Nitration ortho/para ratios

The catalysed nitration of phenol gives chiefly 0- and />-nitrophenol, (< 0-1% of w-nitrophenol is formed), with small quantities of dinitrated compound and condensed products. The ortho para ratio is very dependent on the conditions of reaction and the concentration of nitrous acid. Thus, in aqueous solution containing sulphuric acid (i 75 mol 1 ) and nitric acid (0-5 mol 1 ), the proportion of oriha-substitution decreases from 73 % to 9 % as the concentration of nitrous acid is varied from o-i mol l i. However, when acetic acid is the solvent the proportion of ortAo-substitution changes from 44 % to 74 % on the introduction of dinitrogen tetroxide (4-5 mol 1 ). 

The theory that the catalysed nitration proceeds through nitrosation was supported by the isolation of some />-nitrosophenol from the interrupted nitration of phenol, and from the observation that the ortho.-para ratio (9 91) of strongly catalysed nitration under aqueous conditions was very similar to the corresponding ratio of formation of nitrosophenols in the absence of nitric acid. ... 

As actually performed a 62% yield of a mixture of ortho and para nitration prod ucts has been obtained with an ortho-para ratio of about 1 3... 

The identification of a specific nitrating species can be approached by comparing selectivity with that of nitration under conditions known to involve the nitronium ion. Examination of part B of Table 10.7 shows that the position selectivity exhibited by acetyl nitrate toward toluene and ethylbenzene is not dramatically different from that observed with nitronium ion. The data for i-propylbenzene suggest a lower ortho para ratio for acetyl nitrate nitrations. This could indicate a larger steric factor for nitration by acetyl nitrate. 

The nature of the electrophile in this nitrating mixture is still not wholly agreed upon whereas kinetic evidence can be interpreted as consistent with nitration by nitronium ion, the fact that substituents with lone pairs of electrons or it-electrons give markedly different ortho para ratios from other nitrating mixtures is usually conceded to be consistent with the electrophile being something other than the nitronium ion. The balance of evidence at present is in favour of pro-tonated acetyl nitrate being the electrophile. 

Ortho para ratios for nitration of some aromatics... 

Amines and phenols can be nitrated by dilute (< 5 M) solutions of nitric acid in water or acetic acid provided nitrous acid is present1 °3. A mixture of products is frequently obtained and the ortho para ratio can vary widely according to the nitrous acid concentration104 105, thereby indicating a different electrophile under these conditions. 

Another useful medium for nitration is a solution prepared by dissolving nitric acid in acetic anhydride, which generates acetyl nitrate. This reagent tends to give high ortho. para ratios for some nitrations.2... 

This reagent tends to give high ortho para ratios for some nitrations.2 Another convenient procedure involves reaction of the aromatic compound in chloroform or dichloromethane with a nitrate salt and trifluoroacetic anhydride.3 Presumably, trifluoroacetyl nitrate is generated under these conditions. 

Exclusive ring-nitration occurs with alkylbenzenes. The nitration of toluene in the presence of H-ZSM-5 and molecular oxygen shows a remarkable enhancement of para selectivity (ortho para ratio = 0.08).268 A review is available for the nitration of aromatics by nitrogen oxides on zeolite catalysts.269... 

Due to the industrial importance of the 2-nitrophenol, extensive research has been focused on enhancing the regioselectivity of the nitration of phenol . The regiochemistry of the nitration is dramatically increased, giving an ortho/para ratio of 13.3 with acetyl nitrate as the reagent, when dry sUica gel was used for the catalysis. In chloroform solvent, in the absence of silica gel, a normal ortho/para ratio of 1.8 was obtained . 

The ortho para ratio is also influenced by the nitration medium in a manner which cannot be explained by the Robinson-Ingold theory. The isomer distribution which results from the nitration of aniline and anilides in several nitrating media is shown in Table 4-6. 

The nitration of acetanilide with mixed acid yields nitroacetanilides in which the ortho para ratio is less than 0.1. When the nitration medium is nitric acid, this ratio is 0.7, when acetyl nitrate in acetic anhydride is used, the product is almost entirely o-nitroacetanilide. No satisfactory explanation has been given for these results. The 40-50 per cent yield of m-nitro-aniline that results from the nitration of aniline in mixed acid or in nitric acid can be explained in the following manner. In the strong acids, nitric and sulfuric acid, aniline is largely ionized. 

Whereas it could be argued, that the iscmer distribution differences in the nitration of toluene are not large (see, however, the 50% variation in ortho/para ratios in Table XV) these differences are becoming much more significant when comparing data of nitrations in similar media for -xylene, anlsole, and chlorobenzene (TABLE XVI). 

Explain the following observations (1) The ortho-para ratio of the products obtained by sulfonation of toluene is lower than that of nitration (2) The ortho-para ratio of the products obtained by nitration of isopropylbenzene is lower than that of nitration of toluene. 

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