4-Nitrobenzenediazonium

An increased hydrogen ion concentration, that is a considerably greater amount of acid than the theoretical two equivalents of Scheme 2-1, is necessary in the diazotization of weakly basic amines. The classic example of this is the preparation of 4-nitrobenzenediazonium ions 4-nitroaniline is dissolved in hot 5-10 m HC1 to convert it into the anilinium ion and the solution is either cooled quickly or poured onto ice. In this way the anilinium chloride is precipitated before hydrolysis to the base can occur. On immediate addition of nitrite, smooth diazotization can be obtained. The diazonium salt solution formed should be practically clear and should not become cloudy on standing in the dark. Some practice is necessary, and details can be found in the books emphasizing preparative aspects (Fierz-David and Blangey, 1952 Saunders and Allen, 1985 in Houben-Weyl, Vol. E 16a, Part II, the chapter written by Engel, 1990). These books give a series of detailed prescriptions for specific examples and a useful review of the principal variations of the methods of diazotization. Such reviews have also been written by Putter (1965) and Schank (1975). 

These salts can be made easily since tetrafluoroboric acid (HBF4) and hexa-fluorophosphoric acid (HPF6) are commercially available. However, the main advantage of the diazonium salts with the anions of these acids is their stability, which is significantly higher than that of probably all other diazonium salts. 4-Nitrobenzenediazonium tetrafluoroborate is nowadays even a commercial product. Preparative diazotization methods with these two acids can be found in Organic Syntheses (tetrafluoroborate Starkey, 1943 hexafluorophosphate Rutherford and Redmont, 1973). 

Muzik, 1952 see also Putter, 1965, p. 46). However, by adding a solution of a 4-nitrobenzenediazonium salt (as in Scheme 2-33) or a biphenyl-4,4 -bisdiazonium salt to the diamine, one obtains the mono-diazonium ion (2.57). 

For the first six substituents in Table 5-1 K2 is 3 to 5 powers of 10 greater than Kx. At pH = pATm the maximum equilibrium concentration of the (Z)-diazohydroxide (last column) is very small. For the 2,6-dichloro-4-nitrobenzenediazonium ion, however, Kx is smaller than K2 by a factor of only 101 67 = 37. This factor results in a significantly higher maximum concentration of the (Z)-diazohydroxide. Finally, the benzene-l-diazonium-4-diazohydroxide cation reaches a maximum equilibrium concentration of approximatively 80% at pH = pKm because, as mentioned before, Ki is larger than K2 by a factor of ten. 

Semiquantitatively, the reaction of an aromatic diazonium ion with the methoxide ion occurs in three phases. The first is the extremely rapid formation of the (Z)-diazo methyl ether. This is followed by a second, partitioning, phase which in the case of the 4-nitrobenzenediazonium ion at 30 °C is completed in 60 s (Boyle et al., 1971). During this phase, some of the (Z)-diazo ether decomposes to form dediazoniation products (mainly nitrobenzene via the hydro-de-diazoniation reaction) and the rest is converted into the (Zi)-diazo ether. 

In DMSO as solvent and in the presence of nitrobenzene, aryl-de-diazoniation of the unsubstituted benzenediazonium ion leads mainly via meta substitution to 3-nitrobiphenyl, whereas in the case of the 4-nitrobenzenediazonium ion the formation of o- and -substituted products (2,4 -and 4,4 -dinitrobiphenyl) prevails (Gloor et al., 1972). 

Szele and Zollinger (1978 b) have found that homolytic dediazoniation is favored by an increase in the nucleophilicity of the solvent and by an increase in the elec-trophilicity of the P-nitrogen atom of the arenediazonium ion. In Table 8-2 are listed the products of dediazoniation in various solvents that have been investigated in detail. Products obtained from heterolytic and homolytic intermediates are denoted by C (cationic) and R (radical) respectively for three typical substituted benzenediazonium salts and the unsubstituted salt. A borderline case is dediazoniation in DMSO, where the 4-nitrobenzenediazonium ion follows a homolytic mechanism, but the benzenediazonium ion decomposes heterolytically, as shown by product analyses by Kuokkanen (1989) the homolytic process has an activation volume AF = + (6.4 0.4) xlO-3 m-1, whereas for the heterolytic reaction AF = +(10.4 0.4) x 10 3 m-1. Both values are similar to the corresponding activation volumes found earlier in methanol (Kuokkanen, 1984) and in water (Ishida et al., 1970). 

