Explosive characteristics; GTN and TNT; applications

Characteristics of an explosive

Important characteristics of a good explosive:

1. Stable during storage, without decomposing over time.
2. Safe to handle: not too sensitive to shock, friction or heat.
3. Easily exploded by a detonator or proper initiation.
4. High velocity of detonation and brisance (shattering power).
5. Large volume of gas released per unit mass, with high heat of explosion.
6. Gives little smoke and no poisonous fumes (for use in mines).
7. Cheap, easy to make from available raw materials, and non-hygroscopic.

Preparation of glycerol trinitrate (GTN, nitroglycerine, TNG)

GTN is made by nitrating glycerol with a mixture of conc. HNO₃ and conc. H₂SO₄ (the sulfuric acid absorbs the water formed). The temperature is kept below 10–15 °C by cooling, as higher temperatures can cause explosion.

CH₂OH                      CH₂ONO₂
|                          |
CHOH  + 3HNO₃ --H₂SO₄-->   CHONO₂  + 3H₂O
|                          |
CH₂OH                      CH₂ONO₂

The oily layer of GTN is separated, washed with water and sodium carbonate solution and dried. It is a colourless to pale yellow oily liquid, very sensitive to shock, and explodes violently: 4C₃H₅(ONO₂)₃ → 12CO₂ + 10H₂O + 6N₂ + O₂

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

Industrial applications of explosives

Industrial applications: mining and quarrying, tunnelling, road, dam and canal construction, demolition of old structures, oil-well perforation, metal forming/welding by explosion, and making fireworks and safety fuses.