Diazotization nucleophilic catalysis

Nucleophilic catalysis is also observed with iodide ions. Fluoride ion does not form nitrosyl fluoride under diazotization conditions, as is to be expected from Pearson s hard and soft acids and bases principle which was discussed briefly in Section 3.2. More recently, nucleophilic catalysis has also been shown to occur with thiocyanate ion (SCN ), thiosulfate ion (HS2Of), dimethyl sulfide, and thiourea (H2NCSNH2) or its alkyl derivatives (see below). 

For substituted anilines (Thompson and Williams, 1977) and for 1-naphthylamine and a series of derivatives thereof (Castro et al., 1986a), k2 and the ratio Ar 2/Ar3 have been determined for nucleophilic catalysis with Cl-, Br-, SCN-, and SC(NH2)2. The values of k2 correspond fairly well to those found for the diazotization of aniline, but those of Ar 2/Ar3 increase markedly in the above sequence (Table 3-1). As k3 is expected to be independent of the presence of Cl- or Br- and to show little dependence on that of SCN- or thiourea, the increase in k 2/k3 for this series must be due mainly to 2. Indeed, the value of log(Ar 2/Ar3) shows a linear correlation with Pearson s nucleophilicity parameter n (Pearson et al., 1968). This parameter is based on nucleophilic substitution of iodine (as I-) in methyl iodide by various nucleophiles. The three investigations on nucleophilic catalysis of diazotization demonstrate that Pearson s criteria for bimolecular nucleophilic substitution at sp3 carbon atoms are also applicable to substitution at nitrogen atoms. 

Nucleophilic catalysis of diazotizations by chloride ions was also observed in methanol, first by Schmid (1954) and later in a detailed investigation by Schelly (1972), which included work with methanol/CCl4 mixtures. 

Catalysis by other nucleophiles has also been found for secondary aliphatic amines, e. g., by thiocyanate (Fan and Tannenbaum, 1973), and by thiourea and its tetramethyl derivative (Meyer and Williams, 1988, and earlier investigations mentioned there). The results are comparable to those of aromatic diazotizations (Zollinger, 1994, Sect. 3.3). 

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