Low and high explosives; GTN preparation

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 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

Uses

• Making dynamite (absorbed in kieselguhr), blasting gelatin and gelignite.
• Component of smokeless propellants such as cordite.
• Blasting in mining and tunnelling.
• Dilute form is a medicine (vasodilator) for angina.