Picric acid and benzene

Hphe formation of molecular or addition compounds by organic sub-stances is well known, and many handbooks cite as an example the mixture of benzophenone and diphenylamine to produce a stable equi-molecular addition compound. Another example is the molecular compound formed by benzene and picric acid. This is stable only in the presence of an excess of picric acid. 

Example.— An example of the type of system just discussed is found in the case of benzene and picric acid, These two susbtances combine, in equimolecular proportions, to form a compound which, however, can exist only in contact with solutions containing excess of benzene (Fig. 42). When the temperature is raised a point (K) is... 

The case of multicomponent separation between two immiscible phases requires further consideration if each phase is primarily made up of one species and the remaining components are present in these phases in small amounts. Consider a three-component system consisting of, say, water, benzene and picric acid, the latter being present in very small amounts. Water and benzene... 

OS 31] [R 16c] ]P 23] The levels of dinitrobenzene, dinitrophenol and picric acid in the organic phase during benzene nitration in a micro reactor were monitored [31]. Picric acid levels were no higher than 100 ppm for all experiments conducted. Dinitrobenzene was the largest impurity fraction. A study revealed contents < 1000 ppm up to 34 mass-% on increasing the sulfuric acid content from 70 to 85%. 

Nitric acid in the presence of catalytic amounts of mercury (II) nitrate reacts with some substrates, under certain conditions, to give substimted polynitrophenols. The first example, reported by Boeters and Wolffenstein and known as oxynitration , involved treating benzene with 50-55 % nitric acid in the presence of mercury (II) nitrate. The product is a mixture of unreacted benzene, nitrobenzene, m-dinitrobenzene, 2,4-dinitrophenol and picric acid, from which the latter can be isolated by steam distillation of this crude mixture followed by recrystallization of the residue from hot water. 

Although in part this order may be affected by subsequent reaction of the iodine with the organic solvent under the influence of catalytic material in the charcoal (Schmidt, Z it KoUoid. Ghem. XIV. 242, 1914) yet similar alterations have been noted by Freundlich (ibid. p. 260) with other solutes, e.g. benzoic and picric acids, who found the following order for decreasing adsorption for benzoic acid water, benzene, ethyl ether, acetone for picric acid, water, ethyl, alcohol, benzene. 

Amines, (a) Picrates The most versatile derivative from which the free base can be readily recovered is the picrate. This is very satisfactory for primary and secondary aliphatic amines and aromatic amines and is particularly so for heterocyclic bases. The amine, dissolved in water, alcohol or benzene, is treated with excess of a saturated solution of picric acid in water, alcohol or benzene, respectively, until separation of the picrate is complete. If separation does not occur, the solution is stirred vigorously and warmed for a few minutes, or diluted with a solvent in which the picrate is insoluble. Thus, a solution of the amine and picric acid in ethanol or benzene can be treated with benzene or light petroleum, repectively, to precipitate the picrate. Alternatively, the amine can be dissolved in alcohol and aqueous picric acid added. The picrate is filtered off, washed with water, ethanol or benzene, and recrystallised from boiling water, ethanol, methanol, aqueous ethanol or methanol, chloroform or benzene. The solubility of picric acid in water, ethanol and benzene is 1.4, 6.23 and 5.27% respectively at 20°. 

WOLFFENSTEIN-BOTERS REACTION. Simultaneous oxidation and nitration of aromatic compounds to nitrophcnols with nitric acid or the higher oxides of nitrogen in the presence of a mercury salt as catalyst. Hydroxynitration of benzene yields picric acid. 

Lead picrate is insoluble in water, ether, chloroform, benzene and toluene, and sparingly soluble in acetone and alcohol. It is prepared by precipitation with a solution of lead nitrate in a solution of sodium picrate and picric acid. 

Phenol is liable to undergo extensive oxidation during nitration so that carefully controlled conditions are required it forms 40% o- and 13% p-nitrophenolA solvent like chloroform or acetic acid is recommended. The nitration of p-cresol is carried out in benzene and acetic acid solution at 0°, the product being 3-nitro-4-hydroxytoluene (77%). The nitration of ra-cresol is discussed under method 491. Benzene is oxidized and nitrated (oxynitration) to 2,4-dinitrophenoI (72%) or to picric acid (2,4,6-trinitrophenol) by the action of mercuric nitrate in nitric acid. Aromatic alcohols like / -phenylethanol are nitrated as the esters to avoid oxidation products... 

