Naphthalene, a polyaromatic hydrocarbon, is a crystalline solid known for its distinctive pungent smell. Its structure consists of two fused benzene rings, primarily derived from coal tar, although it is also obtained during the petroleum distillation process. Naphthalene is a versatile compound that has many applications in diverse industries such as agriculture, pharmaceuticals, dyes, resins, and textiles.
What is Naphthalene?
Naphthalene is a white crystalline hydrocarbon compound with the chemical formula C₁₀H₈. It is the simplest polycyclic aromatic hydrocarbon (PAH) composed of two fused benzene rings, resulting in its distinctive aromatic structure. Naphthalene is commonly found in coal tar and is a byproduct of petroleum refining. Naphthalene has a robust and distinct odor often described as mothball-like, a function of it commonly being used as an ingredient in mothballs and moth repellents. Its pungent smell makes it effective in repelling insects and protecting fabrics from damage caused by moth larvae.
Napthalene Chemical Structure – Source: Wikipedia
One of the earliest and most widespread uses of naphthalene has been as an insect repellent, especially for moths. The volatile nature of naphthalene makes it an ideal compound for mothballs, where it sublimes from a solid to a gas toxic to insects. However, due to environmental and health concerns, using naphthalene in mothballs has been increasingly replaced with other alternatives.
How is Naphthalene Produced?
Naphthalene is present in natural resources like crude oil and coal tar because it is formed through the decomposition of organic matter over time. When fossil fuels, or coal, undergoes geological processes such as heat and pressure, the organic compounds within them break down, forming naphthalene and other aromatic hydrocarbons. Historically, significant amounts of naphthalene were produced from heavy petroleum fractions during petroleum refining, but coal tar is the primary source of naphthalene production today. Coal tar is a thick, black liquid that is a byproduct of coal’s carbonization during coke production or coal gasification. Depending on the desired source material and specific requirements, Naphthalene can be produced through different processes. Here are some standard production processes for naphthalene:
Coal Tar Distillation
Coal tar, a byproduct of coal carbonization, is subjected to fractional distillation. This process involves heating the coal tar to separate it into different fractions based on boiling point. As the coal tar is heated, its components vaporize at different temperatures. Naphthalene, having a relatively low boiling point among the components, vaporizes at a specific temperature range of approximately 210 to 220 °C. It is collected as a distillate when the coal tar is heated, and the vapors are condensed. The condensed distillate contains naphthalene along with other volatile components.
The distillate containing naphthalene is further processed through filtration. This step helps remove any solid impurities or residues present in the distillate. The filtered distillate is subjected to crystallization. By cooling the distillate, naphthalene crystallizes as it becomes less soluble in the liquid at lower temperatures. Crystals of naphthalene start forming and can be visually observed. The crystallized naphthalene is separated from the liquid fraction and undergoes a drying process. This step aims to remove any remaining moisture or liquid residues, leaving behind pure naphthalene crystals.
Catalytic Reforming
Naphthalene can be produced by catalytic reforming certain hydrocarbon feedstocks, such as light naphtha. Light naphtha contains various hydrocarbons, including straight-chain and branched alkanes. The selected hydrocarbon feedstock, light naphtha, is introduced into a catalytic reforming reactor. Inside the reactor, the feedstock comes into contact with a catalyst, usually a solid material containing specific metal components, such as platinum or palladium, typically at high temperature and pressure. The reaction promotes rearrangement and conversion of the hydrocarbon molecules, forming naphthalene and other aromatic compounds. After the catalytic reactions, the product mixture is subjected to separation techniques such as fractional distillation or solvent extraction to isolate the naphthalene from other aromatic compounds and byproducts.
Petroleum Refining
Historically, naphthalene was produced from heavy petroleum fractions obtained as byproducts while refining crude oil. These fractions contained naphthalene along with other aromatic hydrocarbons. The heavy petroleum fractions undergo various refining techniques to separate naphthalene from the other aromatic hydrocarbons. Common separation methods include distillation, solvent extraction, or catalytic processes. These techniques exploit the differences in physical or chemical properties to isolate naphthalene selectively.
Industrial Uses of Naphthalene
Production of Phthalic Anhydride
Naphthalene is a key raw material in the production of phthalic anhydride. The production process of phthalic anhydride involves the oxidation of naphthalene with air or oxygen. This reaction forms phthalic anhydride, which is further processed and utilized in different applications. Phthalic anhydride is a valuable compound used primarily as a precursor to produce phthalate ester plasticizers. These plasticizers are extensively employed to enhance various plastics’ flexibility, durability, and workability, including polyvinyl chloride (PVC). Phthalic anhydride is also utilized in the production of alkyd resins, which are widely used in coatings, paints, and adhesives.
Moth Repellent and Deodorizer
Naphthalene is widely known for its use as a moth repellent. It is used in mothballs or flakes to protect clothing and other stored items from moth larvae. Additionally, naphthalene’s strong odor makes it useful as a deodorizer for closets, bathrooms, and other spaces with unwanted odors. Naphthalene derivatives, such as 2-naphthol, are used to produce insecticides and pesticides. These compounds possess insecticidal properties and are employed in agricultural and public health applications.
Dyes and pigments
Naphthalene derivatives are commonly employed in the production of dyes and pigments. They can impart color and improve the lightfastness of the final product. Naphthalene-based dyes and pigments offer several advantages in color intensity, brightness, and stability. The derivatives can be chemically modified to achieve various hues and shades. They are often used in the textile, printing, and coatings industries to provide vibrant and long-lasting colors to fabrics, papers, paints, and other materials.
