A chemical precursor is a reactant that precedes your desired chemical reaction. It serves as the starting point for the chemical products derived from it.
The term precursor also specifically refers to a chemical compound that precedes a metabolic pathway in biochemistry, such as proinsulin, which is a protein precursor of the hormone insulin. Similarly, many coenzymes necessary to various metabolic pathways are synthesised from vitamins as precursors.
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Definition and Role of Precursors in Chemistry
In chemistry, a precursor is a substance from which other substances are either directly or indirectly derived. It could be a stand-alone chemical or a chemical that reacts with other chemicals to produce the final desired product. In many cases, intermediate byproducts are produced.
Chemical precursors are not the same as reagents or catalysts. Reagents are chemicals added to systems to start a reaction. They directly participate in the reaction, but the chemical systems are already preformulated or designed to receive the reagent, such as in the case of virus testing kits.

Catalysts, on the other hand, are used to either speed up or slow down chemical reactions, but they do not participate (chemically changed) in chemical reactions, such as in the case of the rare metals (platinum, palladium, and rhodium) inside a catalytic converter.
Key Takeaways
Precursors are chemicals that serve as the starting point of other chemicals derived from them
Precursors already contain the elements or functional groups necessary to produce the desired final chemical products
They can either be organic or inorganic, and the final desired products may either be more complex or simpler than the precursors
Precursors are used in many different industries, such as agriculture and pharmaceuticals
Safety considerations and regulatory compliance must be followed to minimise risk
Types of Chemical Precursors
Many economically important chemical products, such as medicines, fertilisers, and cosmetics, are typically mixtures of two or more chemicals.
Their active ingredients are produced using organic precursors, and in many cases, inorganic precursors are also used to produce organic products. This is particularly true for biological systems. For example, ammonia is a precursor of amino acids and nucleotides.
There is at least one active ingredient in any chemical product. The rest of the ingredients are intended for other purposes, such as extending the shelf life of the product or maintaining its form. A particular chemical product can be derived from more than one type of precursor.
It’s important for manufacturers to choose the most cost-effective and efficient precursors. For example, acetic acid, the active ingredient of vinegar, can be produced through the fermentation process. However, this is not a very effective method of mass-producing pure concentrations of acetic acid, so methanol is typically used as the precursor instead.
Organic Precursors
Organic precursors are those that contain carbon-hydrogen bonds that are usually in the form of straight, branched, or cyclic chains. They may also contain functional groups such as carboxyl and amino groups.
Organic precursors are commonly used to manufacture organic products. Taking our example above again, methanol is an alcohol used as a precursor to acetic acid. A notorious example of organic precursor is pseudoephedrine, which is a medicine for nasal congestion – and it’s also used in the manufacture of methamphetamine.
Inorganic Precursors
Inorganic precursors are chemicals that do not contain carbon-hydrogen bonds. They are used to produce either inorganic products or precursors of organic products.
For example, ammonia is an inorganic precursor of industrially produced inorganic fertilisers. Another example of inorganic precursor is fly ash, which is used to manufacture concrete and cement products.
Applications of Precursors in Various Industries
Precursors are used in manufacturing a wide range of consumer and industrial products.
Pharmaceuticals
Medicines are made in various ways, such as through the extraction of biochemicals from plants and animals. For example, the antimalarial drug quinine is extracted from the bark of the cinchona tree together with other alkaloids. Some types of medicines are derived from fungi such as antibiotics.
However, other medicines are derived from precursors. For example, sulfa drugs are derived from para-aminobenzenesulfonamide. These drugs kill bacteria by disrupting bacterial metabolic pathways.
Agriculture
Since humans started farming the land, we have been battling pests to protect our crops. For thousands of years, farmers relied on naturally-occurring pesticides such as sulphur and salts. Later mixtures of copper sulphate pentahydrate and lime were invented to combat fungus.
Modern organic pesticides require precursors for mass-production. Glyphosate, for example, is a common herbicide derived from glycine, formaldehyde, and phosphorous acid as precursors.

Electronics
Before the invention of silicon-based semiconductor electronics components, vacuum tubes were used in electronic appliances, such as radio receivers. Silicon-based semiconductors, such as diodes, ICs, and transistors, are the backbone of the electronics industry, and by extension of modern technological infrastructure – from communication to banking/financial systems.
Semiconductors, especially microchips, are layers of silicon and other metals. They are manufactured through chemical vapour deposition and atomic layer deposition processes.
Some of the main precursors used in this process are trimethylsilane (3MS), tetramethylsilane (4MS), and hexachlorodisilane (HCDS).
Textile
The fabric or textile industry is heavily dependent on inexpensive materials, which are mainly synthetic fibres. Sources of natural fibres like cotton and wool are still used, but they are relatively more costly. Synthetic fibres are more resistant to wear and tear, making them ideal for sportswear garments.
Nylon, rayon, and Spandex are some examples of synthetic fabrics derived from precursors. For example, one important precursor of nylon is cyclohexanone oxime. This precursor is used as a captive intermediate in synthesising a monomer caprolactam, which is then used to synthesise the polymer polycaprolactam (Nylon-6) fibres.
Construction
The construction industry heavily relies on building materials like cement and steel. The mass production of these materials require precursors such as oxides of calcium, silicon, and aluminium.
Other materials, such as protective coatings, also require precursors. For example, hexamethyldisiloxane is used as a precursor for the plasma-polymerized coatings on iron.

Safety and Regulatory Considerations for Precursors
Some chemical precursors are toxic, corrosive, flammable, or explosive. They can be dangerous to handle, having detrimental health and environmental hazards. Many of these types of precursors are unstable, needing special storage facilities with internal climate control.
Safety considerations and regulatory compliance are of utmost importance when handling, storing, transporting, or using some precursors.
Handling and Storage Guidelines
Just like handling other hazardous chemicals, there are proper protocols in place for dealing with hazardous precursors.
Highly reactive precursors should be stored separately, away from chemicals with which they might react. They must be properly labelled along with the safety data sheets (SDS).
When handling these precursors, it is crucial that the appropriate personal protective equipment is worn.
Compliance with UK and EU Regulations
Since Brexit, there have been some changes in the regulations about hazardous chemicals, although they are quite similar to EU rules.
Compliance with these regulations are necessary for companies to be authorised to operate. One relevant regulation is the Registration, Evaluation, Authorisation and restriction of Chemicals (REACH) which catalogues and limits potentially harmful chemicals that can have detrimental effects on health and in the environment.
Conclusion
Precursors are the foundational chemicals necessary to synthesise desired chemical products. They may be used as stand-alone chemicals that will undergo processing using catalysts, or they may react with other chemicals. Precursors have a wide range of applications in various industries. Proper safety considerations and regulatory compliance are necessary when handling precursors.