Free radical photo-induced reaction

Irradiation of benzene solutions of 7e-aryl arenetellurocarboxylates with light from a mercury lamp generated radicals via homolytic cleavage of the Te—CO bond. The free radicals subsequently formed aldehydes, diorgano tellurium compounds, and diorgano ditellurium compounds via hydrogen abstraction, dimerization and elimination of tellurium, or dimerization -. The products of the photo-induced reaction of Te-4-methylphenyl 2-methylthiobenzene(tellurocarboxylate) are given as an example . ... 

Bhawe (14) has simulated the periodic operation of a photo-chemically induced free-radical polymerization which has both monomer and solvent transfer steps and a recombination termination reaction. An increase of 50% in the value of Dp was observed over and above the expected value of 2.0. An interesting feature of this work is that when very short period oscillations were employed, virtually time-invariant products were predicted. 

Oxidations initiated by thermally induced electron transfer in an oxygen-CT complex represent the thermal analog of the Frei photo-oxidation and are properly classified as hybrid type IlAOi-type IIaRH oxidations (Fig, 2), Such reactions require either zeolites with high electrostatic fields or substrates with low oxidation potentials. In addition, elevated temperatures are known to promote the thermally initiated electron-transfer step, although the possible intrusion of a classical free-radical initiation chain oxidation at higher temperatures must be considered. 

The photo-initiated addition process appears to have general applicability, although it can require extensive photolysis times [194-196]. Indeed, photolytic generation of RF- from RFI has been the method used to add Rf- to C60 and C70, not for synthetic purpose, but to examine epr spectra of the resulting radical species [197-199]. A good comprehensive review of the early work on thermal and photochemically-induced free radical addition reactions to olefins can be found in Sosnovsky s book [60]. 

The Photo-oxidation of Methoxy Substituted Phenols. As we are now aware of the fact that there are several routes to form phenols in the chemical reactions of lignin, we have looked into the nature of free radical induced photooxidation of phenols (IS). There is no doubt that photolysis of methoxy phenols in solutions or in solid state produces yellow products. Molecular oxygen has often been shown to be a necessary reactant in the oxidation, although the radical nature of the mechanism is not understood (16). 

Appropriate modification of the ESR spectrometer and generation of free radicals by flash photolysis enables time-resolved (TR) ESR spectroscopy [22]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They can be enhanced as much as one-hundredfold and appear as absorption, emission, or a combination of both. Effects of this type are a result of chemically induced dynamic electron polarization (CIDEP) these spectra indicate the intermediacy of radicals whose sublevel populations deviate substantially from equilibrium populations. Significantly, the splitting pattern characteristic of the spin-density distribution of the intermediate remains unaffected thus, the CIDEP enhancement not only facilitates the detection of short-lived radicals at low concentrations, but also aids their identification. Time-resolved ESR techniques cannot be expected to be of much use for electron-transfer reactions from alkanes, because their oxidation potentials are prohibitively high. Even branched alkanes have oxidation potentials well above the excited-state reduction potential of typical photo-... 

A few more free radical reactions of nitro compounds should be mentioned. Some of these reactions are photo-induced and are described in the chapter on photochemistry of nitro compounds. 

They are formed in the following chain of events. First, the initial step, a chain initiation, which happens as a result of thermal or photo-induced breakage of a chemical bond in the polymer. This often happens with the help of a catalyst (water, metal ion) in the vicinity of the reaction site, where the initiation step produces a hydrocarbon free radical, R ... 

The photo-induced substitution reactions of alkenylmercury compounds appear to proceed by a free-radical chain mechanism [e.g. reactions (20)]. Photolysis of... 

Scheme 28, by application of the known thiol ester-thiopyrone phototransformation to selenium-containing systems. 5/f-[l]Benzoselenino[2,3-6]-pyridine, 4.ff-selenolo[2,3-6][l]benzoselenine, and 9/f-seleno[3,2-6][l]benzo-selenine have similarly been obtained by the corresponding selenol ester-seleninone conversion. Phenyl areneselenosulphonates undergo facile photo-induced homolysis of the selenium-sulphur bond in the presence of alkenes, a free-radical chain reaction leads to the formation of -phenylselenosulphones. ... 

Numerous publications have appeared within the year which formally describe light-induced substitution of an aromatic compound. As in previous years, the current account is restricted mainly to reactions in which the light-absorbing species is the aromatic compound and the aromatic ring is directly involved. Reactions in which the substituent undergoes chemical change are only briefly mentioned, and accounts of attack of photo-generated free radicals on the aromatic species are not included. 

---