Nitration electrophilic aromatic

Isomeric product distributions. Isomeric product distributions obtained from toluene and anisole have been the subject of considerable mechanistic discussion in electrophilic aromatic nitration (Schofield, 1980 Olah et al., 1989). As applied to nitrations with iV-nitropyridinium ion, the yellow colour of the EDA complex immediately attendant upon the mixing of toluene and PyN02 in acetonitrile persists for about a day (in the dark), whereas the charge-transfer colour of toluene and Me2PyNOj is discharged within 10 min at 25°C. Both bleached solutions afford an identical product mixture (81), consisting of o- (62%), m- (4%) and p-nitrotoluenes (34%)... 

The electron-transfer mechanism for electrophilic aromatic nitration as presented in Scheme 19 is consistent with the CIDNP observation in related systems, in which the life-time of the radical pair [cf. (87)] is of particular concern (Kaptein, 1975 Clemens et al., 1984, 1985 Keumi et al., 1988 Morkovnik, 1988 Olah et al., 1989 Johnston et al., 1991 Ridd, 1991 Rudakov and Lobachev, 1991). As such, other types of experimental evidence for aromatic cation radicals as intermediates in electrophilic aromatic nitration are to be found only when there is significant competition from rate processes on the timescale of r<10 los. For example, the characteristic C-C bond scission of labile cation radicals is observed only during the electrophilic nitration of aromatic donors such as the dianthracenes and bicumene analogues which produce ArH+- with fragmentation rates of kf> 1010s-1 (Kim et al., 1992a,b). 

The nitration reagents (NO2 Y) for electrophilic aromatic nitration span a wide range and contain anions Y such as nitric acid (Y = OH-), acetyl nitrate (Y = OAc-), dinitrogen pentoxide (Y = NO3-), nitryl chloride (Y = Cl-), TV-nitropyridinium (Y = pyridine) and tetranitromethane [Y = C(N02)3-]. All reagents contain electron-deficient species which can serve as effective electron acceptors and form electron donor-acceptor (EDA) complexes with electron-rich donors including aromatic hydrocarbons107 (ArH, equation 86). Excitation of the EDA complexes by irradiation of the charge-transfer (CT) absorption band results in full electron transfer (equation 87) to form radical ion... 

There are many testimonies for the cation-radical formation during electrophilic aromatic nitration. Positional selectivities are in line with spin-density distributions. In principle, the attack of N02 radical is probably at the position of the aromatic cation-radical, which bears the maximal spin density. 

A. S. Morkovnik, the Oxidation-Reduction Stage in the Nitration Reaction, Russ. Chem. Rev. 57,144 (1988). L. Eberson u. F. Radner, Electron-Transfer Mechanisms in Electrophilic Aromatic Nitration, Acc. Chem. 

Nitronium Ion (N02+). Nitration is one of the most studied and best understood organic reactions.510-512 The species responsible for electrophilic aromatic nitration was shown to be the nitronium ion (N02+) 219. Since the early 1900s, extensive efforts have been directed toward the identification of this ion, whose existence was first shown by Hantzsch and later firmly established by Ingold and Hughes.510... 

The accepted reaction mechanism for the electrophilic aromatic nitration was postulated by Ingold in 1969[3] and involves several steps (Scheme 5.1). Firstly, the nitric acid is protonated by a stronger acid (sulfuric). The protonated nitric acid gives water and the nitronium ion (N02+) which is the electrophilic active species for nitration of aromatics. Nitric acid heterolysis is considered to be accelerated by the polarity of the solvent, and solvation of nitronium ion in different media affects its reactivity and the selectivity of the reaction. Combination of nitronium ion and an aromatic molecule form an intermediate named the Wheland complex or er-complex. The loss of a proton from the er-complex gives the aromatic nitrocompound (Scheme 5.1). 

J. K. Kochi, Inner-Sphere Electron Transfer in Organic Chemistry. Relevance to Electrophilic Aromatic Nitration, Acc. Chem. Res. 1992, 25, 39 17. 

L. Eberson, M. P. Hartshorn, F. Radner, Electrophilic Aromatic Nitration Via Radical Cations Feasible or Not , in Advances in Carbocation Chemistry (J. M. Coxon, Ed.) JAI, Greenwich, CT, 1995. 

The regioselectivity of nitration of toluene with nitronium salts has been successfully altered by their prior complexation with crown ethers. Complexation of NO2 BFJ by 18-C-6 crown ether substantially altered the selectivity in nitration of toluene and benzene as reported by Elsenbaumet and Wasserraan [128]. Similar effect was observed with polyethylene oxides. Savoie al. reported isolation of the IS-C-b-NOa BFJ complex and its characterization [129], Masci carried out the yet most detailed study on the effect of crown ethers on the selectivity of electrophilic aromatic nitration [130]. 

Because of the advantages of using solid superacidic catalysts in electrophilic aromatic nitration and in acid-catalyzed reactions in general, Olah et al. have examined the mercury (Il)-promoted azeotropic nitration of aromatics using Nafion-H solid superacidic catalyst [142]. Azeotropic removal of water accelerates the rate of reaction by mitigating the dilution of nitric acid in a static reaction system. The yield of nitroaromatics varies from 48-77% (Table XXVIII). 

The best route to aromatic primary amines is by reduction of the corresponding nitro compounds, which are in turn prepared by electrophilic aromatic nitration. The nitro group is easily reduced, either catalytically with hydrogen or by chemical-reducing agents. 

Predict the position of major electrophilic aromatic nitration in l-(l-methylethyl)naphthalene. Strategy... 

Predict the position of electrophilic aromatic nitration in (a) 2-nitronaphthalene (b) 5-methoxy-1-nitronaphthalene (c) l,6-bis(l,l-dimethylethyl)naphthalene. (Caution In problem (c), both rings are activated. Hint Consider steiic effects.)... 

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