Nitronium Nitrophenol

The operation of the nitronium ion in these media was later proved conclusively. "- The rates of nitration of 2-phenylethanesulphonate anion ([Aromatic] < c. 0-5 mol l i), toluene-(U-sulphonate anion, p-nitrophenol, A(-methyl-2,4-dinitroaniline and A(-methyl-iV,2,4-trinitro-aniline in aqueous solutions of nitric acid depend on the first power of the concentration of the aromatic. The dependence on acidity of the rate of 0-exchange between nitric acid and water was measured, " and formal first-order rate constants for oxygen exchange were defined by dividing the rates of exchange by the concentration of water. Comparison of these constants with the corresponding results for the reactions of the aromatic compounds yielded the scale of relative reactivities sho-wn in table 2.1. 

Further evidence that the nitronium ion was not the electrophile in the uncatalysed reaction, and yet became effective in the catalysed reaction, came from differences in the orientation of substitution. The nitration of chlorobenzene in the uncatalysed reaction yielded only 43 % of the para compound, whereas, when the catalysed reaction was made important by adding some nitric acid, the ratio of substitution was that usually observed in nitration involving the nitronium ion ( 5.3.4). In the case of the uncatalysed reaction however, the reaction was complicated by the formation of nitrophenols. 

An ipso attack on the fluorine carbon position of 4-fluorophenol at -40 °C affords 4-fluoro-4-mtrocyclohexa-2 5-dienone in addtion to 2-mtrophenol The cyclodienone slowly isomerizes to the 2-nitrophenol Although ipso nitration on 4-fluorophenyl acetate furnishes the same cyclodienone the major by-product is 4 fluoro-2,6-dmitrophenol [25] Under similar conditions, 4-fluoroamsole pnmar lly yields the 2-nitro isomer and 6% of the cyclodienone The isolated 2 mtro isomer is postulated to form by attack of the nitronium ion ipso to the fluorine with concomitant capture of the incipient carbocation by acetic acid Loss of the elements of methyl acetate follows The nitrodienone, being the keto tautomer of the nitrophenol, aromatizes to the isolated product [26] (equation 20) Intramolecular capture of the intermediate carbocation occurs in nitration of 2-(4-fluorophenoxy)-2-methylpropanoic acid at low temperature to give the spiro products 3 3-di-methyl-8 fluoro 8 mtro-1,4 dioxaspiro[4 5]deca 6,9 dien 2 one and the 10-nitro isomer [26] (equation 21)... 

Nitrobenzenes, oxidation of 419 2-Nitro-4-i-butylphenol, synthesis of 639 NitrocaUxarenes, synthesis of 633-635 Nitrocyclohexadienones, formation of 1403 Nitronaphthols, synthesis of 406 Nitronium radical 638 p-Nitrophenol,... 

With the nitronium ion results indicate that the same intermediate phenylnitrate is formed, which then follows the same path to give 55/45 ortho/para mixture of nitrophenols (Scheme 5). It is interesting to note that in organic medium the N2O4 nitration of phenol is much faster than with the nitronium ion. This mechanism involving the initial formation of the phenylnitrate which decomposes homolitycally to the phenoxyradical and NO2 has already been proposed by J.H. Ridd in the case of nitration of paranitrophenol with the nitronium ion (ref. 16). 

The zwitterion, N-dodecylpyridinium-3-carboxylate converted in an acetonitrile suspension with an acetonitrile solution of nitronium tetrafluoroborate to the nitro ester, upon treatment with phenol gave 2-nitrophenol in 95% yield (ref.61). 

Nitrobenzene, dinitrobenzene, nitrophenol, and dinitrophenol have been identified as reaction products in HNO3. The nitration of aromatic compounds has been studied extensively [34-45]. Kargin et al. have studied the anodic nitration of aromatic compounds in acetonitrile and sulfolane on a Pt electrode [34]. They attribute the formation of nitrated BZ to a reaction involving either nitronium ion, N02, or anode-generated H2N03, as shown in Equations 32 and 33, respectively. 

Phenol silyl ether when reacted with nitronium tetrafluonoborate gives nitrophenols (as well as nitrated silyl ethers). 

Surfactant aggregates can also improve selectivity by having different effects on rates of different reactions. In aqueous nitric acid (HNO3) solution, for example, nitration of phenol produces 35% o-nitrophenol and 65% p-nitrophenol. In an oil-in-water microemulsion, however, the corresponding results were 80% and 10% (Chhatre et al., 1993). Solubilized phenol molecules were oriented at the microemulsion droplet surfaces with the hydroxy (OH) group extending toward the aqueous solution. The ortho position was thus most accessible to nitronium ions (NO2+) in the aqueous solution. 

The same pathway can be written for the nitration of phenol with a mixture of nitric (HONO2) and sulfuric (H2SO4) acids (which serve to generate the nitronium ion [N02 ]), and, indeed, in dilute aqueous mixtures of these acids, mononitration occurs to produce p-nitrophenol. However, when concentrated acids are used and there is a large excess of the nitronium ion (N02" ) present, di- and trinitrophenols are formed. Picric acid (2,4,6-trinitrophenol), a material reported to decompose explosively, is formed in this way (Scheme 8.38). 

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