Electrophilic aromatic nitrosation

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

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 chemistr-y attends the nitrosation of tertiar y alkylamines, electrophilic aromatic substitution by nitrosyl cation ( n Q ) takes place with A,A-dialkyl-arylfflnines. 

Other typical electrophilic aromatic substitution reactions—nitration (second entr-y), sul-fonation (fourth entry), and Friedel-Crafts alkylation and acylation (fifth and sixth entries)—take place readily and are synthetically useful. Phenols also undergo electrophilic substitution reactions that are limited to only the most active aromatic compounds these include nitrosation (third entry) and coupling with diazonium salts (seventh entry). 

The C-nitrosation of aromatic compounds is characterized by similar reaction conditions and mechanisms to those discussed earlier in this section. The reaction is normally carried out in a strongly acidic solution, and in most cases it is the nitrosyl ion which attacks the aromatic ring in the manner of an electrophilic aromatic substitution, i. e., via a a-complex as steady-state intermediate (see review by Williams, 1988, p. 58). We mention C-nitrosation here because it may interfere with diazotization of strongly basic aromatic amines if the reaction is carried out in concentrated sulfuric acid. Little information on such unwanted C-nitrosations of aromatic amines has been published (Blangey, 1938 see Sec. 2.2). 

The kinetics of aromatic nitrosation at ring carbon have received little attention. The first attempt to determine the nature of the electrophile was made by Ingold et a/.117, who measured the rates of the nitrous acid-catalysed nitration of 4-chloroanisole by nitric acid in acetic acid which proceeds via initial nitrosation of the aromatic ring. Assuming that the electrophiles are the nitrosonium ion and... 

Aromatic nitrosation with nitrosonium (NO + ) cation - unlike electrophilic nitration with nitronium (NO ) cation - is restricted to very reactive (electron-rich) substrates such as phenols and anilines.241 Electrophilic nitrosation with NO+ is estimated to be about 14 orders of magnitude less effective than nitration with N02+. 242 Such an unusually low reactivity of NO+ toward aromatic donors (as compared to that of NO ) is not a result of the different electron-acceptor strengths of these cationic acceptors since their (reversible) electrochemical reduction potentials are comparable. In order to pinpoint the origin of such a reactivity difference, let us examine the nitrosation reaction in the light of the donor-acceptor association and the electron-transfer paradigm as follows. 

Steric Control of the Inner/Outer-Sphere Electron Transfer 461 Thermal and Photochemical ET in Strongly Coupled CT Complexes 463 Electron-Transfer Paradigm for Arene Transformation via CT Complexes 465 Electron-Transfer Activation of Electrophilic Aromatic Substitution 469 Structural Pre-organization of the Reactants in CT Complexes 470 CT Complexes in Aromatic Nitration and Nitrosation 472 Concluding Summary 475 References 475... 

Ring nitrosation is an electrophilic aromatic substitution reaction, in which the... 

Isotope effects are also useful in providing insight into other aspects of the mechanisms of individual electrophilic aromatic substitution processes. In particular, since primary isotope effects are expected only when the breakdown of the rate-determining, the observation of a substantial kn/ko points to rate-determining deprotonation. Some typical isotope effects are summarized in Table 9.7. While isotope effects are rarely observed for nitration and halogenation, Friedel-Crafts acylation, sulfonation, nitrosation, and diazo coupling provide examples in which the rate of proton abstraction can control the rate of substitution. 

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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