Picric acid (2,
4, 6-trinitrophenol)
(1) Preparation : It is obtained when phenol is treated with conc. . However, the yield is very poor. It
is prepared on an industrial scale :
(i) From chlorobenzene
(ii) From phenol through disulphonic acid
(2) Properties : It is a yellow crystalline solid, melting points 122°C. it is insoluble in cold water but soluble in hot water and in ether. It is bitter in taste. Due to the presence of three electronegative nitro groups, it is a stronger acid than phenol and its properties are
comparable to the carboxylic acid. It neutralises alkalies and decomposes carbonates with evolution of carbon dioxide. Dry picric acid as well as its potassium or ammonium salts explode violently when detonated. It reacts with PCI 5 to form picryl chloride which on shaking with NH3 yields picramide.
When distilled with a paste of bleaching powder, it gets decomposed and yields chloropicrin, CCI3NO2, as one of the products and is thus employed for the manufacture of tear gas. It forms yellow, orange or red coloured molecular compounds called picrates with aromatic hydrocarbons, amines and phenols which are used for characterisation of these compounds. Picrates are explosive in nature and explode violently when heated. These are prepared carefully. Uses : It is used as a yellow dye for silk and wool, as an explosive and as an antiseptic in treatment of burns.
Synthesis of picric acid Picric acid, or 2,4,6-trinitrophenol, is both a useful explosive compound and a precursor to other explosives. It does not see much use now a days as a laboratory reagent or explosive. It is too dangerous for the lab and too costly for the battlefield. I found a very old bottle in the lab once, the bottle had not been used for 5 years and should not have been there. It is stable unless old, it will form a very unstable crystalline structure after many years. I have three different methods of synthesizing picric acid. The first is a laboratory procedure using fairly common chemical reagents, it will provide a better and cheaper product. The second is a lesser used, but economical, catalytic process for possible large scale manufacture, its chemicals are more difficult to get. The third is one of those improvisational procedures using aspirin tablets, it sounds feasible. Picric acid was first mentioned in the alchemical writings of Glauber in 1742. The old timers made it from nitrating animal horn, silk, indigo, natural resin, and the like. Its synthesis from phenol, and the correct determination of its formula, was in 1841. It was not until 1830 did anybody think to use picric acid as an explosive. Before then it was assumed that only the salts of picric acid were explosive, not the acid itself. In 1873 H. Sprengel proved it could be detonated and by 1894 the Russians had worked out a method of manufacture
for artillery shells. Soon after, every military power used picric acid as their primary high explosive material. The 20th century saw the decline of picric acid, the replacement being TNT. The downfall was partially due to the acidic nature and the sensitivity of picric acid. Today picric acid is more suited to detonators or booster charges.
Picric acid
IUPAC
name[hide]
2,4,6-Trinitrophenol
Other names[hide]
Carbazotic Acid; phenol trinitrate; picronitric acid; trinitrophenol; 2,4,6trinitro-1-phenol; 2-hydroxy-1,3,5-trinitrobenzene; TNP; Melinite
Properties
Molecular formula
C6H3 N3O7
Molar mass
229.10 g·mol
Appearance
Colorless to yellow solid
í1
í3
Density
1.763 g·cm , solid
Melting point
122.5 °C
Boiling point
> 300 °C (Explodes)
í1
Solubility in water
14.0 g·L
Acidity (p K a)
0.38 Hazards
EU classification
Explosive (E), Toxic (T)
R-phrases
R1 R4 R11 R23 R24 R25
S-phrases
S28 S35 S37 S45
