Nitroglycerin

Nitroglycerin—also called trinitroglycerol, nitro, glyceryl trinitrate, or 1,2,3-trinitroxypropane—is a thick, oily liquid that is either colorless or pale yellow. It is made by treating glycerol with white fuming nitric acid under conditions that form a nitric acid ester. Despite its name, it is chemically a nitrate ester, not a nitro compound. The substance was first created in 1846 by Ascanio Sobrero. It serves as a key ingredient in explosives like dynamite, used in construction, demolition, and mining. When combined with nitrocellulose, it forms double-based smokeless powder, a propellant for firearms and artillery since the 1880s.

Like many explosives, nitroglycerin becomes more likely to explode on its own as temperature rises. At sea-level pressure, heating it above 218 °C makes it extremely unstable and prone to detonation. In a vacuum, its autoignition temperature is 270 °C. It melts at 12.8 °C, so it is usually encountered as a thick, viscous fluid; when frozen, it turns into a crystalline solid. Pure nitroglycerin is colorless, but leftover nitric oxide impurities from production often give it a slight yellow tint.

Because it has a high boiling point and a very low vapor pressure (0.00026 mmHg at 20 °C), pure nitroglycerin has almost no smell at room temperature. It tastes sweet and burning if ingested. It can accidentally detonate if dropped, shaken, lit on fire, heated quickly, exposed to sunlight or ozone, subjected to sparks or electrical discharges, or handled roughly. This sensitivity has caused many devastating industrial accidents over the years. Adding desensitizing agents makes it less likely to explode. Clay (diatomaceous earth) is one such agent; mixing it with nitroglycerin produces dynamite, which is much safer to handle. Other desensitizing agents yield different dynamite formulations.

Since 1878, nitroglycerin has been used as a medication. It acts as a potent vasodilator, widening blood vessels to treat heart conditions like angina pectoris and chronic heart failure. It was known that these effects come from nitroglycerin being converted into nitric oxide, a strong vasodilator, but the enzyme responsible—mitochondrial aldehyde dehydrogenase (ALDH2)—was only identified in 2002. The drug is available as sublingual tablets, sprays, ointments, and patches.

**History**

Nitroglycerin was the first practical explosive stronger than black powder. Italian chemist Ascanio Sobrero synthesized it in 1846 while working under Théophile-Jules Pelouze at the University of Turin. Sobrero originally called it "pyroglycerin" and strongly warned against using it as an explosive.

Alfred Nobel turned nitroglycerin into a commercially useful explosive. He began experimenting with safer handling methods after his younger brother, Emil Oskar Nobel, and several factory workers died in an explosion at the Nobels' armaments factory in Heleneborg, Sweden, in 1864. The following year, Nobel founded Alfred Nobel and Company in Germany and built an isolated factory in the Krümmel hills near Hamburg. This business exported a liquid mixture of nitroglycerin and gunpowder called "Blasting Oil," but it was extremely unstable and hard to handle, leading to many disasters. The Krümmel factory buildings were destroyed twice.

In April 1866, several crates of nitroglycerin were shipped to California. Three were meant for the Central Pacific Railroad, which planned to use it as a blasting explosive to speed up construction of the 1,659-foot Summit Tunnel through the Sierra Nevada. One of the other crates exploded, destroying a Wells Fargo office in San Francisco and killing 15 people. This led to a complete ban on transporting liquid nitroglycerin in California. For the rest of the First transcontinental railroad’s construction, nitroglycerin had to be made on-site for hard-rock drilling and blasting.

On Christmas Day 1867, an attempt to dispose of nine canisters of Blasting Oil illegally stored at the White Swan Inn in Newcastle upon Tyne caused an explosion on the Town Moor that killed eight people. In June 1869, two one-ton wagons loaded with nitroglycerin—locally called Powder-Oil—exploded while passing through the village of Cwm-y-glo in North Wales. The blast killed six people, injured many, and caused extensive damage. Little trace of the two horses was found. The UK Government, alarmed by the damage and the risk to a city (the two tons were part of a larger shipment from Germany via Liverpool), soon passed the Nitro Glycerine Act 1869. Liquid nitroglycerin was widely banned elsewhere as well. These legal restrictions pushed Alfred Nobel and his company to develop dynamite in 1867, made by mixing nitroglycerin with diatomaceous earth (Kieselguhr) from the Krümmel hills. Other companies created similar mixtures—such as "dualine" (1867), "lithofracteur" (1869), and "gelignite" (1875)—by combining nitroglycerin with different inert absorbents, often trying to circumvent Nobel’s patents.