Photochemical chain reactions initiation

In early attempts to produce an iron-oxo species (20) from typical porphyrins like chloro-a,/3,y,8-tetraphenylporphinatoiron(III) [Fe(III)TPP-Cl] and chloroferriprotoporphyrin(IX)[Fe(III)PPIX-Cl], we examined the reaction of t-butyl hydroperoxide and peroxy-acids with alkanes and olefins in the presence of these catalysts. With peroxyacids, decomposition of the porphyrin ring was observed, while with the f-butyl hydroperoxides, product distributions were indistinguishable from free-radical chain reactions initiated photochem-ically in the absence of any metals. 

Quantum yield, in photochemical reactions, is the ratio of the number of chemical reactions caused by light to the number of photons absorbed by the chemical species. With the exception of some rare photochemical processes in bio-inorganic chemistry, in which chain reactions initiated by absorption of a single photon result in multiple catalytic events and hence quantum yield greater than unity, in the vast majority of photochemical reactions and in all known photobiological reactions such as photosynthesis, vision, and phototropism the quantum yield is less than 1.0. 

Oxidation of ethanol by a chain reaction initiated by the photochemical homolysis of the oxygen-metal bond was reported to proceed with a turnover... 

Fig. 2 Simplified representation of photocatalytic reaction pathways ) photo induced catalytic reaction, N denotes a nominal catalyst which yields photochemically the real catalyst C (2) photoinduced chain reaction, here I means any photoinitiator (3) photoinduced chain reaction initiated by a sacrificial agent I (4) photoassisted reaction with N as nominal catalyst (photo-assistor) (5) photoassisted reaction, the catalyst N is a short-lived intermediate in ground state (6) sensitized photoreaction, N represents a sensitizer (7) catalyzed photo-reaction, C is to be considered as a real catalyst.

Thermal chlorination is preferred, but photochemical or catalytic methods are also employed. The thermal chlorination is a radical chain reaction, initiated by chlorine atoms at temperatures of 350-550 °C... 

A photochemical chain reaction is initiated by the absorption of a photon. 

Accounting for the high yields of the photoacylation of 1 with aromatic aldehydes (Scheme 12), Moore and Waters suggested a photochemical chain reaction of very long chain-length , initiated via hydrogen abstraction from the aldehyde by the excited quinone (see Scheme 14 out-of-cage scenario). However, quantum yield data and effects of inhibitors did not support this proposal as, for example, a quantum yield for the disappearance of the quinone ( d) of Oj = 1 has been reported for irradiation of 1 in benzaldehyde at 436 nm. ... 

Chlorine atoms obtained from the dissociation of chlorine molecules by thermal, photochemical, or chemically initiated processes react with a methane molecule to form hydrogen chloride and a methyl-free radical. The methyl radical reacts with an undissociated chlorine molecule to give methyl chloride and a new chlorine radical necessary to continue the reaction. Other more highly chlorinated products are formed in a similar manner. Chain terrnination may proceed by way of several of the examples cited in equations 6, 7, and 8. The initial radical-producing catalytic process is inhibited by oxygen to an extent that only a few ppm of oxygen can drastically decrease the reaction rate. In some commercial processes, small amounts of air are dehberately added to inhibit chlorination beyond the monochloro stage. 

Sulfoxidation is a photochemical reaction. The radical chain reaction is initiated by triplet sulfur dioxide (3 S02), excited by ultraviolet (UV) light of wavelength longer than 320 nm ... 

The problem of competition of the molecular reaction (direct route) and chain reaction (complicated, multistage route) was firstly considered in the monograph by Semenov [1], The new aspect of this problem appeared recently because the quantum chemistry formulated the rule of conservation of orbital symmetry in chemical and photochemical reactions (Woodward-Hofmann rule [4]). Very often the structure of initial reactants suggests their direct interaction to form the same final products, which are also obtained in the chain reaction, and the thermodynamics does not forbid the reaction with AG < 0. However, the experiment often shows that many reactions of this type occur in a complicated manner through several intermediate stages. For example, the reaction... 