If mercury salts are added, the oxidizing action of dilute nitric acid can be elegantly utilized for simultaneous introduction of hydroxyl and several nitro groups. This oxynitration , discovered by Wolffenstein and Boters,103 permits the one-step conversion of, e.g., benzene into picric acid in about 50% yield104 and of benzoic acid into 3-hydroxy-2,4,6-trinitrobenzoic acid, although in less good yield in the latter case105 (for the mechanism see West-heimer et a/.106). 

Ox3mitration. An interesting reaction occurs between benzene and approximately 50 per cent nitric acid containing 0.2 molar mercuric nitrate which yields up to 85 per cent dinitrophenol and picric acid. This process is known as oxynitration. It has been shown by Westheimer and coworkers that this reaction occurs in the following steps ... 

The benzene is initially converted to phenylmercuric nitrate which reacts with nitrogen dioxide to yield nitrosobenzene Each of these intermediates has been isolated from the reaction mixture. The nitrosobenzene can react in two wajrs. In nitric acid weaker than 50 per cent, it reacts with 2 moles of nitric oxide to form phenyldiazonium nitrate, a reaction first discovered by Bamberger. The diazonium salt is converted by water to phenol, which is nitrated in steps to the final products. In nitric acid of greater than 50 per cent concentration, the nitrosobenzene is converted directly to p-nitro-phenol without going through the diazonium compound. The p-nitro-phenol is then nitrated further to give the dinitrophenol and picric acid. 

In fact, one can go a step further by identifying the benzene extract phase as region 1 and the water raffinate phase as region 2 and working only with the concentrations of the two acids in each region. We then, in effect, have a binary system of benzoic acid and picric acid between two regions, 1 (benzene layer), and 2 (water layer), and one could proceed with the description in the manner of Section 1.1. Thus the description of a four-component separation system may he reduced to that of a two-component separation system provided each of the immiscihle phases is made up of essentially one species... 

It is wrong to assume that no energy is required in separation processes using a mass-separating agent. In the example of Figure 1.2.2, the extract phase containing the solute picric acid in benzene has to be subjected to an ESA process to separate benzene from picric acid. Only then is picric acid recovered in purer form, and the two initial species, water and picric acid, are separated. 

Procedure Ethoxynaphthalene (100 mg) and picric acid (130 mg) are thoroughly mixed in a micro test tube with a glass rod and the mixture is melted by cautious heating. After cooling, the melt is triturated with 0.5 ml of benzene (glass rod) and crystallized from it. Yield, 195 mg, mp, 103-104 °C. 

Picric acid is usually stored damp for safety, and acetone is therefore a better solvent than benzene or this test the solutions musi be almost saturated, however. 

If the amine is soluble in water, mix it with a slight excess (about 25 per cent.) of a saturated solution of picric acid in water (the solubility in cold water is about 1 per cent.). If the amine is insoluble in water, dissolve it by the addition of 2-3 drops of dilute hydrochloric acid (1 1) for each 2-3 ml. of water, then add a sUght excess of the reagent. If a heavy precipitate does not form immediately after the addition of the picric acid solution, allow the mixture to stand for some time and then shake vigorously. Filter off the precipitated picrate and recrystaUise it from boiling water, alcohol or dilute alcohol, boiUng 10 per cent, acetic acid, chloroform or, best, benzene. 

Picrates, Many aromatic hydrocarbons (and other classes of organic compounds) form molecular compounds with picric acid, for example, naphthalene picrate CioHg.CgH2(N02)30H. Some picrates, e.g., anthracene picrate, are so unstable as to be decomposed by many, particularly hydroxylic, solvents they therefore cannot be easily recrystaUised. Their preparation may be accomplished in such non-hydroxylic solvents as chloroform, benzene or ether. The picrates of hydrocarbons can be readily separated into their constituents by warming with dilute ammonia solution and filtering (if the hydrocarbon is a solid) through a moist filter paper. The filtrate contains the picric acid as the ammonium salt, and the hydrocarbon is left on the filter paper. 

Dissolve naphthalene in a little spirit, and add an equal quantity of a solution of picric acid in spirit. On cooling, yellow needles of naphthalene picrate separate, C,oHg.CQH20H(NO,)3. Benzene forms colourless crystals, anthracene, scarlet needles, having a similar composition. Sec Appendix, p. 295. 

In moderately acidic solutions bromocriptine mesilate readily forms ion pairs with anionic dyes such as picric acid, bromothymol blue, methyl orange, which are extractable with an organic solvent. A procedure has been developed both for direct assay and for assay following chromatographic separation from the impurities. Therein bromocriptine mesilate is allowed to react with bromothymol blue at pH 2.5. The resulting ion pair is then extracted with benzene and its concentration determined at 410 nm (25). ... 

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