Nitroglycerin
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Nitroglycerin (also nitroglycerine, trinitroglycerin, or glyceryl trinitrate) is a chemical compound, a heavy, colorless, poisonous, oily, explosive liquid obtained by nitrating glycerol. It is used in the manufacture of explosives, specifically dynamite, and as such is employed in the construction and demolition industries. It is also used medically as a vasodilator to treat heart conditions. It is colored yellow when it is decomposing due to acidic pH.
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Instability and desensitization
In its pure form, it is shock-sensitive (i.e., physical shock can cause it to explode) and degrades over time to even more unstable forms. This makes it highly dangerous to transport or use in its pure form.
Early in the history of this explosive it was discovered that liquid nitroglycerin can be "desensitized" by cooling to 40–50 °F, at which temperature it freezes, contracting upon solidification. However, later thawing can be extremely sensitizing, especially if impurities are present or if warming is too rapid. It is possible to chemically "desensitize" nitroglycerin to a point where it can be considered approximately as "safe" as modern High Explosive formulations, by the addition of approximately 10%–30% ethanol, acetone, or dinitrotoluene (percentage varies with the desensitizing agent used). Desensitization requires extra effort to reconstitute the "pure" product. Failing this, it must be assumed that desensitized nitroglycerine is substantially more difficult to detonate, possibly rendering it useless as an explosive for practical application.
What is detonation?
Nitroglycerin and any or all of the diluents mentioned above can certainly deflagrate, or burn. However, the explosive power of nitroglycerin is derived from detonation: a shock propagates through the fuel-rich medium at a supersonic speed. In other words, the initial burn sets up a pressure gradient that pre-ignites unshocked material, creating a fast-moving transition zone, which (due to the nature of the material) can detonate any appropriate material it encounters. This generates a self-sustained cascade of hyper-instantaneous pressure-induced combustion that grows upon itself exponentially. This is quite unlike deflagration, which depends solely upon available fuel, regardless of pressure or shock.
An explosion is essentially a very fast combustion, and combustion requires fuel and an oxidant. Nitroglycerin, as can be seen from its composition and structure (below), essentially contains both of these components. If it is detonated under pressure, it explodes to form thousands of times its original volume in hot gas.
One of these gases is nitrogen gas. It is very stable so its production is highly exothermic, which is why it is a main constituent of most explosives.
Properties
formula: CH2(ONO2)-CH(ONO2)-CH2(ONO2)
IUPAC Name: 1,2,3-Tris-nitro-oxy-propane
colour: yellow but colourless when pure
aspect: slightly oily liquid
density: 1.13 at 15 °C
melting point: 13.2 °C
molecular weight 227.0872
very sensitive to friction, shock, elevation of temperature, and sparks
Preparation
Nitroglycerin is prepared by nitration of glycerin (glycerol). In the process, glycerin is slowly tipped into a mix of concentrated nitric and sulfuric acids. The solution is slowly mixed. The temperature should never exceed 30 °C, otherwise there is a risk of explosion.
When the reaction is over, the mix is poured into a large amount of water. The nitroglycerin settles and is washed with water and sodium carbonate until it becomes neutral.
Manufacture
The industrial manufacturing process uses a 50:50 mixture of fuming sulphuric acid (fuming means it is very concentrated) and red fuming nitric acid. This produces nitronium ions in situ, which attack glycerin at its negatively charged oxygen atoms. The functional group NO2 is thus added, adding extra oxygen atoms to the flammable substance glycerin.
The use of strong acids almost always results in an exothermic reaction (i.e., heat is produced), and this reaction is no exception. However, if the mixture becomes too hot, it explodes. Thus, the acid mixture is added slowly to the reaction vessel containing the glycerin. The reaction vessel itself is cooled with ice-cold water or some other coolant mixture at about 0 °C. The vessel itself has an emergency trap door at its bottom, which hangs over a large pool of very cold water. If sensors in the mixture detect the temperature rising too rapidly, then the whole mixture can be dumped into the ice-cold water, which prevents an explosion if done in time.
This chemical reaction is extremely risky to attempt except by trained professionals in specially equipped laboratories.
Medical use
In medicine, nitroglycerin (sometimes called Glyceryl trinitrate) is used as a heart medication (under the trade names Nitrospan and Nitrostat). It is used as a medicine for angina pectoris (ischaemic heart disease) in tablets, ointment, or solution for intravenous use.
The principal action of nitroglycerin is vasodilation, that is, widening of the blood vessels. The main effects of nitroglycerin in episodes of angina pectoris are
- chest pain subsides
- blood pressure decreases
- heart rate increases
These effects come about because nitroglycerin is converted to nitric oxide in the body (by a mechanism that is not completely understood), and nitric oxide in turn is a well-known natural vasodilator.
History
Nitroglycerin was discovered by Ascanio Sobrero in 1847, working under TJ Pelouze at the University of Torino. The best manufacturing process was developed by Alfred Nobel in the 1860s. His company exported a liquid combination of nitroglycerin and gunpowder as 'Swedish Blasting Oil', but the extreme danger of using the liquid, as shown in a number of "appalling catastrophes", led to the liquid being widely banned and the development of dynamite (and similar mixtures such as dualine and lithofracteur), mixing the nitroglycerine with inert (Nobel used kieselguhr) or combustible absorbents (e.g. nitrocellulose to produce the yellow gel, blasting gelatine).
External link
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