chain oxidation includes the following steps ... 

Polar effects appear to be of prime importance in determining the effect of quinones. p-Benzoquinone and chloranil (which are electron-poor) act as inhibitors toward electron-rich propagating radicals (vinyl acetate and styrene) but only as retarders toward the electron-poor acrylonitrile and methyl methacrylate propagating radicals. A further observation is that the inhibiting ability of a quinone toward electron-poor monomers can be increased by the addition of an electron-rich third component such as an amine. Thus the presence of triethylamine converts chloranil from a very weak retarder to an inhibitor toward methyl methacrylate. 

As in the case of phenoxyl and nitroxyl radical reactions, the value of Ee0 for the quinone reaction with phenol (AriOH) is much lower than that for the reaction of Q with R1 H (AEdJ 23 kj mol ). Such a difference is the result of the high triplet repulsion in TS of the type C H and low in the TS of the type H O, as in the reactions of the nitroxyl radical. The very low value of Ee0 for the reaction Q with aromatic amine is due to a high difference in electron affinity of N and atoms in TS of the type H N. The values of rate constants of p-benzoquinone with several inhibitors were calculated by the IPM method. The parameters of the IPM model are collected in Table 18.9. 

Nitro compounds, labelled with carbon-14, have been used during the polymerization of styrene (40). It has been shown that picric acid, which is rather an inefficient inhibitor, formally resembles p benzoquinone with this monomer. It becomes incorporated in the polymer but most of it is included in polystyrene of comparatively low molecular weight. Most of the combined picric add can subsequently be removed by treatment with trifluoro-acetic anhydride reagent, showing that the picric acid is not held in the polymer by carbon-carbon bonds. Further, it was shown, by isotope dilution analysis, that most of the picric add removed from the polymer by this treatment appeared as the add. 

Equimolar quantities of tellurium tetrachloride and alkenes produce chloroalkyl tellurium trichlorides (Table 6, p. 302). The reaction of tellurium tetrachloride in chloroform with alkenes such as butenes, 1-decenes, cycloalkenes, and 3-phenoxypropenes usually gives mixtures of products arising from syn- and anti-1,2-addition. Catalytic amounts of p-benzoquinone, a radical inhibitor, and acetonitrile as solvent promoted sy -addition. In chloroform in the absence of a radical scavenger, the regiospecific, concerted syn-addition competes with the radical pathway15. 

Inhibition — is a decrease in the reaction rate caused by a substance (- inhibitor) affecting the concentration of a reactant, catalyst, or reaction intermediate. For example, molecular oxygen and p-benzoquinone can react as inhibitors in many reactions involving radicals as intermediates by virtue of their ability to act as scavengers toward these radicals. If the rate of a reaction in the absence of inhibitor is Vq and that in the presence of a certain amount of inhibitor is v, the degree of inhibition, i, is given by... 

Inhibitors are added in small quantities to increase the shelf life of the compounds. They also help in modifying the cure rate and the magnitude of the exotherm to prevent cracking in thick sections of the molded components. Substituted phenolic derivatives and the quaternary ammonium salts, like hydro-quinone and p-benzoquinone, are two general classes of inhibitors usually used. More information about initiator chemistry is available in the literature. 

Problem 8.23 Explain how the following substances act as inhibitors or retarders in cationic polymerization water, tertiary amines, trialkyl phosphines, and p-benzoquinone. 

Polymerization is inhibited by typical free-radical inhibitors oxygen, diphenylpicrylhydrazyl, p-benzoquinone, hydroquinone, p-naphtylamine, etc. 

In benzoquinone-treated algae, the re-reduction rate of Cyt c could be accelerated by increasing the concentration of reduced DAD (not shown). In this case, addition of myxothiazol also blocked the reaction. Therefore, the inhibition caused by the treatment is not due to its impairing the b-c complex, but rather affects an upstream step, such as ubiquinone reduction. This was actually verified by Prof. R. Douce (personal communication), who found that p-benzoquinone was an inhibitor of the NADH-ubiquinone- reductase in isolated plant mitochondria. The slow re-reduction of Cyt c in Fig. 1 depends on the duration and concentration of the benzoquinone treatment, and as mentioned above, on the added reductant. In this experiment sufficient reduction occurs within each flashing period so that the saturation effect observed in the myxothiazol curve of Fig.2 does not occur. As shown in [l], under such conditions, the mitochondrial response remains linear. 

As shown in Fig. 2 (lanes B, D) DCMU (3-(3 ,4 -dichlorophenyl)-l,l-dimethylauria) and PpBQ (phenyl-p-benzoquinone) completely abolish PPi dependent protein phosphorylation. Inhibition by the PSII inhibitor DCMU confirms that this phosphorylation is electron transport dependent. Inhibition by PpBQ is consistent with its role as an artificial electron acceptor, leaving the plastoquinone in an oxidized state. This indicates that PQ has to be reduced for the activation of the PPi dependent protein kinase, as is known to be the case with ATP. 

Known also as Diphenyl Ketone. p-Benzoquinone -kwi- non (1,4-benzoquinone, chinone) A yellow crystalline compound used, along with many of its derivatives, as an inhibitor in unsaturated polyester resins to prevent premature gelation during storage. 

The sulfhydryl nature of the mammalian glutaminase appears to be well established (853, 854). The following reagents inhibit mercuric chloride, p-chloromercuribenzoate, iodoacetamide, JV-ethylmaleimide, p-benzoquinone, and quinone. p-Chloromercuribenzoate inhibition is independent of phosphate concentration and appears to be a competitive inhibitor of glutamine, indicating that the —SH group of the enzyme is a site of attachment for the substrate (854)-... 

Neelson et al. [138] investigated the emulsion polymerization of vinyl chloride in the presence of inhibitors. The used p-benzoquinone and a stable radical 2,2, 6,6 -tetramethylpyperidine-JV-oxide at concentrations 1 x 10 mol dm After the consumption of inhibitor, the conversion vs. time curve was the same shape as that without inhibitor. In some experiments the inhibition period varies with the emulsifier and initiator concentration. The inhibitor efficiency decreased with increasing concentration of emulsifier. The solubilization of inhibitor is expected to decrease the amount of inhibitor available for reactions with radicals. For this reason, the inhibitor acts more efficiently at the low emulsifier concentration as it was reported in Ref. [139]. 

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