Laboratory and research applications
Naphthalene is used in laboratories for various purposes, such as in chemical reactions, as a solvent, and as a reference standard in analytical techniques. Naphthalene can serve as a solvent in laboratory procedures. It can dissolve various organic compounds and is particularly useful for solubilizing nonpolar or hydrophobic substances.
Properties of Naphthalene
| Chemical formula | C10H8 |
| Molecular weight (g/mol) | 128.1705 |
| Odor | Strong, distinctive, and mothball-like |
| Appearance | White solid crystals/flakes |
| Melting point(°C) | 80.26 |
| Boiling Point (°C) | 218 |
| Density (g/cm³) | 1.14 |
| Refractive Index | 1.5898 |
| Vapor Pressure (Pa at 20 °C) | 8.64 |
| Solubility | Naphthalene is sparingly soluble in water but is highly soluble in organic solvents such as benzene, toluene, ether, and alcohol. |
| Stability | Naphthalene is resistant to most common chemical reactions at room temperature. It is not highly reactive or prone to decomposition under normal circumstances. However, when exposed to high temperatures or potent oxidizing agents, it can undergo reactions under specific conditions, such as combustion or oxidation reactions. |
Naphthalene Derivatives
Naphthalene Sulfonic Acids
Naphthalene sulfonic acids are derivatives of naphthalene obtained through sulfonation, which involves introducing a sulfonic acid group (-SO3H) onto the naphthalene ring. These derivatives have surfactant properties, meaning they can reduce the surface tension between liquids or between a liquid and a solid. This property makes them useful in various applications, including producing detergents, dispersants, and other specialty chemicals.
Naphthols
Naphthols are hydroxyl derivatives of naphthalene. They can be either 1-naphthol or 2-naphthol, depending on the position of the hydroxyl group. Naphthols find applications in producing dyes, pigments, antioxidants, and pharmaceutical intermediates.
Naphthalene Carboxylic Acids
Naphthalene carboxylic acids are derivatives obtained by introducing carboxyl (-COOH) groups onto the naphthalene ring. These derivatives have various applications in industries such as dyes, pharmaceuticals, and other organic compounds.
Naphthylamines
Naphthylamines are amine derivatives of naphthalene. They are organic compounds in which amino (-NH2) groups replace one or more hydrogen atoms of naphthalene. Naphthyl Amines have various applications, mainly as intermediates in synthesizing dyes, pigments, and pharmaceuticals.
Naphthalene Diisocyanates
Naphthalene diisocyanates are derivatives obtained by reacting naphthalene with isocyanates, typically in the presence of a catalyst. The resulting compound contains two isocyanate functional groups attached to the naphthalene ring. Naphthalene diisocyanates find significant applications in producing polyurethane foams, coatings, and adhesives.
Regulatory Considerations
| CAD – Chemical Agents Directive | CAD – Chemical Agents Directive, Article 2(b)(i) – Hazardous Agents |
| Construction Products Regulation | Construction Product Regulation – Annex I (3) – Hazardous Substances |
| EU Ecolabel Regulation | Ecolabel – Restrictions for Hazardous Substances/Mixtures |
| Food Contact Active and Intelligent Materials and Articles Regulation | Active and Intelligent Materials – CMR Substances not allowed for use |
Health Effects of Naphthalene
Direct contact with naphthalene can cause skin irritation, redness, and dermatitis. It can also lead to eye irritation, resulting in redness, tearing, and discomfort. Inhalation or ingestion of naphthalene can cause gastrointestinal symptoms, such as nausea, vomiting, abdominal cramps, and diarrhea. These symptoms may occur due to the irritant effects of naphthalene on the digestive system. Naphthalene exposure, especially at high concentrations, can affect the central nervous system. Neurological symptoms may include confusion, excitement, dizziness, headache, and, in severe cases, convulsions or seizures. Naphthalene has been associated with acute renal shutdown, particularly in significant exposure or ingestion cases. Renal shutdown can result in kidney dysfunction and impairment of renal function. Naphthalene exposure can affect the blood cells. In individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency, naphthalene can cause severe hemolysis (breakdown of red blood cells), leading to features such as icterus (yellowing of the skin and eyes) and severe anemia.
Identification Numbers
| CAS Number | 91-20-3 |
| Chemical Name | Naphthalene |
| RTECS No. | 214-552-7 |
Acceptable Limits or Maximum Usage
| OSHA PEL | The legal airborne permissible exposure limit (PEL) is 10 pm, averaged over an 8-hour work shift. |
| NIOSH PEL | The threshold limit value (TLV) is 2 pm averaged over an 8-hour work shift. |
| ACGIH TLV | The legal airborne permissible exposure limit (PEL) is10 pm, averaged over an 8-hour work shift. |
Fun Facts About Naphthalene
- Chemist John Kidd proposed “naphthaline” for the compound in 1821. The term was derived from “naphtha,” a general term that describes volatile and flammable liquid hydrocarbon mixtures, including coal tar.
- The renowned English scientist Michael Faraday determined the chemical formula of naphthalene, C10H8, in 1826. Faraday’s work contributed significantly to the understanding of naphthalene’s molecular composition.
- Naphthalene has the property of sublimation, which means it can transition directly from a solid state to a gas state without going through a liquid phase. This sublimation process occurs at room temperature and atmospheric pressure. The ability of naphthalene to sublimate quickly allows it to release its characteristic odor into the surrounding environment, even without direct contact. The sublimated gas molecules disperse in the air, carrying the distinctive mothball-like smell for which naphthalene is known. This dispersal is why naphthalene-based products, such as mothballs or air fresheners, can emit their odor without visibly melting or dissolving.