In dimethyl sulfoxide (DMSO) the isomer ratios for aryl-de-diazoniation of 4-nitrobenzenediazonium ions demonstrate that the 4-nitrophenyl radical is the arylating reagent (Gloor et al., 1972 see Sec. 8.2). 

The hydro-de-diazoniation of 4-nitrobenzenediazonium tetrafluoroborate with tri-phenylphosphine in methanol (Yasui et al., 1991) is hardly interesting for synthetic purposes, as the yield of nitrobenzene passes through a narrow maximum (95 %) if 0.5 equivalent of triphenylphosphine is used. 

In this context two observations reported by Rondestvedt (1960, p. 214) should be mentioned (i) Meerwein reactions proceed faster in the presence of small amounts of nitrite ion. Meerwein reactions in which N2 evolution ceased before completion of the reaction can be reinitiated by addition of some NaN02. (ii) Optimal acidity for Meerwein reactions is usually between pH 3 and 4, but lower (pH — 1) with very active diazonium compounds such as the 4-nitrobenzenediazonium ion or the diphenyl-4,4 -bisdiazonium ion. At higher acidities more chloro-de-diazoniation products are formed (Sandmeyer reaction) and in less acidic solutions (pH 6) more diazo tars are formed. 

Laali and Lattimer (1989 see also Laali, 1990) observed arenediazonium ion/crown ether complexes in the gas phase by field desorption (FD) and by fast atom bombardment (FAB) mass spectrometry. The FAB-MS spectrum of benzenediazonium ion/18-crown-6 shows a 1 1 complex. In the FD spectrum, apart from the 1 1 complex, a one-cation/two-crown complex is also detected. Dicyclo-hexano-24-crown-6 appears to complex readily in the gas phase, whereas in solution this crown ether is rather poor for complexation (see earlier in this section) the presence of one or three methyl groups in the 2- or 2,4,6-positions respectively has little effect on the gas-phase complexation. With 4-nitrobenzenediazonium ion, 18-crown-6 even forms a 1 3 complex. The authors assume charge-transfer complexes such as 11.13 for all these species. There is also evidence for hydride ion transfer from the crown host within the 1 1 complex, and for either the arenediazonium ion or the aryl cation formed from it under the reaction conditions in the gas phase in tandem mass spectrometry (Laali, 1990). 

Boldyrev and Grivnak (1984) reported that 4-nitrobenzenediazothiosulfonic acid adducts (4-N02 — C6H4 — N2 — S — S02 — Ar ), which were obtained by reaction of 4-nitrobenzenediazonium salts with benzene- and 4-toluenethiosulfonic acid salts (Ar — S02 — S K+), form azo compounds with 2-naphthol and 1-naphthylamine-4-sulfonic acid. 

Phenol ethers show some, admittedly low, reactivity towards diazonium ions and also undissociated phenols (see Sec. 12.7). An instructive example of the reactivity of phenol ethers was reported by Ronaldson (1981). He found that 1,2-dimethoxy-benzene (veratrole) does not react with the 4-nitrobenzenediazonium ion, but the azo coupling product is formed when the more electrophilic 2,4-dinitrobenzenediazo-nium ion is used. 

Azo coupling reactions are often used for quantitative determination of trace compounds, e. g., for nitrous acid and for compounds that are either diazo or coupling compounds. Spectrophotometric determination of azo dyes formed from these compounds is possible down to concentrations of about 0.01 pg/mL. (An example is the determination of 4-aminophenazone by azo coupling with 4-nitrobenzenediazonium ions see Alwehaid, 1990.)... 

As far as we are aware, the azo coupling of an ethyne derivative was only investigated over half a century ago Ainley and (Sir Robert) Robinson (1937) investigated the reaction of phenylethynes (phenylacetylenes) with diazonium ions (Scheme 12-59). Unsubstituted phenylethyne did not give identifiable products with the 4-nitrobenzenediazonium ion, but with the more nucleophilic 4-methoxyphenyl-ethyne an azo compound (12.119) was formed. On reaction with water it gives an arylhydrazone of an a-ketoaldehyde (12.120). 

