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High and low explosives; TNT preparation

Short answer

Low explosives deflagrate (burn rapidly) and are used as propellants (e.g., black powder). High explosives detonate with a shock wave, shattering effect, and need a detonator (e.g., TNT). TNT is prepared by stepwise nitration of toluene (from benzene + CH₃Cl/AlCl₃) with HNO₃/H₂SO₄; used in shells, bombs, mining.

What are high explosives and low explosives? Write the preparation and uses of TNT.

Low explosives

Low explosives burn rapidly (deflagrate) at a speed of a few metres per second to about 400 m/s, and the gas pressure pushes the material forward rather than shattering it. They are used as propellants and for slow blasting. Examples: black powder (gunpowder: KNO₃ + charcoal + S), smokeless powder, fireworks, safety fuse. Uses: firing bullets and shells, rocket propellant, quarrying soft rock, fireworks, safety fuses.

High explosives

High explosives detonate: the decomposition travels as a shock wave faster than about 1000 m/s (up to 9000 m/s) and has a strong shattering effect (brisance). They need a detonator to start them.

  • Primary (initiating) explosives are very sensitive and used in detonators: lead azide Pb(N₃)₂, mercury fulminate Hg(ONC)₂.
  • Secondary explosives are less sensitive and need a detonator: TNT, picric acid, RDX, PETN, dynamite, nitroglycerine (GTN). Uses: mining, blasting rocks, demolition, military shells and bombs.

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

Uses

  • Filling for shells, bombs, grenades, torpedoes and mines.
  • Mining, quarrying and demolition (alone or as amatol/ammonal mixtures).
  • Raw material for other explosives and dyes.

✓ Answer checked by Claude Opus 5.5 (high effort)

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