246Trinitrophenol

2,4,6-Trinitrophenol, commonly known as picric acid, is an organic compound with the formula C6H3N3O7. It consists of a phenol ring bearing hydroxyl at position 1 and nitro groups at positions 2, 4, and 6. Picric acid is a yellow crystalline solid that can form a variety of picrate salts with metals.

Key properties include a melting point around 122–123°C and partial solubility in organic solvents such as acetone, ethanol, and ether; it is sparingly soluble in water. It is a relatively strong organic acid, with a pKa around 0.3, reflecting the electron-withdrawing effect of the three nitro groups. Its chemistry includes the ability to form stable picrate salts, which can be highly energetic.

Preparation of picric acid is traditionally achieved by the nitration of phenol using a mixture of nitric and sulfuric acids, followed by workup and purification to yield the free acid. The reaction favors substitution at the ortho and para positions, giving the 2,4,6-trinitro derivative.

Historically, picric acid has been significant as an explosive. It was used in early high-explosive formulations (for example, Lyddite shells) before being largely supplanted by more stable or safer materials. Beyond explosives, picric acid has been employed as a dye intermediate and mordant, and it serves as a starting material for various derivatives and picrate salts used in analytical chemistry and pyrotechnics. In modern contexts, its use is largely limited to specialized applications, due to safety considerations.

Safety and handling notes: picric acid is hazardous and, when dry, can be shock-sensitive and explosive. It forms reactive salts and can pose risks of detonation under friction, impact, or high temperature. Proper containment, storage, and disposal procedures are essential in any handling context.