Preparation of Diazonium Salts

Note The object of this and of a number of the experiments which follow is to study the preparation of diazonium salts and their application to organic syntheses. For this purpose a number of choices may be made, depending on the time available, (a) If the time is short, the student may do sections (A) and (B) of this experiment (i ) the student may choose to prepare 0.15 moles of the diazonium salt according to section (C) of this experiment, and then use the semimicro method for the preparation of iodobenzene or chlorobenzene, phenol, phenylhydrazine, and a few azo dyes (Experiments 54, 55, 56, 57, and 58) in such case it is posable to complete the preparations in two laboratory periods (c) if the macro method is used, then selection is made from Experiments 54, 55, 56, and 57. 

Introduction. The action of nitrous acid on alkyl primary amines at ordinary temperatures causes replacement of the amino group by the hydroxyl, while the nitrogen atom of the amino group and the nitrogen atom from nitrous acid are evolved as free molecular nitrogen  

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 it is desired to isolate the diazonium salt, the amine is dissolved in alcohol or glacial acetic acid and diazotized by adding amyl or butyl nitrite and then the acid. Diazonium salts are unstable and explosive, and are not prepared in the dry state in any considerable quantities. Diazotization is usually carried out in aqueous solutions. In most cases the base is suspended in water and 2.2 moles of acid is added. One mole of acid is required to form the amine 

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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There are many preparations of diazonium salts listed in Organic Syntheses, but they are always prepared for use in other reactions. We do not list them here, but under reactions in which they are used. The preparation of aliphatic diazo compounds can be found in OS III, 392 IV, 424. See also OS VI, 840. 

Scheme 31 Preparation of diazonium salt immobilized on T1 triazine linker resin...

NOTE.—In the preparation of diazonium salts it is advisable to keep the reaction-mixture cold during the addition of the nitrite solution, even in the case when the resulting salt is to be decomposed by water in this way the formation of by-products is largely avoided. 

A continuous process for the preparation of diazonium salts has been described by Hupfer.253... 

A. Notes on the Preparation of Diazonium Salt Solutions in Aqueous Media... 

A. NOTES ON THE PREPARATION OF DIAZONIUM SALT SOLUTIONS IN AQUEOUS MEDIA... 

Gattermann (1890) found that the preparation of the cuprous halide may be avoided by making use of the fact that finely-divided copper (e.g., freshly-precipitated or reduced by hydrogen or copper bronze) acts catal3d.ically in the decomposition of solutions of diazonium salts, for example ... 

The diazonium salts 145 are another source of arylpalladium com-plexes[114]. They are the most reactive source of arylpalladium species and the reaction can be carried out at room temperature. In addition, they can be used for alkene insertion in the absence of a phosphine ligand using Pd2(dba)3 as a catalyst. This reaction consists of the indirect substitution reaction of an aromatic nitro group with an alkene. The use of diazonium salts is more convenient and synthetically useful than the use of aryl halides, because many aryl halides are prepared from diazonium salts. Diazotization of the aniline derivative 146 in aqueous solution and subsequent insertion of acrylate catalyzed by Pd(OAc)2 by the addition of MeOH are carried out as a one-pot reaction, affording the cinnamate 147 in good yield[115]. The A-nitroso-jV-arylacetamide 148 is prepared from acetanilides and used as another precursor of arylpalladium intermediate. It is more reactive than aryl iodides and bromides and reacts with alkenes at 40 °C without addition of a phosphine ligandfl 16]. 

The alternative approach is to pad the fabric with the alkaline naphthol and dry, foUowed by printing directly onto this prepared fabric diazonium salts or stabilized diazonium salts. Coupling is instant and the only further treatment needed is to remove aU the uncoupled naphthol and surface azo pigment in a subsequent washing treatment. Because the choice of colors is limited from one naphthol component, other shades are obtained by using other classes of dye alongside the azoic colors, eg, reactives. This approach is widely used in the production of African prints. 

Incidentally, 31 contributes more to the hybrid than 32, as shown by bond-distance measurements. In benzenediazonium chloride, the C—N distance is 1.42 A, and the N—N distance 1.08 A, which values lit more closely to a single and a triple bond than to two double bonds (see Table 1.5). Even aromatic diazonium salts are stable only at low temperatures, usually only below 5°C, though more stable ones, such as the diazonium salt obtained from sulfanilic acid, are stable up to 10 or 15°C. Diazonium salts are usually prepared in aqueous solution and used without isolation, though it is possible to prepare solid diazonium salts if desired (see 13-20). The stability of aryl diazonium salts can be increased by crown ether complexion. ... 

