Ortho- and para-Nitrophenols

From the residue after the steam distillation the non-volatile p-compound is isolated at the same time, at first in the form of its sodium salt. Sodium hydroxide solution (2 N) is added until the reaction to Congo paper has just disappeared, 100 c.c. more of the hydroxide solution are then poured in, and the mixture is boiled with a little animal charcoal while steam is again passed through. The solution is now filtered through a folded paper and the filtrate is 

The ease with which the phenols are nitrated has already been discussed. The process is not satisfactory, however, even when dilute nitric acid is used, because resinous by-products are formed as a result of oxidation and condensation. Nitration with nitrogen peroxide in non-aqueous solvents such as benzene and petrol ether gives better results (Ber., 1921, 54, 1776). 

By further nitration with more concentrated acid o- and p-nitro-phenols are converted into the same 2 4-dinitrophenol, and finally into picric acid. Polynitro-derivatives of benzene, such as picric acid and trinitrotoluene, can be caused to explode by detonation with mercury fulminate or lead azide. (The formulae of these two compounds should be written.) They are endothermic, i.e. the oxygen of the nitro-group can oxidise carbon and hydrogen within the molecule and heat is liberated. This intramolecular combustion is rather considerable in the case of picric acid, which is decomposed in accordance with the equation  

In the pure state m- and p-nitrophenols are colourless, but the o-com-pound, on the other hand, is yellow. The salts of all three, however, are intensely coloured, the o- and wi-compounds being orange-red and orange-yellow and the p-compound deep yellow. (Use of p-nitro-phenol as an indicator.) 

An attempt has been made to explain the strong colours of the salts of the nitrophenols as a rearrangement involving the formation of a 

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A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

Ortho and para nitrophenols are more acidic than phenol. Draw the resonance structures of the corresponding phenoxlde ions. 

Phenol, when treated with dilute nitric acid at room temperature, forms ortho- and para-nitrophenol. 

The product is a mixture of ortho- and para-nitrophenol from which the ortho compound can be separated by steam distillation. A strong intramolecular hydrogen bond reduces the availability of the OH group for intermolecular hydrogen bonds so the ortho compound has a lower boiling point. The remaining pnra-nitrophenol is used in the manufacture of the painkiller paracetamol. 

Phenol reacts with NaN02 on wet Si02 at room temperature to give mono- or dinitrosa-tion products, which are in situ oxidized by oxone to give the ortho- and para-nitrophenols in high yields, depending on the reaction conditions . 

Conversion of Phenols to Amines. Aniline and some diphenylamine are formed when phenol and NHs solution are heated under pressure in the presence of FeCh, Al(OH)s, or Fe(OH)j. When NH and phenol or ortho-or para-cresols are reacted in the vapor phase over an AljO catalyst, yields of up to 88 per cent of the corresponding amines are obtained. However, these amines are customarily obtained by reducing the parent nitro compound, except in cases where it is difficult to obtain the required nitro isomer. For example, it is considered that the amination of symr xylenol is the best method of preparing sym-xylidine (l-amino-3,5-di-methylbenzene). When sym-xylenol is heated under pressure to 320 C with ammonium chloride, about equal amounts of sj/m-xylidine and sym-dixylylamine (5-imino-bis-l,3-dimethylbenzene) are formed. The ortho-and para-nitrophenols and nitrocresols can be aminat more readily. 2-Nitro-p-cresol [OH(l), N02(2), CHj(4)] and o-nitrophenol have been aminated in aqueous ammonia containing ammonium salts of weak acids to inhibit decomposition. Phosphoric, boric, carbonic, and formic acids were used. In one case it is claimed that 55-65 per cent yields of 2-nitro-p-toluidine (MNPT of commerce) were obtained when 2-nitro-p-cresol, 28 per cent aqueous NHj, and monoammonium phosphate, 1 11.5 0.2 molar ratio, were heated under pressure for 10 hr at 140-150°C and then 5 hr at about 160 C. Earlier workers, employing somewhat similar conditions, claimed excellent yields of MNPT when 1 mole of ammonium formate was used per mole of 2-nitro-p-cresol. ... 

The ortho and para isomers can be separated by steam distillation. o-Nitrophenoi is steam voiatile due to intramolecular hydrogen bonding whiie p-nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules. 

Examjdes.—Aniline is rapidly tribrominated at room temperature, the end point is very sharp, and this estimation of aniline is preferable to the nitrous acid method. At ordinary temperature ortho- and para-toluidines are dibrominated and meta-toluidine is tribrominated. Dimethylaniline is monobrominated at 0°—5°, dibrominated at 40°—50° and tribrominated at 60°—70°, an interesting example of the effect of temperature. Sulphanilic acid at 60°—70° yields tribromoaniline, and at this temperature p-nitroaniline reacts quantitatively. Phenol at about 22° yields tribromophenol. For reference to the estimation of m-diamines, m-dihydroxy compounds, R salt, cresols, p nitroaniline, diphenylamine, p-nitrophenol, and other notes, see J. S. C. I., 41, 161. 

Nitrobenzene gives m- dinitrobenzene along with small quantities of the ortho-and para- isomers, a mixture of o-, m- and p- nitrophenols and a small quantity of 3,3 -dinitrodiphenyl. 

The reaction yields the ortho and para isomers (2-nitrophenol, 4-nitrophen-ol), with a slight excess of the former. 

This review will illustrate examples of computer projected models of inclusion complexes of structural isomers (ortho, meta, para nitrophenol), enantiomers (d- and 1- propranolol) and diastereomers [cis and trans. l(p-B-dimethylaminoethoxy-phenyl-butene), tamoxifen] in either a- or B-cyclodextrin. The use of these computer projections of the crystal structures of these complexes allows for the demonstration and prediction of the chromatographic behavior of these agents on immobilized cyclodextrin. 

The yield is about 40 grams each of the ortho and para isomers. It is bad practice to treat the crude nitrophenols with caustic soda, as called for in some procedures, because the caustic has an immediate resinifying action. 

The nitration of aromatic ethers leads to a mixture of nitro ethers and nitrophenols in proportions which depend upon experimental conditions. Benzoyl nitrate favors almost exclusively the formation of o-nitrophene-tole however, detailed directions are lacking. Treatment of diphenyl ether with nitric acid in acetic anhydride-acetic acid gives a se[>arable mixture of the ortho and para isomers (86% total). ... 

The photochemical destruction of ortho, meta and para-nitrophenols induced by ultra-violet light illumination of aqueous slurries of titanium dioxide has been monitored by electronic absorption spectroscopy the products are said to have been identified as dihydroxynitrobenzene isomers by coupled gas chromatography-mass spectrometry although no details are supplied. Nitrophenols have also been identified in the fog shrouding the University of Bayreuth. They are presumed to be the products of photochemical nitration and the possible precursors (phenol, cresol and nitrate) were also detected. 

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