Nitrosation of Arylamines

Nitrous acid deamination of 2,2-dimethylpropylamine, (CH3)3CCH2NH2, gives the same products as from 1,1-dimethylpropylamine. Surest a mechanism for their formation from 2,2-dimethylpropylamine. 

Aryl diazonium ions, prepared by nitrous acid diazotization of primary arylamines, are substantially more stable than alkyl diazonium ions and are of enormous synthetic value. Their use in the S5mthesis of substituted aromatic compounds is described in the following two sections. 

The nitrosation of tertiary alkylamines is rather complicated, and no generally useful chemistry is associated with reactions of this type. 

We learned in the preceding section that different reactions are observed when the various classes of alkylamines— primary, secondary, and tertiary— react with nitrosating agents. 

Although no useful chemistry attends the nitrosation of tertiary alkylamines, electrophilic 

Nitrosyl cation is a relatively weak electrophile and attacks only very strongly activated aromatic rings. 

Y-Alkylarylamines resemble secondary alkylamines in that they form Af-nitroso compounds on reaction with nitrons acid. 

Primary arylamines, like primary alkylamines, form diazoninm ion salts on nitrosation. Aryl diazonium ions are considerably more stable than their alkyl counterparts. Whereas aUcyl diazonium ions decompose under the conditions of their formation, aryl diazonium salts are stable enough to be stored in aqneons solntion at 0-5°C for reasonable periods of time. Loss of nitrogen from an aryl diazoninm ion generates an nnstable aryl cation and is much slower than loss of nitrogen from an alkyl diazoninm ion. 

FIGURE 22.6 Flowchart showing the synthetic origin of aryl diazonium ions and their most useful transformations. 

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We learned m the preceding section that different reactions are observed when the var lous classes of alkylammes—primary secondary and tertiary—react with mtrosatmg agents Although no useful chemistry attends the nitrosation of tertiary alkylammes elec trophilic aromatic substitution by mtrosyl cation ( N=0 ) takes place with A A dialkyl arylamines... 

Tertiary alkylamines illustrate no useful chemistry on nitrosation. Tertiary arylamines undergo nitrosation of the ring by electrophilic aromatic substitution. 

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). 

Alkyl nitrites have been used to effect nitrosation or diazotisation in aprotic solvents. Typically, arylamines react in benzene solution to give biaryls (Cadogan et al., 1966). Friedman and coworkers have examined the products from alkyl nitrites and amines in a number of solvents. No mechanistic study aimed at identifying the details of the nitrosation reaction was attempted rather the emphasis was on the intermediacy of a diazoalkane and subsequent reaction of carbocations (Friedman and Bayless, 1969 Friedman et al., 1969). 

Tertiary arylamines react with nitrous acid to form C-nitroso aromatic compounds. Nitrosation takes place almost exclusively at the para position if it is open and, if not, at the ortho position. The reaction (see Practice Problem 20.9) is another example of electrophilic aromatic substitution. 

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