Nitroglycerin

Overview
Nitroglycerin (NG), also known as nitroglycerine, trinitroglycerin, and glyceryl trinitrate, is a chemical compound. It is a heavy, colorless, 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, and as a plasticizer in some solid propellants. Nitroglycerin is also used medically as a vasodilator to treat heart conditions; it is a venous dilator.

History
Nitroglycerin was discovered by chemist 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 it was extremely dangerous as a result of its extreme instability, as shown in numerous "appalling catastrophes," such as the explosion that destroyed a Wells Fargo office in San Francisco in 1866. The liquid was widely banned, and this led to the development of dynamite (and similar mixtures such as dualine and lithofracteur), by mixing the nitroglycerine with inert absorbents (e.g., nitrocellulose gel], blasting gelatine) (Nobel used kieselguhr.)

Instability and desensitization
In its pure form, it is a contact explosive (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 this undiluted form it is one of the most powerful high explosives, comparable to the military explosives RDX and PETN (which are not used in munitions at full concentration because of their sensitivity) as well as the plastic explosive C-4.

Early in the history of this explosive it was discovered that liquid nitroglycerin can be "desensitized" by cooling to 5 to 10 °C (40 to 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 nitroglycerin is substantially more difficult to detonate, possibly rendering it useless as an explosive for practical application.

A serious problem in the use of nitroglycerin results from its high freezing point 13 °C (55 °F). Solid nitroglycerin is much less sensitive to shock than the liquid, a feature common in explosives; in the past it was often shipped in the frozen state, but this resulted in a high number of accidents during the thawing process by the end user just prior to use. This disadvantage is overcome by using mixtures of nitroglycerin with other polynitrates; for example, a mixture of nitroglycerin and ethylene glycol dinitrate freezes at -29 °C (-20 °F).

Detonation
Nitroglycerin and any or all of the dilutents used can certainly deflagrate or burn. However, the explosive power of nitroglycerin is derived from detonation: energy from the initial decomposition causes a pressure gradient that detonates the surrounding fuel. This can generate a self-sustained shock-wave that propagates through the fuel-rich medium at or above the speed of sound as a cascade of near-instantaneous pressure-induced decomposition of the fuel into gas. This is quite unlike deflagration, which depends solely upon available fuel, regardless of pressure or shock.

Manufacturing
The industrial manufacturing process often uses a nearly 50:50 mixture of sulfuric acid and nitric acid. This can be produced by mixing white fuming nitric acid (quite costly pure nitric acid in which oxides of nitrogen have been removed, as opposed to red fuming nitric acid) and concentrated sulfuric acid. More often, this mixture is attained by the cheaper method of mixing fuming sulfuric acid (sulfuric acid containing excess sulfur trioxide) and azeotropic nitric acid (consisting of around 70% nitric acid, the rest being water).

The sulfuric acid produces protonated nitric acid species, which are attacked by glycerin's nucleophilic oxygen atoms. The nitro group is thus added as an ester C-O-NO2 and water is produced. This is different from an aromatic nitration reaction in which nitronium ions are the active species in an electrophilic attack of the molecules ring system.

The addition of glycerin results in an exothermic reaction (i.e., heat is produced), as usual for mixed acid nitrations. However, if the mixture becomes too hot, it results in runaway, a state of accelerated nitration accompanied by the destructive oxidizing of organic materials of nitric acid and the release of very poisonous brown nitrogen dioxide gas at high risk of an explosion. Thus, the glycerin mixture is added slowly to the reaction vessel containing the mixed acid (not acid to glycerin). The nitrator is cooled with cold water or some other coolant mixture and maintained throughout the glycerin addition at about 22 °C, much below which the esterification occurs too slowly to be useful. The nitrator vessel, often constructed of iron or lead and generally stirred with compressed air, has an emergency trap door at its base, which hangs over a large pool of very cold water and into which the whole reaction mixture (called the charge) can be dumped to prevent an explosion, a process referred to as drowning. If the temperature of the charge exceeds about 10 °C (actual value varying by country) or brown fumes are seen in the nitrators vent, then it is immediately drowned.

Because of the great dangers associated with its production, most nitroglycerin production facilities are in offshore rigs or very remote locations.

Medical use
Nitroglycerin in medicine, where it is generally called glyceryl trinitrate, is used as a heart medication (under the trade names Nitrospan®, Nitrostat®, and Tridil®, amongst others). It is used as a medicine for angina pectoris (ischaemic heart disease) in tablets, ointment, solution for intravenous use, transdermal patches (Transderm Nitro®, Nitro-Dur®), or sprays administered sublingually (Nitrolingual Pump Spray®, Natispray®). The principal action of nitroglycerin is vasodilation&mdash;widening of the blood vessels. Nitroglycerin will dilate veins more than arteries, decreasing cardiac preload and leading to the following therapeutic effects during episodes of angina pectoris:
 * subsiding of chest pain
 * decrease of blood pressure
 * increase of heart rate.
 * orthostatic hypotension

These effects arise because nitroglycerin is converted to nitric oxide in the body (by a mechanism that is not completely understood), and nitric oxide is a natural vasodilator. Recently, it has also become popular in an off-label use at reduced (0.2%) concentration in ointment form as an effective treatment for anal fissure.

A recent medical development will include a small amount of nitroglycerin in the tip of a new Durex condom to stimulate erection during intercourse. "The CSD500 condom contains a chemical in its teat, called glyceryl trinitrate (GTN), which is absorbed by the skin and causes blood vessels to dilate." According to anecdotal evidence, Nitroglycerin patches have also found use as treatment for the bite of the Brown recluse spider, which has a vasoconstricting venom. However, research has suggested that nitroglycerin has negligible benefits and might even increase inflammation of the bite wound.

Infrequent exposure to high doses of nitroglycerin can cause severe headaches known as "NG head". These headaches can be severe enough to incapacitate some people; however, humans develop a tolerance and dependence to nitroglycerin after long-term exposure. Withdrawal can (rarely) be fatal; withdrawal symptoms include headaches and heart problems; with re-exposure to nitroglycerin, these symptoms may disappear. For workers in nitroglycerin manufacturing facilities, this can result in a "Monday Morning Headache" phenomenon for those who experience regular nitroglycerin exposure in the workplace; over the weekend they develop symptoms of withdrawal, which are then countered by reexposure on the next work day.