Phenols, flash-photolysis

Light can effect the coupling of phenols. For example, Joschek and Miller (22) found that phenoxyphenols could be produced in the flash photolysis of phenol, but although sought, no dioxin was detected in the reaction products. 

The actual limit value of rr, below which the time constraint is met for a given transducer, is somewhat ambiguous. For a 0.5 MHz transducer (response time 2 xs), Mulder et al. [297] set this limit at 60 ns, based on the observation of a maximum of amplitude of the photoacoustic wave with the concentration of phenol and calculating rr from the rate constant of reaction 13.24, k = 3.3 x 108 mol-1 dm3 s-1 [298]. Later, Wayner et al. [293] empirically choose 100 ns as that limit and used laser flash photolysis results to adjust the phenol concentration until the lifetime of reaction 13.24 was less than that limit. In any case, the safest way of ensuring that the time constraint is being met is to verify it experimentally by varying the concentration of substrate until the observed waveform reaches a maximum (or, equivalently, until the final A0bs77 value reaches a maximum). 

Flash photolysis has proven to be a useful technique for investigating the reactivity of flavin radicals. This approach takes advantage of the fact that illumination of flavins in the presence of suitable reducing agents (e.g. EDTA, phenols, indoles) leads to a one-electron reduction of the flavin triplet state Through the... 

Following the classical observation of Lewis and his school (29, 30, 34) of the photoionization of aromatic molecules in rigid solvents, Land, Porter and Strachan (25) proved such processes in the flash photolysis of aqueous solutions of phenols. It was suggested (7, 15, 25, 38) that the primary photochemical act involves electron ejection. 

The enol tautomers of many ketones and aldehydes, carboxylic acids, esters and amides, ketenes, as well as the keto tautomers of phenols have since all been generated by flash photolysis to determine the pH rate profiles for keto-enol interconversion. Equilibrium constants of enolization, KB, were determined accurately as the ratio of the rate constants of enolization, kE, and of ketonization, kK, Equation (1). 

Banks JT, Ingold Kl, Lusztyk J (1996) Measurement of the equilibrium constants for complex formation between phenol and hydrogen-bond acceptors by kinetic laser flash photolysis. J Am Chem Soc 118 6790-6791... 

The high value for the quenching of 3,4-dimethoxyacetophenone by phenol suggests that it is probable that within the lignin structure hydroxyl groups are able to quench carbonyls by a static mechanism to yield phenoxy-ketyl radical pairs which decay on a timescales faster than the time resolution of our laser flash photolysis apparatus. Intersystem crossing rate constants for triplet radical pairs in the restricted environments of micelles have been demonstrated to be of the order of 2 -5 x 106 s-1 (25, 24). However, in the lignin matrix where diffusional processes are likely to be... 

The kinetics of combination of radical pairs formed by quenching of the triplet excited state of B by 2,4,6-trimethylphenol (HR) in films of rigid and plasticized polyvinylchloride (PVC) were studied by the method of laser flash photolysis by Levin et al. At high concentrations of the phenol quencher ( 15 wt%), most of the triplet excited state of the ketone is quenched and the formation and disappearance of the ketyl and phenoxyl radicals can be followed from changes in their characteristic transient absorptions at 540 and 390 nm, respectively. 

A very useful method in transforming phenols, even unstable ones, into phenoxyl radicals is flash photolysis 176,177>. When 2,4,6-triphenylphenol (la) in benzene was photolysed the radical 3a as well as its dimer 4a were observed. The K-value 3a 4a, determined by flash photolysis, agrees well with the K-value of the colorimetric determination, mentioned below. Flash photolysis can also be used to evaluate the rates of dimerization or dissociation of 3a -> 4a (k being in the order of 0.3 sec mole-1), as well as determining the activation parameters 177). 

The phenol ArOH is always a side product, resulting from some ArO that leaks from the solvent cage and abstracts a hydrogen atom from a neighboring molecule. When the reaction was performed on phenyl acetate in the gas phase, where there are no solvent molecules to form a cage (but in the presence of isobutane as a source of abstractable hydrogens), phenol was the chief product and virtually no o- or p-hydroxyacetophenone was found. Other evidence for the mechanism is that CIDNP has been observed during the course of the reaction and that the ArO radical has been detected by flash photolysis and by nanosecond time-resolved Raman spectroscopy. ... 

The flash photolysis of hexaarylbiimidazole produces imidazolyl radicals " which have been shown to be more nearly planar than the parent dimers ort/io-substituents in the aryl rings decrease the radical stability. The radicals oxidize electron-rich substrates by rapid electron abstraction from tertiary amines, iodide, and metal ions, and by hydrogen abstraction from phenols, mercaptans, secondary amines, and active methylene com-pounds." " Studies have been made of the photooxidation of /euco-triphenyl-methane dyes by these radicals." " ... 

H-CIDNP and ESR techniques. A laser flash photolysis and time-resolved ESR study of the formation of phosphinoyl radicals from benzoyldiphenyl-phosphine oxide and 257 has appeared. The addition of dialkyIphosphoryl radicals to a fullerene system has also been studied. Interest in adducts of phosphine oxides with proton donors, notably phenols and other solvent guests ° has continued. 

The two keto tautomers of phenol 3, i.e. 4 and 5, were generated by flash photolysis of polycyclic precursors 6-8 in aqueous solution, and the pH-rate profiles of their 4 3 and 5 3 enolization reactions were measured. The rates of the reverse reactions, 3 4 and 3 5, were determined from the rates of acid-catalyzed hydrogen exchange at the ortho- and para-positions of phenol 3 (equation 3). 

Optical absorption spectra have not been utilized to measure these pA a values, although the radical cations of several phenols have been observed by pulse radiolysis and by laser flash photolysis in organic solvents. The spectra were found to be different than those of phenoxyl radicals, and in the presence of water they underwent very rapid deprotonation to form the corresponding phenoxyl radicals. 

Poly(alkyl and aromatic ethers) - Using laser flash photolysis poly(2,6-dimethyl-l,4-phenylene oxide) undergoes scission at the phenolic link to give phenoxy radicals . Poly(vinyl methyl ether) has been shown to undergo a complex series of photoprocesses as shown in Scheme 2. ... 

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