Nitroglycerin

Nitroglycerin is a dense, oily liquid that has played a significant role in both industrial explosives and modern medicine. Although traditionally regarded as a nitro compound, it is chemically classified as a nitrate ester formed by the nitration of glycerol. Since its discovery in the mid-nineteenth century, it has been central to developments in construction, mining, warfare, and cardiovascular treatment, giving the substance an unusual dual identity as both a powerful explosive and an essential vasodilatory drug.
Nitroglycerin appears colourless in its pure state, though trace nitrogen oxide impurities frequently give it a faint yellow tint. Possessing a high boiling point and extremely low vapour pressure, it is almost odourless but has a distinctively sweet and burning taste. The compound is highly sensitive to physical shock, heat, friction, or contamination, and this instability has historically been the cause of numerous industrial accidents. Its behaviour varies markedly with temperature: it freezes at about 13 °C, becoming a crystalline solid, and its sensitivity often increases during thawing. Despite these challenges, the development of desensitised forms such as dynamite enabled its widespread industrial application.

Chemical Characteristics and Physical Properties

Nitroglycerin is typically produced through the controlled nitration of glycerol using white fuming nitric acid, under strictly regulated conditions to form the nitric acid ester. As a nitrate ester, its explosive properties arise from rapid decomposition into gases such as nitrogen, carbon dioxide, and water vapour, releasing large volumes of energy in a very short time.
The compound becomes progressively more unstable with rising temperatures. Above approximately 21–22 °C at standard atmospheric pressure, it can become dangerously sensitive, whereas in a vacuum its auto-ignition temperature increases to around 270 °C. The process of freezing and thawing is particularly hazardous: while frozen nitroglycerin may appear less reactive to certain stimuli, it is prone to unpredictable detonation when crushed or broken.
To mitigate these risks, desensitising agents such as clay or diatomaceous earth are added, producing safer mixtures. This principle underpinned the creation of dynamite, which revolutionised the handling of the substance. Other additives, including ethanol, acetone, or dinitrotoluene, have been used to reduce sensitivity further, altering its stability for different applications.

Industrial and Commercial Uses

Nitroglycerin was the first powerful explosive to surpass black powder in practical utility. Its introduction into mining, tunnelling, railway construction, and civil engineering dramatically improved the efficiency and scale of nineteenth-century infrastructure projects. When combined with nitrocellulose, it forms part of double-based smokeless propellants used in artillery and firearms, a role it has filled since the 1880s.
Its extreme sensitivity initially imposed major limitations on transport and storage. Liquid nitroglycerin was notoriously dangerous to ship, and numerous accidents—including explosions in San Francisco, Newcastle upon Tyne, and Cwmyglo in Wales—led many governments to ban its transport in liquid form. The requirement for on-site manufacturing of the explosive during major engineering projects, such as the construction of the transcontinental railways, stemmed from these strict regulations.
The invention of dynamite by Alfred Nobel in 1867 marked a turning point. By absorbing nitroglycerin into diatomaceous earth, Nobel created a substance that retained explosive power while being far more stable. This desensitised form allowed for wider adoption and safer industrial practice. Subsequent innovations yielded a variety of formulations, including gelignite, which incorporates nitrocellulose to produce a gelatinised, more manageable explosive.

Historical Development

Nitroglycerin was first synthesised in 1846 by the Italian chemist Ascanio Sobrero at the University of Turin. Sobrero initially named the compound “pyroglycerine” and expressed strong concerns about its volatility, doubting its suitability for practical use. His reservations were justified, as early attempts to handle and transport liquid nitroglycerin led to multiple fatal incidents.
Alfred Nobel’s involvement began after an explosion at his family’s armaments factory in Sweden killed his brother and several workers. Recognising both the dangers and potential value of the substance, Nobel devoted his efforts to creating safer handling methods. His establishment of Dynamit Nobel AG and the construction of factories in isolated locations allowed more systematic production and experimentation. The early export of a nitroglycerin-gunpowder mixture known as “blasting oil” proved disastrous, prompting stricter regulatory measures around the world and accelerating the search for safer compositions.
In the United Kingdom, catastrophic explosions involving wagons carrying nitroglycerin triggered legislative action, most notably the Nitro-Glycerine Act of 1869. Similar restrictions elsewhere motivated widespread adoption of dynamite and related desensitised explosives. Throughout the late nineteenth century, numerous companies attempted to develop alternatives that circumvented Nobel’s patents, resulting in many new explosive blends based on the nitrate ester.

Wartime Significance

The two World Wars dramatically increased demand for nitroglycerin, particularly for use in cordite, a key propellant. Production facilities such as HM Factory Gretna in the United Kingdom, the Royal Navy Cordite Factory at Holton Heath, and purpose-built Canadian factories produced hundreds of tonnes of nitroglycerin each month. Large-scale manufacture required extensive safety protocols due to the compound’s inherently unstable nature.
Cordite, typically produced as a mixture of nitroglycerin, nitrocellulose, and stabilisers, became one of the most important military propellants of the twentieth century. Its reliability and energy output made it essential for artillery shells, naval guns, and small-arms ammunition, ensuring nitroglycerin’s pivotal place in military engineering.

Medical Applications and Pharmacological Role

Beyond its industrial uses, nitroglycerin has a long history in medicine as a vasodilator. The discovery that it alleviated chest pain led to experiments with diluted solutions in the 1870s. Physician William Murrell played a crucial role in demonstrating its therapeutic value for angina pectoris and hypertension. His findings were widely accepted after publication, and nitroglycerin became an early example of a scientifically validated cardiovascular treatment.
In the body, nitroglycerin acts by releasing nitric oxide, a potent signalling molecule that relaxes smooth muscle tissue in blood vessel walls, thereby reducing cardiac workload. The specific enzyme responsible for this conversion—mitochondrial aldehyde dehydrogenase (ALDH2)—was identified only in the early twenty-first century, expanding scientific understanding of the drug’s mechanism of action.
Modern formulations of medical nitroglycerin include sublingual tablets, oral sprays, transdermal patches, and topical ointments. These delivery methods allow rapid absorption and controlled dosing, supporting its continued use in treating angina and certain forms of heart failure. Despite sharing the same chemical identity as the industrial explosive, pharmaceutical preparations are highly diluted and stabilised, rendering them safe under normal medical supervision.

Stability, Degradation, and Desensitisation

The instability of nitroglycerin remains a defining feature of the compound. If manufacturing processes do not adequately remove impurities, the chemical can degrade over time, forming by-products that increase its sensitivity. This degradation has historically been linked to unexpected detonations during storage or transport.
Desensitisation strategies have been developed to improve safety. Absorbent materials, such as clay or diatomaceous earth, physically stabilise the liquid, while chemical additives reduce susceptibility to shock and heat. Freezing can temporarily phlegmatise nitroglycerin, though thawing introduces renewed hazards. Modern high explosives utilise chemical modifications and stabilisers that make handling substantially safer than in the nineteenth century.
Throughout its history, nitroglycerin has occupied a unique position at the intersection of chemistry, engineering, warfare, and medicine. Its influence spans from the acceleration of global industrial development through dynamite to the lifesaving therapeutic interventions of cardiovascular medicine. Despite the inherent challenges posed by its instability, the compound remains a substance of enduring scientific and practical significance.