Plastic explosive; TNT preparation; detonator

Plastic explosive

Plastic explosives are high explosives (such as RDX or PETN) mixed with a plasticiser (oil, wax, polymer) to form a putty-like mass that can be moulded by hand into any shape. They are safe to handle, stable, waterproof, and need a detonator. Examples: C-4 (about 91% RDX), Semtex, PE-4.

Preparation of TNT (2,4,6-trinitrotoluene)

TNT is made by the stepwise nitration of toluene with a mixture of conc. HNO₃ and conc. H₂SO₄ (H₂SO₄ is a dehydrating agent, giving NO₂⁺). Toluene itself is obtained from benzene by Friedel–Crafts methylation (benzene + CH₃Cl, anhydrous AlCl₃).

C₆H₆ + CH₃Cl --AlCl₃--> C₆H₅CH₃ + HCl            (toluene)
C₆H₅CH₃ --HNO₃/H₂SO₄, 30°C-->  mononitrotoluene
        --HNO₃/H₂SO₄, 60°C-->  dinitrotoluene
        --HNO₃/oleum, 100-110°C--> 2,4,6-trinitrotoluene

Structure of TNT (ring carbons C1 to C6)
           CH₃
            |
       .----C1---.
 O₂N--C2         C6--NO₂
       |         |
       C3        C5
       '---C4----'
            |
           NO₂

The product is washed with water and dilute sodium carbonate and recrystallised from alcohol. It is a pale yellow crystalline solid, m.p. 81 °C. Decomposition: 2C₇H₅N₃O₆ → 3N₂ + 5H₂O + 7CO + 7C

Need for a detonator

TNT is very stable and insensitive to heat, shock and friction. If lit, it burns quietly with a sooty flame and does not explode. The activation energy for its explosive decomposition is high, so a strong shock wave is needed to start it. A detonator (containing a sensitive primary explosive such as lead azide or mercury fulminate) gives this initial shock, which starts the detonation of the whole mass.