Rotating parallel reaction

The rotating ring-disc electrode has been much used in the study of electron transfer in consecutive and parallel reactions or a mixture of both. Each of these situations is now examined in detail. 

Based on their isomerization experiments in polar solvents, Whitten et al. stated that thermal Z—> E isomerization of donor/acceptor-substituted azobenzenes proceeds by rotation. Later, Shin and Whitten modified this point of view and saw a dual mechanism active dependent on solvent and donor strength, Asano and coworkers,from their experiments under pressure, inferred a dependence of the mechanism on solvent and even parallel reaction paths along these mechanistic coordinates. 

The efficiency of such a DKR-based synthesis depends on the kinetic parameters of the parallel reaction [kg and kj) and racemization (kj ). This is determined not only by the structural parameters of the catalyst and substrate, but also by the conditions that facilitate or enforce in situ racemization of the chiral substrate. In this review, examples of DKR have been classified into five categories according to the method of racemization (i) tautomerization, (ii) internal motion (pyramidal inversion, and deformation and rotation), (iii) elimination-addition and addition-elimination, (iv) nucleophilic substitution and (v) others. Representative equilibrium for each type of racemization is shown in Figure 5.2. 

The experiment is performed as follows (Fig. 13.1). The reaction mixture is placed in a thermostatted vessel 6 with transparent planar-parallel walls. The vessel is irradiated with the light with such a wavelength that generates radicals. Disk 4 is placed in the point where the beams are focused and is rotated. The reaction rate is measured by this or another method from experiment to experiment, and the empirical dependence of the v/v ratio on logrj is plotted, td is found from the rotation velocity of the disk and the ratio between the sizes of the dark and light sectors (usually r - 3). This empirical dependence is compared with the theoretical one, and 2k, is determined by comparison, and from this 2k, is calculated. The initiation rate is measured by the methods of inhibitors (see above) or through the chain reaction rate and the kp/2k, ratio. 

Electrochemical reactors (cells, tanks) are used for the practical realization of electrolysis or the electrochemical generation of electrical energy. In developing such reactors one must take into account the purpose of the reactor as well as the special features of the reactions employed in it. Most common is the classical reactor type with plane-parallel electrodes in which positive and negative electrodes alternate and all electrodes having the same polarity are connected in parallel. Reactors in which the electrodes are concentric cylinders and convection of the liquid electrolyte can be realized by rotation of one of the electrodes are less common. In batteries, occasionally the electrodes are in the form of two long ribbons with a separator in between which are wound up as a double spiral. 

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