Quenching agent

Process development of the use of hydrogen as a radical quenching agent for the primary pyrolysis was conducted (37). This process was carried out in a fluidized-bed reactor at pressures from 3.7 to 6.9 MPa (540—1000 psi), and a temperature of 566°C. The pyrolysis reactor was designed to minimize vapor residence time in order to prevent cracking of coal volatiles, thus maximizing yield of tars. Average residence times for gas and soHds were quoted as 25 seconds and 5—10 rninutes. A typical yield stmcture for hydropyrolysis of a subbiturninous coal at 6.9 MPa (1000 psi) total pressure was char 38.4, oil... 

Reactions (d) and (e) occur much less frequently than reactions (a)-(c) but do so at a sufficient rate to cause changes in most polymers. There are three methods available for combating such effects light screens, ultraviolet absorbers and quenching agents. 

In the above scheme A represents the excited polymer, Q the quenching agent and [A.Q] an excited complex. 

The use of quenching agents in polymers is a recent development. Of particular interest are the nickel(II) chelates in polypropylene film and fibre and the even newer hindered amines which appear to combine the roles of antioxidant cmd quenching agent. 

To improve the resistance to ultraviolet light carbon black is often useful as a light screen. Its use in fibres and films is clearly very restricted and in these instances ultraviolet absorbers and/or quenching agents are used. Recent developments include the greater use of hindered amine and nickel compounds. 

In contrast to 6-azidobenzo[/)]thiophene, which yields only benzo[i]thiophen-6-amine (9 %) and JVh,Ar(1-diethylbenzo[/)]thiopheiie-6,7-diamine (25 % bp 175-177 C/0.7 Torr), 6-azido-2,3-dibromobcnzojhjthiophene (1 a, R = R2 = Br) on irradiation in diethylamine in the presence of pyrene, a triplet nitrene quenching agent, yields a mixture of 2,3-dibromo-./V6,./V6-diethyl-benzo[5]thiophene-6,7-diamine (2a, R1 = R2 = Br 13%) and the 8W-thieno[2,3-r]azepine 3a.14<1 Likewise, methyl 6-azidobenzo[6]thiophene-2-carboxylate (lb, R1 = C02Me R2 = H) yields the thienoazepine ester 3b.147... 

In studying two-component polymerization catalysts, beginning with Feldman and Perry (161), a radioactive label was introduced into the growing polymer chain by quenching the polymerization with tritiated alcohols. The use of these quenching agents is based on the concept of the anionic coordination mechanism of olefin polymerization occurring... 

However, in olefin polymerization by two-component catalysts during polymerization not only active transition metal-polymer bonds are formed, but also inactive aluminum-polymer ones, as a result of the transfer process with the participation of a co-catalyst (11, 162-164). The aluminum-polymer bonds are quenched by tritiated alcohol according to the scheme (25), so an additional tagging of the polymer occurs. The use of iodine (165, 166) as a quenching agent also results in decomposing inactive metal-polymer bonds. 

Several determinations of the number of propagation centers by the quenching technique have been carried out (98, 111). As a quenching agent methanol, labeled C14 in the alkoxyl group, proved to be suitable in this case. The number of active centers determined by this technique at relatively low polymerization rates (up to 5 X 102 g C2H4/mmole Cr hr at 75° and about 16 kg/cm2) (98, 111, 168) in catalysts on silica was about... 

Furthermore volatile SbX3 decomposes in the heat of the flame to generate fine particles of Sb203, which form sites for radical recombination. A problem with these gas-phase radical quenching agents is that the volatiles emitted are acidic and corrosive. 

Using quenching agents like Nile Blue and Sudan Black are effective in suppressing AF for confocal or widefield microscopy... 

Remove excess reactant by gel filtration using a desalting resin with PBS, pH 7.4. Some protocols use a quenching agent to remove excess periodate prior to gel filtration. This can be done by adding glycerol to a final concentration of 0.1 M. 

Centrifuge and wash the particles at least 3 times with buffer to remove unreacted protein and quenching agent. Finally, suspend the particles in a suitable storage buffer containing a preservative. 

Wrighton and coworkers101. The basic photochemistry is thought to involve the formation of an excited state charge transfer complex [R3E+-Re(CO)3L-]. This in turn cleaves to form R3E- and [Re(CO)3L]-. In the presence of suitable quenching agents, the excited state can be quenched. 

To demonstrate the ability of the system to perform a matrix experiment as described above, concentrations of enzyme, substrate, and ATP were varied across the 24 wells in a row of an SBS 384-well microtiter plate. Results of these types of evaluations for the optimization of an assay for a protein kinase A and Kemptide system were presented by Wu et al.12 All the reactions were carried out in lOOmM HEPES, pH 7.4, lOmM MgCl2, lOmM DTT, and 0.015% Brij-35. No quenching agent was used. A sample from each of the 24 wells was analyzed in parallel every 6.5 min as the 24 enzymatic reactions progressed. 

From the same zirconocene intermediates, Huang and co-workers have prepared vinyl sulfides [24,35] and sulfones [26] through use of the appropriate quenching agents (Scheme 4.6). Treatment of vinyl zirconocenes with an equivalent of a disulfide in THF at 60 °C affords, after work-up and purification, (E)-vinyl sulfides in good isolated yields. Vinyl sulfones, which as a class are generally useful as Michael acceptors and Diels—Alder dienophiles, are obtained in about two hours upon treatment of (fc)-vinyl zirconocenes with various sulfonyl chlorides in THF at 40°C. 

Unfortunately, many reactions do not occur with quantitative conversion and in near absolute purity. The work-up and purification of most chemical processes probably takes up most of a bench chemists time. Therefore techniques that simplify and accelerate these operations not only free up valuable time, but allow greater creativity and increased levels of output. Here again, supported systems can be used to aid the chemist in the guise of scavengers, quenching agents and catch and release systems. 

Supported scavengers are reactive species that selectively quench and/or sequester by-products of the reaction or remove any excess starting materials, and are subsequently removed by filtration (Fig. 2.2). These species, which allow hquid-solid phase extraction are also often referred to as sequestering agents or quenching agents . 

---