The Kinetics of Reaction of 2,2-Diphenyl-1-Picrylhydrazyl With Phenols

Abstract

The ki~letics of reaction betireen 2,2-diphellyl-1-picrylhyclrazyl ( D P P H ) and a wide variety of phenols have been studied. 'The rate of disappearance of D P P H is of first order with respect to both the D P P H and the reacting phenol. The rates of reaction can be roughly correlated with the Hammett u value of the phenol substituent in the range -0.4 < u < 0.2, a p value of -6 being obtained. I-Butyl groups in both ortho positions of the phenol give rise to steric hindrance, the reductio~l in rate being largely due to a reduction in the A factor. Hydrogen abstraction fron~ the less reactive phenols is stroilgly retarded by the product, %,2-diphenyl-1-picrylhydrazine.The rate-determining step probably involves the abstraction of a hydrogen atom from the phenol by the DPPM to give diphenylpicryll~ydrazine and a phenoxy radical. The retardation by diphe~~ylpicryll~yclrazi~~e is readily explained if this primary step is reversible.Hydrogen-abstraction reactions involving 2,2-diphenyl-1-picrylhydrazyl ( D P P H ) as the acceptor molecule are relatively easily studied and a number of recent publications have been concerned with the rates of reaction of this radical with hydrogen donors. Inforination has been obtained concerning the reactivities of restricted ranges of donors containing C-IH (I), N-I-I (2, 3), 0-H (4, 5 , G), and S-I-I (7, 8) bonds. Phenols are well suited to an investigation of hydrogen abstraction froin 0-I-I-containing compouilds (3). A wide range of phenols is available and a detailed study of the effect of substituents is possible.During the course of the worli, a paper b y McGowan, Powell, and Raw was published (4) giving rate constants for the second-order reactioils of DPPIH with a large number of phenols, showii~g that substituents inay exert coilsiderable polar and steric influences on the reaction. Their measurements were normally made a t room temperature using carboil tetrachloride as solvent, but with three phenols, n~easurements were made over a teinperature range. In the work described here benzene was used as solvent, activation energies and A factors were obtained for all the phenols studied, and the effect of the product 2,2-diphenyl-1-picrylhydrazine on the rate of reaction was determined. Soine additional evidence was obtained that the primary reaction i~lvolves the hydrogen atom of the 0-H group, and the stoichiometry of the process was investigated.ESPERIRIES?'.AL iWaterials 2,2-Diphenyl-1-picrylhydrazine was prepared by the reaction of picryl chloride and 1,l-diphe11)rlhydrazine (9, 10). DPPIH was prepared by the oxidation of diphenylpicrylhydraziile with lead dioxide (9). Co~nplexed solveilt was re~noved by heating ilz vaci~o a t GO" for several hours.2-h4ethyla1tisole and 1-and 2-~~~ethoxynaphthalene (Eastman Kodali) were used without further purification.Phenols were obtained froin the Aldrich Chemical Coinpany or from the Eastinan I<odalr Company. The liquids were distilled and the solicls were either subliined a t reduced pressure or recrystallized; 2,G-di-t-butylphen...

The Inhibited Autoxidation of Styrene: Part Ii. The Relative Inhibiting Efficiencies of Meta- And Para-Substituted Phenols

“…Reaction [7] may possibly be an example of this phenomenon. Similarly, the fact that reaction [15] definitely follows u+ constants whereas reactions [13] and [14] tend more to favor a constants would also be in line with this suggestion.…”

“…With the exception of reaction [2], compounds containing one or two ortho substituents have not been included in the determination of p. The original data have been recalculated in most cases* and the best correlation, i.e., that giving the smallest standard deviation (root mean square in loglo units), is given in heavy type. I t can be seen that in aromatic solvents and in CC14 only reactions [7] and [14] are better represented by u than by a+ and that for reactions [4] and [13] no distinction can properly be made.…”

Methods of measurement and evaluation of natural antioxidant capacity/activity (IUPAC Technical Report)

“…[25]. ANTIRADICAL ACTION DPPH • quenching reactions have been used to elucidate structural effects on chemical mechanisms and reactivity of simple phenols since the late 1950s [26][27][28][29]. Antioxidant activity assays for complex polyphenols were initiated in a 1985 pivotal study in which Wayner et al used the water-soluble azide azobis (2-amidinopropane hydrochloride) to generate radicals in plasma, then followed inhibition of oxygen consumption to investigate compounds contributing to antioxidant capacity [30].…”

“…Natural antioxidant capacity/activity 975 3.5 DPPH assays 3.5.1 Experimental approach DPPH is a stable radical with a deep purple colour whose reaction with other radicals, reducing agents, or compounds capable of HAT leads to loss of colour at 515 nm and loss of its electron paramagnetic resonance (EPR) free radical signal [26,29]. Like ABTS +• , DPPH • reacts with both electron and hydrogen donors [75,76], though more slowly, and steric accessibility to the radical site is a clear issue [27,28]. No antioxidant assay is simpler or less expensive to run than the DPPH assay, which accounts for its popularity and extensive use.…”

Phytochemical andin vitrototal antioxidant capacity analyses of peel extracts of different cultivars ofCucumis meloandCitrullus lanatus

“…The degree of decolorization indicates the scavenging potential of the antioxidant compound (Ali et al, 2011). The stoichiometry of the reaction of DPPH radical with various antioxidants has been reported (Hogg et al, 1961;Cuvelier et al, 1992). Natural antioxidants have different rates of reaction with DPPH free radicals depending on their site of occurrence in the cell, solubility in water or fat, etc.…”