Quantitative studies based on kinetic measurements using strongly electrophilic diazonium ions and, as coupling components, 1-naphthol, 2-naphthol-6-sulfonic acid, and resorcinol in aqueous acid were made by Sterba and coworkers (Kropacova et al., 1970 Kavalek et al., 1970 Sterba and Valter, 1972 Machackova et al., 1972a). In a typical case (2,6-dichloro-4-nitrobenzenediazonium ion and 1-naphthol) the dependence of the logarithm of the measured rate constant (ks) on pH was linear with a slope of 1. At pH < 1, however, a practically constant value of ks was obtained. The measured rate constants therefore correspond to Scheme 12-62, in which the first term relates to the reaction of the naphthoxide ion and the second to that of the undissociated naphthol Ka is the acidity constant of 1-naphthol. 

The only really different case is the azo coupling reaction of nitroethane investigated by Sterba and coworkers (Machacek et al., 1968a, 1968b). With the 4-nitrobenzenediazonium ion the reaction is zero-order with respect to diazonium ion and first-order in both nitroethane and base. Obviously the rate-limiting step is the dissociation of nitroethane the formation of the anion is slower than its subsequent reaction with this diazonium ion. For reactions with diazonium ions of lower reactivity it was found necessary to use the reaction system of Scheme 12-64 with the nitroethane anion as steady state intermediate (Machacek et al., 1968b). 

The secondary a-deuterium isotope effects on azo coupling reactions are small, i.e., km/kiv is very close to unity. For the reaction of the 4-nitrobenzenediazonium ion with the trianion of l-D-2-naphthol-6,8-disulfonic acid catalyzed by pyridine, km/kiv = 1.06 0.04 (Hanna et al., 1974). 

In the reaction of the strongly electrophilic 4-nitrobenzenediazonium ion with 2-naphthol-6,8-disulfonic acid, which yields a sterically hindered o-complex, Roller and Zollinger (1970) actually observed the rapid formation of a 7T-complex spec-trophotometrically at low pH. The concentration of the 7T-complex decreases slowly and at the same rate as that of the formation of the azo product. H NMR data indicate that the 7t-complex is not localized. All 7T-electrons of the benzene and the naphthalene system are involved in the complex formation to a similar degree, in... 

Bagal et al. (1975) investigated in more detail the role of donor-acceptor complexes in the azo coupling reaction of the 4-nitrobenzenediazonium ion with 2-naphthylamine-3,6-disulfonic acid and that of the 4-chlorobenzenediazonium ion with 2-naphthol-6-sulfonic acid. Their kinetic results are, as would be expected, compatible with the mechanisms shown in Schemes 12-74 or 12-75. 

More recently, Bagal and coworkers (Luchkevich et al., 1991) obtained similar results in a kinetic investigation of the coupling reactions of some substituted benzenediazonium ions with 1,4-naphtholsulfonic acid, and with 1,3,6-, 2,6,8-, and 2,3,6-naphtholdisulfonic acids. The kinetic results are consistent with the transient formation of an intermediate associative product. The maximum concentration of this product reaches up to 94% of the diazonium salt used in the case of the reaction of the 4-nitrobenzenediazonium ion with 1,4-naphtholsulfonic acid (pH 2-4, exact value not given). The authors assume that this intermediate is present in a side equilibrium, i. e., the mechanism of Scheme 12-77 mentioned above rather than that of Scheme 12-76, and that the intermediate is the O-azo ether. 

Another interesting observation was made by Bagal et al. a year later (1992). In the reaction of 4-nitrobenzenediazonium ions with various 4-phenylazophenols, with or without substituents in the 2- and 3-positions of the phenolic ring and in the 4 -position of the phenylazo ring, in addition to azo coupling in the 6-position they obtained a product that had the same atomic composition as 2,4-bis(4 -nitrophenyl-azo)-phenol (Ci8Hi4N605), but whose 13C NMR spectrum clearly showed a tetrahedral and a carbonyl carbon in the 4- and 1-positions. This product must therefore be the compound 12.153. 

Micellar catalysis of azo coupling reactions was first studied by Poindexter and McKay (1972). They investigated the reaction of a 4-nitrobenzenediazonium salt with 2-naphthol-6-sulfonic and 2-naphthol-3,6-disulfonic acid in the presence of sodium dodecylsulfate or hexadecyltrimethylammonium bromide. With both the anionic and cationic additives an inhibition (up to 15-fold) was observed. This result was to be expected on the basis of the principles of micellar catalysis, since the charges of the two reacting species are opposite. This is due to the fact that either of the reagents will, for electrostatic reasons, be excluded from the micelle. 

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