Cyclic processes, drawback

The ability of varying the rate of the mass transport by agitating the solution (or the working electrode) constitutes the basis of hydrodynamic methods (hydrodynamics = liquids in motion), which are a further support to the study of electrode kinetics. Nevertheless we wish to cite them here simply to cover a drawback of cyclic voltammetry. In fact, cyclic voltammetry is unable to discriminate between oxidation and reduction processes, and vice-versa. 

It is remarkable that better enantioselectivities are achieved when CALB-catalyzed acylations of the alcohol are carried out in organic solvent rather than in water. Excellent enantioselectivities are obtained when the process is carried out with vinyl esters [22]. However, in some cases the use of vinyl or alkyl esters as acyl donors has the drawback of the separation of the ester (product) and the alcohol (substrate). A practical strategy to avoid this problem is the use of cyclic anhydrides [23]. In this case an acid is obtained as product, which can be readily separated from the unreacted alcohol by a simple aqueous base-organic solvent liquid-liquid extraction. This methodology has been successfully used for the synthesis of (-)-paroxetine as indicated in Scheme 10.11 [24]. 

A major drawback of cyclic Fischer projections is the unrealistic manner in which the structures are depicted. In 1929, Haworth designed a representation to address this deficiency. Haworth projections provide a simple way to represent cyclic monosaccharides with a three-dimensional perspective. The following process allows the conversion of a Fischer projection into a Haworth representation ... 

The catalytic liquid-phase oxidation of aqueous phenol solution, carried out in a variety of reactor systems, demonstrates that phenol can be transformed to nontoxic compounds at milder reaction conditions than used in the thermal processes. The present study indicates that it is advantageous to conduct the reaction in a trickle-bed reactor with partial wetting of catalyst particles, perhaps with cyclic operation, since a direct contact between the catalyst surface and gas-phase increases the concentration of active sites for phenol oxidation. Furthermore, the reaction selectivity in a trickle-bed reactor is higher than that in a slurry reactor. The main drawback of the investigated process is dissolution of metal ions into the liquid-phase, which calls for more stable catalysts. 

Cyclic voltammograms of electrochemical systems can often be much more complicated than the traces presented here. It often takes some ingenuity and persistence to determine which peaks belong to which species or processes. Despite these minor drawbacks, the cyclic voltammetric technique is a versatile, and relatively sensitive, electroanalytical method appropriate to the analysis of systems of interest to battery development. The technique will identify reversible couples (desirable for secondary batteries), it provides a method for measuring the rate constant and transfer coefficient of an electrode process (a fast rate constant indicates a process of possible interest for battery development), and it can provide a tool to help unravel complex electrochemical systems. 

A novel voltammetric technique based on the simultaneous dynamic potential control of both the disk and the ring electrodes of an RRDE is presented. The method of dual cyclic voltammetry has proven to be an especially promising method for studying the mechanisms of electrochemical processes. The new 3D representation of the data can be effectively used in order to reveal the formation of electroactive species at the disk electrode. By using appropriate potential programs, the selectivity and sensitivity of RRDE systems can be significantly increased. The results of some selected e>q)eriments have been discussed and some drawbacks of the technique have been pointed out. Numerical simulations have been carried out in order to study the cross-talk effects, and a method has been suggested for their reduction. 

Intermolecular cyclization of diyne with monoynes or intermolecular cyclization of triynes is a general method for the synthesis of substituted benzenes [51]. But a serious drawback of this reaction is the presence of a secondary process of diyne dimerization or monoyne trimerization. The selectivity and yield of the cyclic products strongly depend on the structure of the reactants, solvent, catalyst, etc. A highly efficient and selective [2-I-2-I-2] cyclization of diynes and monoynes was described. A rhodium catalyst allowed the reaction to proceed at room... 

---