Diazonium salts decomposition mechanism

The effect of substituents on the rates of diazonium salt decompositions can be accommodated by the SnI mechanism. The essential facts are as follows (10,12,14,15,21) ... 

Two mechanisms are among those that have been postulated for decomposition of aryl diazonium salts in aqueous solution containing nucleophilic anions, A ... 

Reactions involving diazo groups are also affected by heavy metals such as cuprous ion. For example, in the well known Sandemeyer reaction, cuprous ion can catalyze the decomposition of aryl diazonium salts. Although the exact mechanism is unknown and may involve free radical processes, the anion of the cuprous salt replaces the nitrogen of the diazonium salt. 

More than one mechanism has been proposed to explain the catalytic activity of diazonium salts in initiating polymerization of cyclic ethers. Dreyfuss and Dreyfuss (1 ) postulated that initiation involves hydrogen abstraction from the cyclic ether by a carbenium ion formed via decomposition of the diazonium salt, followed by polymerization via tertiary oxonium ions associated with PFe" counterions. The polymerization reactions studied by Dreyfuss and Dreyfuss were initiated by thermal decomposition of... 

A mechanism for the formation of compound 140 could be the following. An aromatic radical 151 formed by thermal decomposition from the diazonium salt 150 abstracts a hydrogen radical from the C-1 position of the isoquinoline skeleton, and the resulting aliphatic radical 152 is attacked by the hydroxy radical to give alcohol 153. The radical at the C-1 position in the 3,4-dihydroisoquinoline ring system 154, which formed from 153, is attacked by the radical formed in the phenyl group to give spiroisoquinoline 140. 

A famous example of a reaction which must be kept cold is the diazotization of anilines to make diazonium salts. The reaction involves treating the amine with nitrous acid (HONO) made from NaN02 and HCl. You need not think about the mechanism at this stage— you will meet it in Chapter 22—but the key point is that the product is a rather unstable but very useful diazonium salt. The diazotization takes place readily at room temperature, but unfortunately so does the decomposition of the product to give a phenol. By lowering the temperature, we supply insufficient energy for the phenol formation, but the diazotization still works just fine. 

The radical-generating step is a special case of the decomposition of an azo compound. An important use for this reaction is in the synthesis of biphenyls, by reactions in which a second aromatic molecule is attacked by the aryl radical. Under these conditions, hydrogen abstraction from the intermediate arylcyclohexadienyl radical becomes part of the chain mechanism, with the aryl diazonium ion oxidizing the radical intermediate to give the biphenyl. Aryl diazonium ions generated in the usual way by diazotization of aryl amines can also serve as sources of aryl radicals. Substituted biphenyls can be synthesized by base-catalyzed decomposition of the diazonium salt, usually in the presence of an excess of the aromatic substrate. 

A particular challenge in azo couplings is the hazardous potential of diazonium salts, which tend to undergo abrupt decomposition or even explosion when exposed to light, heat or mechanical impact. Therefore, a major driver for using microreactors is to ensure safe processing of potentially hazardous azo couplings. 

The most widely used illustration for the 5 1 mechanism is the uncatalyzed decomposition of diazonium salts. Such substrates carrying amino substituents have scarcely been investigated, except for the magnetochemical study on the photodecomposition of p- N,N-dimethylamino)benzenediazonium chloride . Tlie result of this and other work indicated a free-radical mechanism. 

The third possible mechanism we believe, accounts for all our observations, and in addition, it is generally applicable to the triazene and nitrous acid methods of deamination. Based on the structures of aromatic diazonium salts, the formation of ion pair 42 seems to be a logical first step in the reaction. By analogy to other ion pairs, the N—O bond in 42 could have some covalent character and the nitrogen could be bonded to one of the oxygens in particular . However, for the decomposition in a solvent as polar as acetic acid (Table 8), it appears that the C—and N—O bonds (34) are either broken simultaneously or in rapid succession in view of the independence of the scrambling with respect to the carbonium ion character of R. 

Replacement of diazonium groups by halide is a very valuable synthetic alternative to direct halogenation for preparation of aryl halides. There are three broad types of procedures decomposition of aryl diazonium-halide adducts with expulsion of nitrogen, copper-mediated redox processes, and thermal processes proceeding via aryl radicals. The first type of process is probably involved in the reaction of aryl diazonium salts with iodide ion. Smooth high-yield transformation takes place in the absence of any metal catalyst. The mechanism of the reaction... 

Three general types of mechanisms are involved. The fimt or SnI mechanism is exemplified by the decomposition of diazonium salts in solution. 

Within this class of reactions the most detailed studies of mechanism available are for the decomposition of diazonium salts in aqueous acid. Here, a large body of evidence has been offered to support an S 1 mechanism, in which the rate-determining step is the decomposition of the diazonium ion to give nitrogen and the highly reactive phenyl cation. 

Dissolve 34 g (0.25 mol) of o-nitroaniline in a warm mixture of 63 ml of concentrated hydrochloric acid and 63 ml of water contained in a 600-ml beaker. Place the beaker in an ice-salt bath, and cool to 0-5 °C while stirring mechanically the o-nitroaniline hydrochloride will separate in a finely divided crystalline form. Add a cold solution of 18 g (0.26 mol) of sodium nitrite in 40 ml of water slowly and with stirring to an end-point with potassium iodide-starch paper do not allow the temperature to rise above 5-7°C. Introduce, while stirring vigorously, a solution of 40 g (0.36 mol) of sodium fluoroborate in 80 ml of water. Stir for a further 10 minutes, and filter the solid diazonium fluoroborate with suction on a Buchner funnel. Wash it immediately once with 25 ml of cold 5 per cent sodium fluoroborate solution, then twice with 15 ml portions of rectified (or industrial) spirit and several times with ether in each washing stir the fluoroborate well before applying suction. The o-nitrobenzenediazonium fluoroborate weighs about 50 g (86%) the pure substance melts with decomposition at 135 °C. 

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