Arylamines Diazonium salts

Aniline, 407, 859. See also Arylamines Diazonium salts basicity of, 866—868 electrostatic potential map, 862 isolation, 859... 

Primary arylamines like primary alkylammes form diazonium ion salts on nitro sation Aryl diazonium 10ns are considerably more stable than their alkyl counterparts Whereas alkyl diazonium 10ns decompose under the conditions of their formation aryl diazonium salts are stable enough to be stored m aqueous solution at 0-5°C for a rea sonable time Loss of nitrogen from an aryl diazonium ion generates an unstable aryl cation and is much slower than loss of nitrogen from an alkyl diazonium ion... 

A reaction of aryl diazonium salts that does not involve loss of nitrogen takes place when they react with phenols and arylamines Aryl diazonium ions are relatively weak elec trophiles but have sufficient reactivity to attack strongly activated aromatic rings The reaction is known as azo coupling two aryl groups are joined together by an azo (—N=N—) function... 

Reductive deamination of primary arylamines The ammo substituent of an arylamine can be replaced by hydrogen by treatment of its de rived diazonium salt with ethanol or with hypophosphorous acid... 

Diazotization of a primary arylamine followed by treatment of the diazonium salt with copper(l) bromide or copper(l) chloride yields the corresponding aryl bromide or aryl chloride. 

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ X-. The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Another possibility for ring closure to annulated 1,3-diazocines employs the thermal decomposition of the diazonium salt derived from arylamine 3.7... 

Research into the mechanism of diazotization was based on Bamberger s supposition (1894 b) that the reaction corresponds to the formation of A-nitroso-A-alkyl-arylamines. The TV-nitrosation of secondary amines finishes at the nitrosoamine stage (because protolysis is not possible), but primary nitrosoamines are quickly transformed into diazo compounds in a moderately to strongly acidic medium. The process probably takes place by a prototropic rearrangement to the diazohydroxide, which is then attacked by a hydroxonium ion to yield the diazonium salt (Scheme 3-1 see also Sec. 3.4). 

The Pd°-catalyzed arylations using arenediazonium tetrafluoroborates are limited to those diazonium salts that can be manipulated at room temperature. The reaction can, if necessary, be performed at temperatures up to 50 °C by using a mixture of an arylamine and tert-butyl nitrite in chloroacetic acid or in a mixture of chloroacetic and acetic acid (Kikukawa et al., 1981a). Styrene reacted with fourteen arylamines in the presence of 5 mol-% Pd(dba)2 to give the corresponding substituted stilbenes in yields of 46-97%. It is important for good yields to carry out these reactions in an acidic system. Without acid the yield was low (11%), and diazo tars were also formed. 

A vast array of arylamines can be converted into the corresponding diazonium salts from these diazo components and the host of available coupling components an almost limitless range of azo dyes is accessible. 

Sandmeyer-type reactions are a useful route to polynitroarylenes with unusual substitution patterns. In these reactions an arylamine is treated with a source of nitrous acid to form an intermediate diazonium salt which is readily displaced on reaction with a suitable nucleophile. Many substituents can be incorporated into the aromatic ring via this method, including the nitro group. 

Diazonium salt formation Primary arylamines react with nitrous acid (HNO2) to yield stable arenediazonium salts, Ar—N+=NX . Alkylamines also react with nitrous acid, but the alkanediazonium salts are so reactive that they cannot be isolated. 

Aniline is a primary arylamine, and it reacts with nitrous HNO2 to yield stable benzene diazonium salt (Ph—N+=NX ). 

Palladium(II) salts apparently oxidize arylamines to arylpalladium salts since alkenes are arylated by reaction with only an aromatic amine and a palladium salt. However, yields are generally low.100 Much better yields are obtained if /-butyl nitrite is added and, of course, this forms the diazonium salt in situ. This not only saves a step but some diazonium salts which are too unstable to be isolated may be used as well. The reactions are carried out in the presence of acetic or chloroacetic acid with 5-10% bis(di-benzylideneacetone)palladium as catalyst (equation 41).101... 

J) Primary arylamines are converted to aryl diazonium salts on treatment with sodium nitrite in aqueous acid. When the aqueous acidic solution containing the diazonium salt is heated, a phenol is formed. 

B-4. Which of the following arylamines will not form a diazonium salt on reaction with sodium nitrite in hydrochloric acid ... 

The most useful reaction of amines with nitrous acid is the reaction of arylamines to form arenediazonium salts. We consider next how these diazonium salts may be used as synthetic intermediates. 

Use diazonium salts in reactions involving arylamines, including diazo coupling reactions. 

Deaminative substitution. Arylamines undergo various substitution reactions in the presence of 1 (slight excess). The reactions proceed through a diazonium salt to form an aryl radical. 

Salts of primary aromatic amines react with nitrous acid to produce diazonium salts. The reaction is usually performed by adding a cold solution of sodium nitrite to a cold solution of the arylamine in aqueous mineral acid. The end point of the reaction is conveniently determined by the detection of excess nitrous acid with potassium iodide-starch paper. Sulfamic acid has long been used both in industry and in the laboratory to remove excess nitrous acid. It has been found to react with the more active diazo compounds. In most cases, high temperatures are avoided to prevent the formation of phenols and the decomposition of the unstable nitrous acid. An excess of mineral acid is necessary to prevent coupling between the diazonium salt and unreacted amine (cf. method 494). If the amine salt is somewhat insoluble, a fine crystalline form, which is produced by rapid crystallization from a warm aqueous solution, may be employed. ... 

Nitrous acid reacts with 1 ° alkylamines and arylamines to form diazonium salts. This reaction is called diazotization. 

Diazotization reaction (Section 25.13A) A reaction that converts 1 ° alkylamines and arylamines to diazonium salts. 

Primary arylamines rea t with nitrous acid, HNOg, to yield stable arene-diazonium salts, Ar-NsNX". This diazotization reaction is compatible with the presence of a wide variety of substituents on the aromatic ring. 

This behavior is typical of alkylamines. The arylamines behave similarly at higher than room temperatures. However, at temperatures of 5° or lower, the intermediate compounds, known as diazonium salts, are formed. Because of their great reactivity they are used in the preparation of a large and diversified number of compounds. The formation of the diazonium salts is known as diazotization of the arylamine ... 

If the base is slightly soluble in acid, as in the case of those arylamines which have very feeble basic properties, a large excess of acid is used, or a suspension of the finely divided base is diazo-tized. The base dissolves as fast as that portion which is in solution is diazotized. The aminosulfonic and aminocarboxylic acids are diazotized by first dissolving in alkali to form the sodium salt and then adding the calculated amount of nitrite. The mixture is cooled and added slowly to a solution of acid. The end of all diazotizations is recognized by the presence of,free nitrous acid in the solution. As a test of this, a drop of the solution of the diazonium salt placed on starch-iodide paper produces a dark blue stain immediately, due to the oxidation of iodide by nitrous acid to iodine and action of the latter on starch. 

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