Picrate

Identification of Aromatic Hydrocarbons. Picric acid combines with many aromatic hydrocarbons, giving addition products of definite m.p. Thus with naphthalene it gives yellow naphthalene picrate, C oHg,(N08)jCeHiOH, m.p. 152°, and with anthracene it gives red anthracene picrate, C 4Hio,(NOj)jCeHjOH, m.p. 138 . For practical details, see p. 394. 

Identification of Amines. Picric acid combines with many amines to give crystalline picrates, of general formula B,(NO )aCeHjOH, where B is a molecule of a monacidic base. These picrates have usually sharp melting- or decomposition-points, and serve to characterise the amines concerned. They may be formed either by (a) direct union of the acid and the base in a suitable solvent, or (6) by the interaction of sodium picrate and a salt of the amine in aqueous solution. 

If an ethanolic solution of picric acid is similarly added to one of aniline, no precipitation occurs, owing to the high solubility of aniline picrate in ethanol. If, however, a cold aqueous solution of aniline hydrochloride is added to a similar solution of sodium picrate and the mixture shaken, yellow crystals of aniline picrate, m.p. 165 , soon separate. 

Detection of Potassium in the presence of Sodium. Add a cold saturated aqueous solution of sodium picrate to a solution of potassium chloride. A rapid precipitation of the less soluble potassium picrate occurs, even from a i°o solution of potassium chloride. 

If cold saturated ethanolic solutions of the recrystallised tetrahydrocarbazole and of picric acid are mixed and stirred, the chocolate-brown picrate of the carbazole slowly crystallises. After it has been filtered off at the pump, washed with a small quantity of ethanol, and dried, it has m.p. 145-146°. 

Dissolve a small quantity of the methiodide in cold water and add it with shaking to an excess of cold saturated aqueous sodium picrate solution. A yellow dovible salt of quinaldine niethoi icrate ami sodium picrafe. 

I or examples of other similar methopicriitcs uniting tvith picric acid or sodium picrate to livc "double piciatcs" ot larioiis stiibilifics, see b. (i. M inii... 

A) Picrates. Attempt precipitation of picrates by (a) mixing cold ethanolic solutions of the amine and of picric acid (saturated) or... 

Note. PRIMARY ALIPHATIC AMINES. The lower amines are gases or low-boiling liquids (b.ps. CHjNH, 7 CiHjNH, 17 CH,(CH2,>,NH 49 (CHg)jCHNHa, 34 ) but may be encountered in aqueous or alcoholic solution, or as their crystalline salts. They are best identified as their benzoyl, or toluene-/>-sulphonyl derivatives (c/. (C) above), and as their picrates when these are not too soluble. This applies also to benzylamine, CjHsCHjNH, b.p. 185 also to ethylenediamine, usually encountered as the hydrate, NHj (CHj)j NH2,HjO, b.p. 116 , for which a moderate excess of the reagent should be used to obtain the di-acyl derivative. (M.ps., pp. 55 55 )... 

A) Picrates (except diphenylamine). (B) Acetyl Derivatives. (C) Benzoyl, Toluene-p sulphonyl and Benzenesulphonyl Derivatives. (D)... 

Note. Secondary amines, etc. in Class (ii) can be identified by their derivatives (C) and (D) above, and by their picrates when these can be readily isolated. The preparation of derivatives (D) requires the dry amine, whereas water does not aifect that of derivatives (C). 

A) Picrates. These are usually precipitated when ethanolic solutions... 

Form picrates that of naphthalene can be readily isolated. 

Picrates. The picrates of benzene and toluene are isolated only with extreme difficulty and are not used for purposes of identification. 

Make a concentrated solution of anthracene in hot acetone. To about 2 ml. of this solution add a cold concentrated acetone solution of picric acid drop by drop, and note the formation of a red coloration which becomes deeper on further addition of the acid. If excess of picric acid is added, however, the solution becomes paler in colour, and this is to be avoided if possible. Boil to ensure that both components are in solution and then transfer to a small porcelain basin or watch-glass ruby-red crystals of anthracene picrate separate out on cooling. The product, however, is often contaminated with an excess of either anthracene or of picric acid, which appear as yellowish crystals. 

B) Picrates. Very useful for ethers containing an aromatic group. 

Primary and Secondary Amines. Picrates (pp. 374, 376), Acetyl derivatives (pp. 373, 376), Benzoyl derivatives (pp. 374, 376), Toluene-p-sulphonyl and benzene-sulphonyl derivatives (pp. 374, 376), Phenylurea derivatives (pp. 374, 377)-... 

Tertiary Amines. Picrates (p. 378), Methiodides (p. 377), />-Nitroso-deriva-tives (p. 378). 

Ethers. 3,5-Dinitrobenzoates (p. 396), Picrates (with aryl ethers) (p. 397). 

M.p. B.p. Picrate M.p. Acetyl deiiv. M.p. Benzoyl deriv. H.p. Benzene sulph. Onyl deriv. M.p. Toluene p-sulph> onyl deny. M.p. Phenyl urea deriv, M.p. 

The conversion of the compounds under investigation into coloured derivatives (e.g., the separation of carbonyl compounds by conversion into their 2 4-dinitrophenylhydrazones, etc. of hydrocarbons through their picrates of alcohols through their 3 5-dinitrobenzoates of glucose, fructose and other simple sugars through their p-phenylazobenzoyl esters). 

S-Alkyl-iso-thiuronium picrates. Alkyl bromides or iodides react with thiourea in alcoholic solution to produce S-alkyl-iso-thiuronium salts, which yield picrates of sharp melting point ... 

Place a mixture of 0-5 g. of finely powdered thiourea, 0-5 g. of the alkyl halide and 5 ml. of alcohol in a test-tube or small flask equipped with a reflux condenser. Reflux the mixture for a j)eriod depending upon the nature of the halide primary alkyl bromides and iodides, 10-20 minutes (according to the molecular weight) secondary alkyl bromides or iodides, 2-3 hours alkyl chlorides, 3-5 hours polymethy lene dibromides or di-iodides, 20-50 minutes. Then add 0 5 g. of picric acid, boil until a clear solution is obtained, and cool. If no precipitate is obtained, add a few drops of water. RecrystaUise the resulting S-alkyl-iso-thiuronium picrate from alcohol. 

Picrates of p-naphthyl alkyl ethers. Alkyl halides react with the sodium or potassium derivative of p-naphthol in alcoholic solution to yield the corresponding alkyl p-naphthyl ethers (which are usually low m.p. solids) and the latter are converted by alcoholic picric acid into the crystalline picrates ... 

---