Diphenylpicrylhydrazyl

Scavengers such as diphenylpicrylhydrazyl radicals [II] react with other radicals and thus provide an indirect method for analysis of the number of free radicals in a system ... 

The diphenylpicrylhydrazyl radical itself is readily followed spectrophotometri-cally, since it loses an intense purple color on reacting. Unfortunately this reaction is not always quantitative. 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Examples of radicals which are reported to meet these criteria are diphenylpicrylhydrazyl [DPPH, (22)], Koelsch radical (26), nitroxides [e.g. TEMPO (23), Fremy s Salt (24)], triphenylmethyl (25), galvinoxyl (27), and verdazyl radicals [e.g. triphenylverdazyl (28)]. These reagents have seen practical application in a number of contexts. They have been widely utilized in the determination of initiator efficiency (Section 3.3.1.1.3) and in mechanistic investigations (Section 3.5.2). 

The biradical catalysts described previously for double-base propints (Ref 80) are also effective for hydrocarbon propints. Table 34 shows how p,p,-biphenylene-bis(diphenylmethyl) compares to n butyl ferrocene as a catalyst in a carboxy-terminated polybutadiene. These catalysts are claimed to overcome all of the processing difficulties, chemical stability and volatility disadvantages attributed to catalysts based on ferrocene and carborane derivatives. Another somewhat similar functioning catalyst, the free radical compd, 2,2-diphenylpicrylhydrazyl,... 

There is some evidence in favor ° of the captodative effect, some of it is from ESR studies. However, there is also experimental and theoretical evidence against it. There is evidence that while FCH2 and p2CH are more stable than CH3, the radical Cp3- is less stable that is, the presence of the third F destabilizes the radical. " Certain radicals with the unpaired electron not on a carbon are akso very stable. Diphenylpicrylhydrazyl is a solid that can be kept for years. We have already mentioned nitroxide radicals. Compound 29 is a nitroxide radical so stable that reactions can be performed on it without affecting the unpaired electron (the same is true for some of the chlorinated triarylmethyl radicals mentioned above ). ot-Trichloromethylbenzyl(rer/-butyl)aminoxyl (30) is extremely stable. In... 

The ability of the stable free radical diphenylpicrylhydrazyl (DPPH) to act as an efficient trap for reactive radicals such as 804 and OH- has been utilised by Bawn and Margerison in their examination of the Ag -S20g couple. The disappearance of the intensely coloured DPPH gave excellent zero-order kinetics the rate as a whole was identical with that found by Fronaeus and Ostman and 2 was given by 3.1 x 10 exp(—17.9x lO /RT) l.mole sec A Sengar and Gupta have also determined Arrhenius parameters for this reduction and have compared them with those for some redox processes (Table 23). 

AC ADME ANS AUC BA/BE BBB BBM BBLM BCS BLM BSA CE CHO CMC CPC CPZ CTAB CV DA DOPC DPPC DPPH aminocoumarin absorption, distribution, metabolism, excretion anilinonaphthalenesulfonic acid area under the curve bioavailability-bioequivalence blood-brain barrier brush-border membrane brush-border lipid membrane biopharmaceutics classification system black lipid membrane bovine serum albumin capillary electrophoresis caroboxaldehyde critical micelle concentration centrifugal partition chromatography chlorpromazine cetyltrimethylammonium bromide cyclic votammetry dodecylcarboxylic acid dioleylphosphatidylcholine dipalmitoylphosphatidylcholine diphenylpicrylhydrazyl... 

These products are thought to result from attack of an intermediate p-nitro-phenyl radical on the solvent. Evidence for this intermediate was obtained by scavenging the radical with diphenylpicrylhydrazyl, halogens, and nitric oxide.(62) However, the presence of the p-nitrophenetole in the products formed in the presence of iodine, which effectively eliminates most other radical products, suggests that another mechanism involving a phenyl carbonium ion may be also operative ... 

By extrapolation from the behavior of diphenylpicrylhydrazyl, the dissociation of tetraaryl hydrazines into colored, reactive substances in solution at high temperatures is probably a radical reaction. It is only partly reversible, owing to disproportionation. 

