Picric acid reagent

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. 

Esbach s reagent (estimation of proteins) dissolve 10 g of picric acid and 20 g of citric acid in water and dilute to 1 liter. 

Hager s reagent (for alkaloids) this reagent is a saturated solution of picric acid in water. 

Uses Organic synthesis photographic agent manufacture of pesticides, herbicides, explosives, and wood preservatives yellow dyes preparation of picric acid and diaminophenol (photographic developer) indicator analytical reagent for potassium and ammonium ions insecticide. 

Uses Antiseptic and disinfectant pharmaceuticals dyes indicators slimicide phenolic resins epoxy resins (bisphenol-A) nylon-6 (caprolactum) 2,4-D solvent for refining lubricating oils preparation of adipic acid, salicylic acid, phenolphthalein, pentachlorophenol, acetophenetidin, picric acid, anisole, phenoxyacetic acid, phenyl benzoate, 2-phenolsulfonic acid, 4-phenolsulfonic acid, 2-nitrophenol, 4-nitrophenol, 2,4,6-tribromophenol, 4-bromophenol, 4-/ert-butylphenol, salicylaldehyde, and many other organic compounds germicidal paints laboratory reagent. 

Reactions. -Hydroxybenzoic acid undergoes the typical reactions of the carboxyl and hydroxyl moieties. When heated above its melting point, it decomposes almost completely into phenol and carbon dioxide. It reacts with electrophilic reagents in the predicted manner and does not undergo the Friedel-Crafts reaction. Nitration, halogenation, and sulfonation afford the 3-substituted products. Heating -hydroxybenzoic acid with 8 IV-nitric acid results in a 95% yield of picric acid. In a similar fashion, treatment with chlorine water yields 2,4,6-trichlorophenol (50). 

Problem 19.8 Outline reactions and reagents for industrial syntheses of the following from benzene, and naphthalene (NpH) and inorganic reagents (u) catechol, (h) resorcinol, (c) picric acid (2,4,6-trinitrophenol), (d) /3-naphthol ( -NpOH). <... 

H- 1,4-Thiazines and benzothiazines undergo oxidation to dehydro dimers (83) and (84) which are of considerable interest as they are the parent chromophores of the trichosiderin (trichochrome) pigments which occur in mammalian red hair and in the feathers of some birds (74T2781). In the case of monocyclic thiazines, reagents such as nitrobenzene or picric acid are required, but air oxidation is sufficient in the bicyclic series. It is curious that whereas aerial oxidation of the ester (85) gives both the dehydro dimers (86) and (87), ethyl azodicarboxylate only yields the tautomer (87) <70AC(R)35l). 

Major uses of phenol include production of phenolic resins, epoxy resins, and 2,4-D (regulated in many countries) as a selective solvent for refining lubricating oils in the manufacture of adipic acid, salicylic acid, phenophthalein, pentachlorophenol, acetophenetidine, picric acid germicidal paints, and pharmaceuticals as well as use as a laboratory reagent. Special uses include dyes and indicators, and slimicides. 

Nitro compounds, labelled with carbon-14, have been used during the polymerization of styrene (40). It has been shown that picric acid, which is rather an inefficient inhibitor, formally resembles p benzoquinone with this monomer. It becomes incorporated in the polymer but most of it is included in polystyrene of comparatively low molecular weight. Most of the combined picric add can subsequently be removed by treatment with trifluoro-acetic anhydride reagent, showing that the picric acid is not held in the polymer by carbon-carbon bonds. Further, it was shown, by isotope dilution analysis, that most of the picric add removed from the polymer by this treatment appeared as the add. 

Picrates are usually prepared by adding a hot solution of the compound in ethanol to a cold saturated ethanolic solution of picric acid, warming and allowing to cool the derivative separates in a crystalline condition. It is filtered off, washed with a little ether and pressed on a porous tile. If the picrate is stable, it is recrystallised from ethanol, ethyl acetate, benzene or ether. Do not mistake the recrystallised reagent (m.p. 122 °C) for a picrate. 

Most tropolones give sparingly soluble, yellow or orange sodium salts, green cupric chelates, and colored ferric complexes. Although easily acetyl-ated or methylated and frequently precipitated by picric acid, tropolones only exceptionally react with carbonyl reagents. Electrophilic substitution reactions occur readily however, sulfonation or nitration is inhibited... 

Colored reagents to follow the appearance or the disappearance of a functional group have been widely used to monitor reactions in classical organic chemistry, particularly in TLC analysis. This technique has been successfully adapted to SPS for example, ninhydrin (118), bromophenol blue (119), nitrophenyl isothiocyanate-O-trityl (120), picric acid (121), and malachite green isothiocyanate (122) have all been used to show the presence or the absence of free resin-bound amines. The presence of free resin-bound thiol groups can also be detected (123). 

Reagent. To a saturated solution of picric acid add sufficient sodium carbonate to make the solution strongly alkaline. 

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