Catalysis diazo coupling

In fact, when either chloride ion or water is added to the reaction mixture, both the rate and the product composition change markedly. For example, the rate increases by fourteen times and the product composition changes from 9% of the N-coupled diazoamino compound to 79% of the diazoamino compound when the chloride ion concentration is increased from zero to 1.4 x 10" M. The amount of the N-coupled diazoamino product also increased at all three temperatures used in the study as the amount of amine (base) increased. On the basis of the base catalysis and the hydrogen-deuterium kinetic isotope effects and their studies of other diazo coupling reactions, the authors concluded that the N-coupling and the C-coupling reactions both proceed by an S 2 mechanism with the proton transfer from the cr-complex rate-determining. The results are best explained by the mechanism presented in Scheme 3. 

If one eliminates specific hydroxonium and hydroxide ion catalysis, which arise from effects on pre-equilibria, in the measured overall rate constants of diazo coupling reactions, one recognizes that there are some reactions which do and some which do not show general base catalysis. A base-catalysed combination, namely the reaction of p-chlorobenzene diazonium ion with 2-naphtholate-6,8-disulphonate anion has been investigated with respect to catalysis (Zollinger, 1955a, c). A non-linear dependence between rate and the concentration of pyridine, 2-, 3-and 4-picoline and 2,6-lutidine was found, corresponding to a ratio = 1 30 for pyridine (B = pyridine). 

The diazo-coupling of o-diazophenols with naphthols and the coupling reaction of >-nitrodiazobenzene with N,N-diethylaniline possibly belong to one of these sub-groups as it is known that these reactions are subject to general base catalysis (Kozlov and Stepanov, 1952 Putter, 1951 Zollinger, 1961b). 

As no general acid catalysis has been reported for the iodination of p-nitrophenol and the diazo-coupling reactions mentioned, it can be concluded that in these reactions mechanism (21) does not operate or that the solvent molecules (water) solvate the carbonyl oxygen in the intermediate sufficiently so as to increase the rate of proton transfer at the C—H bond (tentative transition state 3) for the iodination of p-nitrophenol (analogous for diazo coupling reactions). The explanation... 

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