Photochemical reaction consecutive

Sunbathing to obtain a tan, or simply to soak up the heat, is an inadvertent means of studying photochemical reactions in the skin. It is also a good example of a consecutive reaction for which k(i) < k(2. ... 

Figure 5.1. Various adiabatic photochemical reaction mechanisms (see text for details), (a) Simple case of dual fluorescence (b) illumination changes sample (i.e., photochemistry) (c) strong fluorescence quenching (photochemical funnel) (d) competitively coupled product species (e) consecutively coupled product species.

Andre JC, Bouchy M, Kossany J (1983) Computer Analysis of Mixing Problems Arising in Consecutive Photochemical Reactions,/. Photochem. 22 213-221. 

It is obvious that these relationships can be deduced from the stoichiometric scheme explained in Sections 2.1.1.1 and 2.1.3.3. A comparison with the consecutive reaction given in Example 2.7 makes it obvious how similar the discussions of thermal and photochemical reactions are. For this reason in Table 2.1 some of the kinetic parameters discussed in the previous sections are compared. 

All the equations used above can be taken in principle to describe photochemical reactions also. However, the relationships become more complicated, since the amount of light absorbed has to be considered. Furthermore the quantum yield can depend on the concentration which makes the formulae more complex. Therefore in a simple example of a consecutive photoreaction, the correlation is demonstrated. Details are discussed in Chapter 3. 

Both are considered to be used as laser dyes. The substituted coumarin 7,7 -diethylamino-4-tnethyl-coumarin (DEMC) had been mentioned before and it has been the focus of intense kinetic examinations. The flucN escence reaction spectrum demonstrates a consecutive photochemical reaction via a second flum escent intermediate (see Fig. S.43). Looking at the reaction spectrum, the same information is not so obvious. 

For most unsaturated hydrocarbons, addition of OH is the first and rate-limiting step of the photochemical reaction chain. In the case of aromatics, which are emitted from automobiles, forest fires and fuel wood burning [11], the addition reaction is reversible at atmospheric temperatures. The effective rate constant for removal of the aromatic depends on consecutive reactions of the adduct. Prior to LACTOZ, consecutive reactions with O2 had not been detected for benzene -OH... 

The discovery of chain reactions was a result of intensive studies of photochemical reactions. In 1912 Einstein formulated the law about the interaction of a photon with a molecule, according to which the quantum yield of the photochemical reaction cannot exceed unity. M. Bodenstein studied a series of reactions that occur under irradiation and discovered that the reaction of chlorine with hydrogen occurs with a huge quantum yield to million of molecules per absorbed quantum. He proposed that the reaction occurs as a chain of consecutive transformations the photon knocks out an electron from the molecule. The electron induces the chain of consecutive transformations of H2 and into HCl. However, measurements of electroconductivity... 

The side-chain substitution of toluene, p-chlorotoluene, etc. is industrially practised. This reaction is carried out in a photochemical reactor. It is an exothermic reaction in which HCl is produced. The reaction is consecutive, and hence CL first reacts with toluene reacts to form the desired benzyl chloride, which is then converted to benzal chloride, and finally benzotrichloride. We may, however, well be interested in the selectivity to benzyl chloride. An additional complication arises due to nuclear chlorination, which is most undesirable. A distillation-column reactor can offer advantages (Xu and Dudukovic, 1999). 

When irradiated in the presence of norbornadiene and high-pressure synthesis gas, rhodium chloride is converted to a catalyst which is active for a variety of reactions. /2A/. The salt is probably converted photochemically to the rhodium norbornadiene complex 9. This dimer may undergo a consecutive photoreaction to give the monomeric hydrido complex 10, which is the actual catalyst for polymerisation, hydrogenation, and hydroformylation reactions. 

The chain unit in the thermal and photochemical oxidation of aldehydes by molecular dioxygen consists of two consecutive reactions addition of dioxygen to the acyl radical and abstraction reaction of the acylperoxyl radical with aldehyde. Experiments confirmed that the primary product of the oxidation of aldehyde is the corresponding peroxyacid. Thus, in the oxidation of n-heptaldehyde [10,16,17], acetaldehyde [4,18], benzaldehyde [13,14,18], p-tolualdehyde [19], and other aldehydes, up to 90-95% of the corresponding peroxyacid were detected in the initial stages. In the oxidation of acetaldehyde in acetic acid [20], chain propagation includes not only the reactions of RC (0) with 02 and RC(0)00 with RC(0)H, but also the exchange of radicals with solvent molecules (R = CH3). 

The kinetics of the thermally induced homogeneous decomposition of phosphine (PH3) have not yet been studied. The species PH2, PH and P2 are formed on flash photolysis of PH3 and could be identified by their absorption spectra63. There are proposals as to the mechanism of the consecutive process after the photochemical primary step, but nothing is known about the kinetic parameters of these reactions. With arsine and antimony hydride only the heterogeneous decomposition has been studied64,65. 

A first point to consider is that thermal or photochemical decomposition of a precursor often does not lead to a single product, due to parallel or consecutive secondary reactions. Since absorption spectroscopy invariably probes all components of a mixture, the problem of how to distinguish between these components may arise in the context of studies on reactive intermediates. This problem can be... 

The net result of a photochemical redox reaction often gives very little information on the quantum yield of the primary electron transfer reaction since this is in many cases compensated by reverse electron transfer between the primary reaction products. This is equally so in homogeneous as well as in heterogeneous reactions. While the reverse process in homogeneous reactions can only by suppressed by consecutive irreversible chemical steps, one has a chance of preventing the reverse reaction in heterogeneous electron transfer processes by applying suitable electric fields. We shall see that this can best be done with semiconductor or insulator electrodes and that there it is possible to study photochemical primary processes with the help of such electrochemical techniques 5-G>7>. 

Compounds 143a-d could be isolated with a diastereomeric excess of up to 90% [90]. An initial [2 + 2] photocycloaddition involved in this reaction leads to intermediates Q. A consecutive thermal rearrangement produces 143a-d. Finally, compounds 144a-d are formed via further photochemical rearrangement. However, the thermal reversibility of the last step allows the formation of 143a-d in yields of up to 90%. 

Photochemical cyclobutane annealings are much more promising and find their way into synthesis to a greater extent. Photochemical cycloaddition reactions to enolized 1,3-diketones and consecutive ring openings via retroaldol reactions have been applied in ring expansion reactions numerous times. 

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