Tetra-/>-anisy lhy drazine is green in benzene at room temperature.185 On the other hand, 1,2-diphenyl-1,2-di- >-nitrophenylhydrazine appears not to be dissociated at all. If this difference in degree of dissociation is real, and not a specious one due merely to a difference in color or reactivity of the radicals, it presents a puzzling contrast to the case of diphenylpicrylhydrazyl and to the triphenylmethyl series in which both kinds of substituent stabilize the radical. 

An example (1S4) of the Overhauser effect in a nonmetallic system was afforded by diphenylpicrylhydrazyl (DPPH), an organic free radical. For partial saturation of the EPR of the radical, an approximately tenfold enhancement of the proton resonance from the ring protons of the DPPH molecule was observed. The Overhauser effect has also been observed in charcoals (proton resonance) and graphite (O resonance) 1S5), and natural crude oils (1S6). 

A third method involves the use of radical scavengers, which count the number of radicals in a system by rapidly stopping their growth. Three groups of radical scavengers are used [Bamford, 1988 Koenig and Fischer, 1973]. The first group consists of stable radicals such as 2,2-diphenylpicrylhydrazyl (DPPH) obtained by oxidation of diphenylpicrylhydra-zine ... 

Detection of Radical Anion by ESR Spectroscopy. The ESR measurements of the rate of free radical formation by electron transfer from fluorene to nitroaromatics were obtained by use of the flow system and U-type mixing cells described previously (18, 20). Concentrations were estimated by comparison of the total area of overmodulated first-derivative spectra with solutions of diphenylpicrylhydrazyl under identical solvent and instrumental conditions. Relative concentrations within a given experiment are considered accurate to within a few per cent, while absolute concentrations are considered to be accurate to 30%. 

Certain radicals with the unpaired electron not on a carbon are also very stable.169 Diphenylpicrylhydrazyl is a solid that can be kept for years. We have already mentioned nitroxide radicals. 19 is a nitroxide radical so stable that reactions can be performed on it... 

EPR Measurements. All intensity measurements were made with a Varian 4500 (100 kc. field modulation) spectrometer. Spin concentrations were estimated by comparison with solid diphenylpicrylhydrazyl. The number of radicals was assumed to be proportional to signal height times signal width squared. 

In practice, all inhibitors show some deviations from ideal behaviour for example when diphenylpicrylhydrazyl is used as a radical scavenger it is converted to products which are reactive towards radicals as shown by the fact that polymerizations occurring after the induction periods are obviously retarded. The use of 14C-hydrazyl in the radical polymerization of styrene has shown that substantial quantities of the material are combined in the polymer 36). 

Conversion to /V,/V-diphenylpicrylhydrazyl. Dissolve 3.95 g (0.01 mol) of the above hydrazine in 60 ml of dry dichloromethane, add 50 g of lead dioxide and 4 g of anhydrous sodium sulphate and shake the mixture mechanically for 1 hour. Filter, and concentrate the deep-violet filtrate on a rotary evaporator. Dilute the residual solution with two volumes of ether, and allow the product to crystallise. Filter off the large black-violet crystals of diphenylpicrylhydrazyl and wash them with ether the yield is 3.5 g (89%), m.p. 137-138°C. The product may be recrystallised from a mixture of dichloromethane and ether. 

Cyclopentadiene undergoes ionic polymerisation by Friedel-Crafts catalysts32. Its polymerisation by y-rays is markedly suppressed by ammonia or amines but much less by diphenylpicrylhydrazyl or oxygen33. This again points to the ionic rather than the free radical nature of the radiation-induced polymerisation. The quenching effect of ammonia was postulated as due to reactions of the type... 

The decolorisation of solutions of the stable free radical diphenylpicrylhydrazyl (DPPH) in a wide variety of organic solvents upon irradiation has been known for some time and attributed to capture of radicals produced by the action of the radiation on the solvent by DPPH... 

Bergstrom, G. and Lagercrantz, C., Diphenylpicrylhydrazyl as a reagent for terpenes and other substances in thin-layer chromatography, Acta Chem. Scand., 18, 560, 1964 Chem. Abs., 61, 2491h, 1964. 

Diphenylpicrylhydrazyl (0.06% in chloroform) Yellow spots on purple background after heating (110°C, 5 min) 3... 

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