Photochemical consecutive

Reactions with a difference in the dependence on intensity In the assumed photochemical consecutive mechanism... 

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). 

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. ... 

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. 

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.

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>. 

Photoexcitation of trans,trans-l,5-di-9-anthrylpentadienone 60 gives isomer 63 whose formation by two consecutive photoreactions, i.e geometrical isomerization to give the cis-trans-isomer 61, followed by geometrically favored intramolecular Diels-Alder addition, has been established by quenching experiments. In the presence of molecular oxygen, the photochem-... 

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%. 

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

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. 

In related work, evaporation of silicon and consecutive co-condensation of the generated atoms with acetylene in an argon matrix at 20 K yields also, as the primary product, silacyclopropenyldene 81, which photochemically interconverts into the CzHzSi species, 82, 19, and 84 (Scheme 25) <1998EJ01313>. The photochemical interconversion of 81 into the other CaHaSi isomers, 82, 19, and 84, was examined via infrared spectroscopy and found to be reproducible, as previously reported <1995JA12712>. 

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