Picric acid with aromatic amines

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, dissolv in water or alcohol, is treated with excess of a saturated solution of picric acid in water or alcohol, 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 can be treated with petroleum ether 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 or ethanol and recrystallised from boiling water, ethanol, methanol, aqueous ethanol, methanol or chloroform. The solubility of picric acid in water and ethanol is 1.4 and 6.23 % respectively at 20°. 

Examples of such charge-transfer complexes include the complex formed between a metal ion and a -n orbital of a double bond or an aromatic system , the complex formed between polynitro aromatics (such as picric acid) and other w-orbital-containing molecules, and complexes of I2 and Br2 with amines, ketones, aromatics, etc ° Phenols and quinones also form charge-transfer complexes ... 

Aromatic hydrocarbons can be purified as their picrates using the procedures described for amines. Instead of picric acid, 1,3,5-trinitrobenzene or 2,4,7-trinitrofluorenone can also be used. In all these cases, following recrystallisation, the hydrocarbon can be isolated either as described for amines or by passing a solution of the adduct through an activated alumina column and eluting with toluene or petroleum ether. The picric acid and nitro compounds are more strongly adsorbed on the column. 

Picric acid, in common with several other polynitrophenols, is an explosive material in its own right and is usually stored as a water-wet paste. Several dust explosions of dry material have been reported [1]. It forms salts with many metals, some of which (lead, mercury, copper or zinc) are rather sensitive to heat, friction or impact. The salts with ammonia and amines, and the molecular complexes with aromatic hydrocarbons, etc. are, in general, not so sensitive [2], Contact of picric acid with concrete floors may form the friction-sensitive calcium salt [3], Contact of molten picric acid with metallic zinc or lead forms the metal picrates which can detonate the acid. Picrates of lead, iron, zinc, nickel, copper, etc. should be considered dangerously sensitive. Dry picric acid has little effect on these metals at ambient temperature. Picric acid of sufficient purity is of the same order of stability as TNT, and is not considered unduly hazardous in regard to sensitivity [4], Details of handling and disposal procedures have been collected and summarised [5],... 

We have mentioned above the prevalence of chromoisomeric effects in two-component systems forming solid charge-transfer complexes. This was studied first by Hertel (120) and labeled by him complex isomerism. In a system such as picric acid with an aromatic amine, there are a variety of structural possibilities. There will probably be intermolecular hydrogen bonds, which are associated with short lateral contacts between the near-planar molecules. In addition, there... 

The presence of three nitro groups on the aromatic ring of picryl chloride makes the chloro group extremely reactive towards nucleophiles. Picryl chloride (87) is hydrolyzed to picric acid (4) in the presence of hot water or aqueous sodium hydroxide. Aminolysis of picryl chloride in the presence of primary and secondary amines is complete in minutes at room temperature. Picryl chloride is therefore a very useful starting material for the synthesis of a range of other picryl derivatives. The reaction of picryl chloride (87) with ammonia can be used to synthesize 2,4,6-trinitroaniline (53) (picramide). Treatment of picryl chloride with alcohols under reflux forms picric acid and the alkyl chloride of the corresponding alcohol, whereas the same reaction in the presence of alkali metal hydroxides, or the alkoxide anion of... 

The largest group of organic molecular compounds, in which hydrogen bond formation plays no part, are the compounds, usually in the ratio 1 1, between on the one hand aliphatic and aromatic nitro compounds (nitromethane, tetranitro-methane, chloropicrin CC13N02, nitrobenzene, s-trinitrobenzene, picric acid), quinones, anhydrides (phthalic acid-and maleic acid anhydride) and ketones with on the other hand especially aliphatic and aromatic amines (aniline, pyridine), unsaturated aliphatic and aromatic hydrocarbons, ethers etc. 

A nitrophenol, such as picric acid, forms strongly coloured complexes, of the type discussed here, with the weaker basic aromatic amines (bathochromic shift p. 241). 

Negative substituents enhance the acidic properties of phenols, an effect opposite to that produced with aromatic amines. o and p-Chloro-phenols are considerably stronger acids than phenol itself, and o- and p-nitrophenols are still stronger. Trinitrophenol, picric acid, is a strong acid whose salts are neutral and not decomposed by carbonic acid or by ammonium salts. These salts of picric acid can be salted out of neutral solutions by sodium or potassium chloride. With negatively substituted phenols, it may be possible to separate the phenolate from solutions which are neutral or weakly alkaline to litmus. In doubtful cases, just as with the amines, the precipitated material must be studied to determine whether it is the free phenol or one of its salts. The color of the precipitate gives an indication in the case of the nitrophenols, since the free phenols have only a weak yellow color, whereas the alkali salts are deep yellow. Solubility tests with indififerent solvents may be used in the case of uncolored compounds. Only the free phenol can be separated from acidic solutions. 

As pointed out in Vol. 1, irinitrobenzene and its derivatives possess the ability to form addition compounds, especially with aromatic hydrocarbons composed of condensed rings, such as naphthalene, anthracene, phenanthrene, acenaphthene etc. Addition compounds of picric acid have been known since 18S8 [82] and those of sym-trinitrobenzene since 1882 [83]. Complexes of sym-trinitro-benzene with aromatic amines were also described in 1882 [84]. 

Inhibitors are compounds, which inhibit the polymerization process during the initiation and growth of the chain. Typical inhibitors include hydroquinone, pyrocatechol and its derivatives, aromatic p-phenylenedi-amine amines, N-phenyl-2-naphthylamine, and also trinitrobenzene, picric acid, copper salts, and others. The reaction of free radical with a monomer molecule depends on the reactivity of double bonds of Ihe latter. 

The reaction of acyl chlorides with ammonia, primary, or secondary amines is probably the most generally applicable method of preparing amides in the laboratory. A large variey of acid chlorides are available commercially, others are readily prepared from the acids. Not only halides of carboxylic acids but also those of sulfonic and phosphonic acids and of picric acid may be converted to amides. Both aliphatic and aromatic amines may be subjected to acylation with acid halides. The reactions of aliphatic acid chlorides have been extensively reviewed [8]. 

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