Benzene diazonium ions, reaction

This reaction is important because it provides a method to place an amino substituent onto the benzene ring, a substitution that cannot be accomplished directly by electrophilic attack. And, as illustrated in the following example, this opens all of the substitution reactions that can be accomplished through diazonium ion reactions. 

Primary aromatic amines differ from primary aliphatic amines in their reaction with nitrous acid. Whereas the latter yield the corresponding alcohols (RNHj — ROH) without formation of intermediate products see Section 111,123, test (i), primary aromatic amines 3neld diazonium salts. Thus aniline gives phcnyldiazonium chloride (sometimes termed benzene-diazonium chloride) CjHbNj- +C1 the exact mode of formation is not known, but a possible route is through the phenjdnitrosoammonium ion tlius ... 

The conductometric results of Meerwein et al. (1957 b) mentioned above demonstrate that, in contrast to other products of the coupling of nucleophiles to arenediazonium ions, the diazosulfones are characterized by a relatively weak and polarized covalent bond between the p-nitrogen and the nucleophilic atom of the nucleophile. This also becomes evident in the ambidentate solvent effects found in the thermal decomposition of methyl benzenediazosulfone by Kice and Gabrielson (1970). In apolar solvents such as benzene or diphenylmethane, they were able to isolate decomposition products arising via a mechanism involving homolytic dissociation of the N — S bond. In a polar, aprotic solvent (acetonitrile), however, the primary product was acetanilide. The latter is thought to arise via an initial hetero-lytic dissociation and reaction of the diazonium ion with the solvent (Scheme 6-11). 

However, in a re-investigation of the reaction of benzyne with benzene 39>, it was shown that the original method of isolating benzene-diazonium-2-carboxylate resulted in the contamination of the zwitter-ion with silver salts 40>41). In the absence of silver ions biphenylene, and... 

There are some inherent limits to the usefulness of such reactions. Radical substitutions are only moderately sensitive to substituent directing effects, so that substituted reactants usually give a mixture of products. This means that the practical utility is limited to symmetrical reactants, such as benzene, where the position of attack is immaterial. The best sources of aryl radicals for the reaction are aryl diazonium ions and A -nitroso-acetanilides. In the presence of base, diazonium ions form diazoxides, which decompose to aryl radicals.151... 

This reaction is highly sensitive to the nature of the substituent X, and coupling to benzene derivatives normally occurs only when X is a strongly electron-donating group such as —(). —N(CH3)2, and —OH. However, coupling with X = —OCH3 may take place with particularly active diazonium ions. 

Macrae and Wright (96) demonstrated that visible light irradiation of xanthene dyes (eosin, erythrosin, rhodamine B, or RB) in ethanolic solutions of 4-(N,N-diethylamino)benzene-diazonium chloride (as the zinc chloride double salt) resulted in decomposition of the diazonium salt. Electron transfer from the dye excited state(s) to the diazonium salt was postulated and dye-diazonium salt ion pair formation in the ground state was shown to be important. Similar dyes and diazonium salts were claimed by Cerwonka (97) in a photopolymerization process in which vinyl monomers (vinylpyrrolidone, bis(acrylamide)) were crosslinked by visible light. Initiation occurs by the sequence of reactions in eqs. 40-42 ... 

Benzene derivatives with two nucleofuges have been used in the preparation of polymeric materials with varying degrees of success. Poly(l,4-phenylene sulfide) has been prepared by condensation of p-di-chlorobenzene with sodium sulfide,99,100 and in a related process, diazonium ions have been shown to initiate the polymerization of p-halobenzenethiolate ions.101 In a preliminary study, poorly characterized polymers were obtained from reaction of equimolar amounts of p-dihalobenzenes and the enolate ions from ketones in the presence of excess base. When an excess of the ketone enolates was used, the normal p-disubstituted derivatives were formed.102... 

It is well known that nitrite diazotises aromatic amines in acidic media and that the resulting diazonium ions are reduced by hypophosphorous acid to form benzene derivatives. The reactions are formulated as follows. 

First the amino group was converted to a hydroxy group via a diazonium ion (Section 17.10). The benzene ring was reduced with hydrogen and a catalyst to produce cyclohexanol. Oxidation with potassium dichromate (Section 10.14) gave cyclohexanone. The bonds between the carbonyl carbon and both a-carbons were then cleaved by a series of reactions not covered in this book. The carbon of the carbonyl group was converted to carbon dioxide in this process. One-half of the original radioactivity was found in the carbon dioxide, and the other one-half was found in the other product, 1,5-pentanediamine. Additional experiments showed that the 14C in the diamine product was located at C-l or C-5. 

A more interesting problem than the influence of substituents in the electrophilic reagent of azo coupling is the extremely high selectivity of the C-coupling reactions, relative to other electrophilic aromatic substitutions. Unsubstituted benzene does not react with any arenediazonium ion, 1,3,5-trimethoxybenzene reacts very slowly with strongly electrophilic diazonium ions only aromatic amines (e.g. N,N-dimethyl-aniline) or phenolate ions react very fast, in some cases close to diffusion control. 

Rearrangements which proceed readily with solvated diazonium ions are often minimized by ion pairing. For example, the reaction of 1-diazopropane with benzoic acid in benzene gives nearly pure 1-propyl benzoate, but the action of aqueous per-... 

Some of the typical benzene electrophihc substitution reactions do not occur at all Friedel-Crafts alkylation and acylation fail because pyridines form complexes with the Lewis-acid catalyst required, involving donation of the nitrogen lone pair to the metal centre. Milder electrophilic species, such as Mannich cations, diazonium ions or nitrous acid, which in any case require activated benzenes for success, naturally fail with pyridines. 

Scrambling of the nitrogen atoms during decomposition of aryldiazonium ions has been observed a study of the reaction in which secondary deuterium isotope measurements were carried out excluded the possibility of a benzene spirodiazirine similar to 193, and favoured a carbocation with closely associated nitrogen, which can collapse to reform the diazonium ion. 

The last two reactions, which are extremely fast, account for the stoichiometric relationship 2 ArN2 + H2Q. The reaction is second order kinetically, however, because only one molecule of diazonium ion is consumed up to the rate-determining step of the reaction. The solvent isotope effect is attributed to the fact that the for deuterated hydroquinone, DO—CgH4—OD, is 4.16 times less than that of hydroquinone. The absence of deuterium in the nitrobenzene when the reaction is done in D2O indicates that the decomposition of the Ar—N2 to product does not involve hydroquinone and that hydroquinone is not the source of the hydrogen that is added to the benzene ring in the final step of the reaction. In fact, the partial deuteration of the nitrobenzene suggests that the decomposition of the aryldiazenyl radical occurs by two pathways (equation 56), where SH is the solvent for the reaction. 

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