One of the best methods for the introduction of iodine into aromatic rings is the reaction of diazonium salts with iodide ions. Analogous reactions with chloride, bromide, and fluoride ions give poorer results, and 14-25 and 13-20 are preferred for the preparation of aryl chlorides, bromides, and fluorides. However, when other diazonium reactions are carried out in the presence of these ions, halides are usually side products. Aniline has also been converted to fluorobenzene by treatment with t-BuONO and Sip4 followed by heating. A related reaction between PhN=N—N C4Hg and iodine gave iodobenzene. ... 

Treatment of diazonium salts with cuprous chloride or bromide leads to aryl chlorides or bromides, respectively. In either case the reaction is called the Sandmeyer reaction The reaction can also be carried out with copper and HBr or HCl, in which case it is called the Gatterman reaction (not to be confused with 11-16). The Sandmeyer reaction is not useful for the preparation of fluorides or iodides, but for bromides and chlorides it is of wide scope and is probably the best way of introducing bromine or chlorine into an aromatic ring. The yields are usually high. 

One very common example is the displacement of N2 in the reactions of diazonium salts, ArN2 , a very useful preparative series ... 

The high reactivity of diazonium salts enables them to enter into a great many reactions other than the azo coupling reaction, but these fall outside the scope of the present chapter. Various other methods can be used for the preparation of azo compounds, although these are of minor importance compared with the diazo coupling reaction and they will only be touched on briefly where appropriate. 

As with resorcinol, the 1,3-diamines of the benzene series are useful in the preparation of disazo dyes that can form the basis of brown shades. Generally the 2,4-disazo compound is the product of coupling with two equivalents of diazonium salt, but at higher pH values formation of the 4,6-isomer can be significant [31]. Again, the 1,2- and 1,4-diamines reduce diazonium salts on attempted coupling. 1,3,5-Triaminobenzene has been found to couple three times [32]. 

Whilst azo compounds prepared from diazonium salts and phenolic or keto-enol coupling components are often depicted in the hydroxyazo form (4.11), an alternative tautomeric structure can be drawn for such compounds (Scheme 4.19). This ketohydrazone tautomer (4.21) can, in cases where the azo and hydroxy groups are located on adjacent carbon atoms, exhibit hydrogen bonding between the two groups as shown. Similar pairs of structures, but without hydrogen bonding, can be drawn for p-hydroxyazo compounds. 

The most feasible method consists in the reduction of diazonium salts with sodium sulfite. Although this method is given in several laboratory manuals, the results were not found entirely satisfactory. The present directions provide for a lengthy but essential heating of the diazonium-sulfite mixture, omit the useless zinc dust reduction, and supply exact details for preparation on a fairly large laboratory scale. 

There is a long history of the preparation of explosive solids or oils from interaction of diazonium salts with solutions of various sulfides and related derivatives. Such products have arisen from benzene- and toluene-diazonium salts with hydrogen, ammonium, or sodium sulfides [1,5] 2- or 3-chlorobenzene-, 4-chloro-2-methylbenzene-, 2- or 4-nitrobenzene- or 1- or 2-naphthalene-diazonium solutions with hydrogen sulfide, sodium hydrogen sulfide or sodium mono-, di- or poly-sulfides [l]-[4,7], 4-Bromobenzenediazonium solutions gave with hydrogen sulfide at -5°C a product which exploded under water at 0°C [2], and every addition of a drop of 3-chlorobenzenediazonium solution to sodium disulfide solution... 

The products produced by interaction of diazonium salts and iodides are unstable and liable to be explosive in the solid state. They are usually the triiodides, but monoiodides have been isolated under specific conditions from diazotised aniline and o-toluidine. Products prepared from diazotised o-, m- or p-nitroanilmes, m-chloro-, -methoxy-, or -methyl-aniline are too unstable to isolate, decomposing below 0°C. 

Alcohols may be prepared (1) by hydration of alkenes (1) in presence of an acid and (11) by hydroboratlon-oxidatlon reaction (2) from carbonyl compounds by (1) catalytic reduction and (11) the action of Grignard reagents. Phenols may be prepared by (1) substitution of (1) halogen atom In haloarenes and (11) sulphonic acid group In aiyl sulphonic acids, by -OH group (2) by hydrolysis of diazonium salts and (3) industrially from cumene. 

The formed mercury dichloride is separated as a crystalline complex with dioxane. Since the starting arylmercury chlorides are easily prepared from diazonium salts, this method allows the conversion of anilines into aryltellurium trichlorides. 

T.J. Tewson, M.J. Welch, Preparation of F-18 aryl fluorides—Piperidyl triazenes as a source of diazonium salts, J. Chem. Soc. Chem. Comm. 24 (1979) 1149-1150. 

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