High and plastic explosives; nitrocellulose, TNT [partly illegible]

Reading: the question is "Define/Differentiate high explosive and plastic explosive with examples. Give the preparation of (a) nitrocellulose and (b) TNT."

High explosive

High explosives detonate: the decomposition travels through the material as a shock wave at more than about 1000 m/s (up to about 9000 m/s) and has a strong shattering effect (brisance).

• Primary (initiating) explosives are very sensitive and explode with a small spark or shock; they are used in detonators: lead azide Pb(N₃)₂, mercury fulminate Hg(ONC)₂.
• Secondary explosives are less sensitive and need a detonator to set them off: TNT, picric acid, RDX, PETN, dynamite, nitroglycerine (GTN). Uses: mining, blasting rocks, demolition, military shells and bombs.

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.

(a) Nitrocellulose (guncotton)

Preparation: cellulose (cotton) is nitrated with a mixture of conc. HNO₃ and conc. H₂SO₄ at about 30–40 °C for a few hours, then washed free of acid and dried.

[C₆H₇O₂(OH)₃]n + 3n HNO₃ --H₂SO₄--> [C₆H₇O₂(ONO₂)₃]n + 3n H₂O

Properties: white fibrous solid that looks like cotton; insoluble in water; burns rapidly and cleanly without ash; explodes when detonated or struck. The highly nitrated form (guncotton, ~13% N) is a high explosive; the less nitrated form (pyroxylin) is soluble in ether–alcohol. Uses: smokeless powder and cordite (with nitroglycerine), blasting gelatin, celluloid, lacquers and artificial leather, photographic film.

(b) 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.