Coupled Reactions. Chemical Induction

From the set of two simultaneously proceeding reactions coupled reactions are of particular interest. They are specific in that one (A + B2) can occur only in the presence of another (A + Bj). In classical chemistry this phenomenon was called chemical induction. The inducing reaction (A + i) is referred to as primary, and the reaction induced by it (A + B2) secondary. 

Coupled chemical reactions are very frequent. An example is the combined oxidation of carbon monoxide and hydrogen. Unlike the reaction 2H2 + O2 - 2H2O, the reaction 2 CO -f- O2 2CO2 of the pure reactants (in the absence of impurities) does not occur up to very high temperatures, but readily in the presence of hydrogen [230] (see Section 1.3). 

The specificities of the mechanisms and kinetic characteristics of coupled reactions were elucidated by studying oxidation reactions in solution which provide a particularly large number of examples of coupled reactions. The phenomenon of coupling, i.e. the interrelation of two reactions, was found to be based on intermediates generated by the primary reaction and directly transferring the inductive effect of the primary to the secondary reaction, i.e. representing a link between the two reactions. 

From these considerations, in a simple case, the coupled reaction can be expressed as a combination of two elementary processes 

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Thus, the accumulation of chemical energy of the reaction in the form of highly active intermediate compounds happens with the energy consumption. For this purpose photosensibilization, light exposure (photochemical reactions), catalysis (catalytic decay) and chemical induction (couples processes) are used. 

Other examples have been provided by the anodically initiated isomerization of several decarbene metal carbonyl couples [197]. Electrochemical induction of chemical reactions can also be successfully used for conversion of alcoolates into ketones with simultaneous reduction of aromatic halides [198], the tetramerization of aziridines [199], or ligand substitution [200] in organometallic compounds. A useful review on this subject was published [201]. 

The dependence of reaction rates on pH and on the relative and absolute concentrations of reacting species, coupled with the possibility of autocatalysis and induction periods, has led to the discovery of some spectacular kinetic effects such as H. Landolt s chemical clock (1885) an acidified solution of Na2S03 is reacted with an excess of iodic acid solution in the presence of starch indicator — the induction period before the appearance of the deep-blue starch-iodine colour can be increased systematically from seconds to minutes by appropriate dilution of the solutions before mixing. With an excess of sulfite, free iodine may appear and then disappear as a single pulse due to the following sequence of reactions ... 

Recently there has been an increasing interest in self-oscillatory phenomena and also in formation of spatio-temporal structure, accompanied by the rapid development of theory concerning dynamics of such systems under nonlinear, nonequilibrium conditions. The discovery of model chemical reactions to produce self-oscillations and spatio-temporal structures has accelerated the studies on nonlinear dynamics in chemistry. The Belousov-Zhabotinskii(B-Z) reaction is the most famous among such types of oscillatory chemical reactions, and has been studied most frequently during the past couple of decades [1,2]. The B-Z reaction has attracted much interest from scientists with various discipline, because in this reaction, the rhythmic change between oxidation and reduction states can be easily observed in a test tube. As the reproducibility of the amplitude, period and some other experimental measures is rather high under a found condition, the mechanism of the B-Z reaction has been almost fully understood until now. The most important step in the induction of oscillations is the existence of auto-catalytic process in the reaction network. 

Typically, the reaction is performed in a liquid-liquid biphasic system where the substrates and products (upper phase) are not miscible with the catalyst/ionic liquid solution (lower phase). The SiH-functional polydimethylsiloxane and the olefin are placed in the reaction vessel and heated up to 90 °C. Then the precious metal catalyst (20 ppm) and the ionic liquid (1 %) are added. After complete SiH conversion, the reaction mixture is cooled to room temperature and the products are removed from the reaction mixture by either simple decantation or filtration (in case of non-room-temperature ionic liquids). The recovered catalyst/ionic liquid solution can be reused several times without any significant change in catalytic activity. A treatment or workup of the ionic liquid-catalyst solution after each reaction cycle is not necessary. The metal content of the products was analyzed by ICP-OES (Inductively coupled plasma optical emission spectroscopy) and the chemical identity of the organomodified polydimethylsiloxane was verified by NMR spectroscopy. 

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