In some reactions involving gases, the rate of reaction estimated by the simple collision theory in terms of the usually infened species is much lower than observed. Examples of these reactions are the oxidation of H2 and of hydrocarbons, and the formation of HC1 and of HBr. These are examples of chain reactions in which very reactive species (chain carriers) are initially produced, either thermally (i.e., by collision) or photochemically (by absorption of incident radiation), and regenerated by subsequent steps, so that reaction can occur in chain-fashion relatively rapidly. In extreme cases these become explosions, but not all chain reactions are so rapid as to be termed explosions. The chain... 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

The hydrogen-chlorine chain reaction has proved to be one of the most controversial systems yet studied. After thirty years of investigation Bodenstein43 was able to say in 1931 that every worker on the photochemical synthesis of HC1 had produced his own mechanism even as late as 1940 little positive information had been obtained. However, the accumulated techniques and experience had firmly established the importance of atom chain reactions. The mechanism of photo-initiation and propagation is the same as for the hydrogen bromide photosynthesis, a non-branching chain reaction... 

Photochemically generated radicals in chain reactions are less familiar to synthetic chemists [8,21]. The above mentioned peroxides have been used in the presence of light to initiate radical chain reactions at room or lower temperatures. Azo compounds are also known to decompose photo-lytically to afford alkyl radicals. AIBN has rarely been used under such conditions. 

Furthermore this chapter deals chiefly with polymerizations which are catalyzed by acid-acting catalysts. A comprehensive discussion of not only the thermal but even the photochemical and free radical-initiated polymerizations is outside its scope. The free radical-initiated reactions include those which are induced by metal alkylies, peroxides, oxygen and certain other substances. They depend on free radical initiation of a chain reaction whether or not these free radicals should be considered to be catalysts has been questioned because the radicals enter into the reaction chain and are part of the reaction product. 

Chlorine combines with hydrogen forming hydrogen chloride, HCl. The reaction occurs rapidly when exposed to hght, involving a photochemical chain initiation step. 

Two examples of the chain reaction involving the a-methylene radical are the irradiation of acetone in the presence of norbornene,107 and that of cyclohexanone in the presence of cyclohexene.108 106 These reactions predominate so that the other products derived from the photochemical reactions were not initially characterized. 

The quantum yield of photochemical processes can vary from a low fractional value to over a million (Section 1.2). High quantum yields are due to secondary processes. An initially excited molecule may start a chain reaction and give rise to a great number of product molecules before the chain is finally terminated. For nonchain reactions, the quantum yields for various competitive photophysical and photochemical processes must add up to unity for a monophotonic process if the reaction occurs from the singlet state only ... 

In recent years visible photoinitiators for the formation of polymers via a radical chain reaction have also been developed. These absorb light which is blue, green, or red and also cause the polymerization of polyolacrylates, in some instances, such as encapsulated systems, with speed which is near photographic. Some of these photoinitiators provide the photochemical backbone of the nonsilver, near-photographic speed, imaging processes such as the Cycolor processes invented by the Mead Corporation. Cycolor initiators are cyanine dye, borate ion salts (4)—so-called ( +, —) ion pair... 

These photoinitiation processes which depend on the formation of free radicals in some photochemical reaction lead to chain reactions, since each molecule of initiator can promote the addition of many monomer units to a polymer chain. The quantum yield of monomer addition can therefore be much larger than unity, but it cannot be controlled since the growth of a polymer chain is then limited by termination reactions in which two free radicals react to produce closed-shell molecules. 

A free radical chain reaction proceeds through a succession of free radicals. In the photochemical chlorination of an alkane, the initiating step is the homolytic lission of chlorine molecules to produce chloroalkanc molecules and chlorine free radicals. These two reactions constitute the propagating step. However, the chlorine free radicals may also combine to form chlorine molecules or react with the alkane free radicals to form chloroalkane molecules. Both of these reactions constitute terminating steps of the chain reaction. Il should be noted, however, that the foregoing sequence cannot take place in the dark. Exposure to light allows the series of reactions then to proceed rather violently. 

One first selects a chain reaction that can be initiated photochemically and that is terminated by the recombination and disproportionation of interest. An example would be the tin hydride reduction of an alkyl bromide, which proceeds according to Scheme 5. Kinetic analysis (see p. 493) yields a relation between rate... 

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