Mixing and Diffusion Effects

The formation of this product of a consecutive disubstitution is apparently not easy to understand. It is known that an arylazo residue diecreases the reactivity of a phenol or a naphthol for substitution by a second diazonium ion by 3 to 5 orders of ten. In phenols and naphthol the favorable effect of promoting the dissociation of a proton from the substrate does not work it was found only for C-acids (see discussion in the preceding Sect. 4.1). 

An explanation for this strange effect could be given only 86 years later It is based on the a n( pt that intrinsic rates of consecutive chonical reactions can be disguised by microdiffusion effects if molecular diffusion is slower than the intrinsic rates of chemical reaction. 

This qualitative explanation could ba verified by Rys et al. for such 

This model predicts that in these reactions the relative yields of the secondary products are dependent on four parameters, namely the stoichiometric ratio of the reacts A and B at time zero (A is the reagent which can be substituted twice), the volume ratio of the solutions of A and B, and the two moduli b.i and b,2- These moduli are characterized by the mean radius R of the eddies, the initial concentration 

The computation results say that if l(Pk2 and the term 2 kas a munerical value 1, mixing effects are probable. Thus with an initial volume ratio of 1, initial concentrations [A]o = [B]o = lAf, D = 10 cm s , disguised kinetics are likely to occur if kj has values of 11 mol s or more. 

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