Petroleum ether
Petroleum ether is a volatile, colorless liquid composed of a mixture of low-boiling aliphatic hydrocarbons, primarily pentane and hexane isomers, obtained through the fractional distillation of petroleum.[1][2] Despite its name, it is not a true ether but a historical misnomer due to its solvent properties resembling those of diethyl ether.[3] It typically has a boiling range of 40–60 °C, a density of approximately 0.65–0.67 g/mL, and is insoluble in water, making it a non-polar solvent ideal for laboratory and industrial applications.[1][4] The composition of petroleum ether can vary by fraction, generally including C5 to C7 hydrocarbons with minimal aromatic content, and it exhibits a gasoline- or kerosene-like odor.[1] It is highly flammable, with a flash point below -20 °C and vapor pressure of approximately 280 mmHg (20 °C), necessitating storage in cool, well-ventilated areas away from ignition sources.[1][5] Health risks include skin and eye irritation upon contact, respiratory effects from inhalation, and potential aspiration hazards if swallowed, classifying it as a hazardous substance under regulatory guidelines.[1] In organic chemistry, petroleum ether is extensively used for extractions of non-polar compounds such as lipids, fats, oils, and waxes from natural sources, as well as for recrystallizations and as a mobile phase in thin-layer and column chromatography.[2][6] Industrially, it finds applications in pharmaceutical processing for isolating active ingredients, in paint and varnish thinning, and in the purification of organic compounds via absorption chromatography.[7][8] Its low toxicity relative to other solvents and ability to dissolve a wide range of non-polar substances contribute to its continued prominence despite environmental concerns over petroleum-derived products.[2]Overview
Definition
Petroleum ether is a volatile, non-polar mixture of aliphatic hydrocarbons obtained through the fractional distillation of petroleum, with a typical boiling range of 35–60 °C.[9][1] It serves primarily as a laboratory solvent, valued for its ability to dissolve non-polar substances effectively.[10] Unlike true ethers, which contain an oxygen atom linking two alkyl or aryl groups, petroleum ether is entirely hydrocarbon-based and lacks any ether functional group.[9] The name "petroleum ether" derives from its origin in petroleum and its ease of evaporation, which resembles the volatile nature of diethyl ether, despite the chemical dissimilarity.[9] This historical nomenclature highlights its solvent-like behavior rather than its composition.[11]Nomenclature and Synonyms
Petroleum ether is known by several synonyms that reflect its composition as a light hydrocarbon fraction and its use as a solvent. Common alternative names include ligroin, benzine (distinct from benzene), light naphtha, and VM&P naphtha (varnish makers' and painters' naphtha), with ligroin specifically referring to fractions boiling in the 60–110°C range and often considered interchangeable with petroleum ether.[12][13] The term "petroleum ether" is a historical designation originating in the 19th century, adopted to highlight its volatile, ether-like properties akin to diethyl ether, despite lacking an ether functional group; informal abbreviations such as "pet ether" persist in laboratory contexts.[14][15] Regional variations in nomenclature further distinguish the substance: in the United Kingdom, it is commonly termed petroleum spirit, while in German-speaking areas, equivalents like Siedegrenzenbenzine or Benzin denote similar low-boiling petroleum distillates.[12]Composition and Production
Chemical Composition
Petroleum ether is a complex mixture of predominantly aliphatic hydrocarbons, consisting mainly of C5 to C7 alkanes such as pentanes, hexanes, and heptanes in straight-chain, branched, and cyclic forms.[16] The mixture is characterized by low aromatic content, typically less than 1% benzene, to minimize health risks associated with aromatic compounds.[12] The composition of petroleum ether varies based on the crude oil source and refining processes used in its production. Commercial grades may contain up to 5% n-hexane in standard formulations, though low-n-hexane variants are available with less than 5 wt% to reduce neurotoxicity concerns.[17] Additionally, many grades undergo hydrodesulfurization to eliminate sulfur compounds, resulting in sulfur levels below 0.5 wt%.[16] Refined petroleum ether lacks olefins, being composed exclusively of saturated hydrocarbons, which contributes to its stability as a solvent. The proportions of individual hydrocarbons can shift depending on the specific boiling point specification.Production Methods
Petroleum ether is primarily obtained through fractional distillation of crude oil in petroleum refineries. Crude oil is heated to approximately 350–400 °C in a furnace and introduced as vapor into a fractionating column operating at atmospheric pressure, where components separate based on boiling points. The light naphtha fraction, boiling in the range of 35–60 °C, is collected as the overhead product, consisting mainly of aliphatic hydrocarbons suitable for solvent use.[18][19] Following initial distillation, the light naphtha undergoes refining to meet purity standards for industrial and laboratory applications. Hydrodesulfurization removes sulfur compounds using a cobalt-molybdenum catalyst at 315–430 °C and 300–1000 psi hydrogen pressure, reducing sulfur content to below 0.1% by converting thiophenes and mercaptans to hydrogen sulfide. Hydrogenation then saturates aromatic hydrocarbons, minimizing their concentration to less than 1%, followed by redistillation to precisely narrow the boiling range and remove impurities. Alternative sources include byproducts from thermal or catalytic cracking processes and gasoline refining residues, which are similarly treated to isolate the desired fraction.[20] On an industrial scale, petroleum ether is produced as a side product in oil refineries worldwide, with global naphtha output of approximately 280 million metric tons annually as of 2025.[21] Recent innovations focus on sustainability, such as solvent extraction techniques applied to oil refinery sludge to recover light hydrocarbon fractions, reducing waste and environmental impact while supplementing traditional production.[18][22]Properties
Physical Properties
Petroleum ether is a colorless liquid with a characteristic gasoline-like or kerosene-like odor.[1] This appearance and smell arise from its composition as a volatile hydrocarbon mixture, making it suitable for applications requiring rapid evaporation.[8] The boiling point of petroleum ether varies depending on the specific grade and distillation range, typically falling between 35–60 °C at standard atmospheric pressure.[23] Denser grades may have slightly higher ranges, such as 40–60 °C or 42–62 °C, reflecting differences in the hydrocarbon chain lengths present.[24] Its density is approximately 0.63–0.66 g/cm³ at 20 °C, which contributes to its low viscosity and ease of handling in laboratory settings.[25] The refractive index is around 1.36–1.37 at 20 °C, a value consistent with its non-polar aliphatic nature.[26] Petroleum ether exhibits high volatility, with a vapor pressure of approximately 200–400 mmHg at 20 °C, enabling quick dispersion into the air.[27] Its flash point is below -20 °C, often as low as -30 °C to -40 °C, indicating extreme flammability under ambient conditions.[23] In terms of solubility, it is immiscible with water but fully miscible with most organic solvents, such as ethanol and chloroform, due to its hydrophobic character.[1] The evaporation rate of petroleum ether facilitates its use in extractions where rapid solvent removal is desired.[28]Chemical Properties
Petroleum ether, consisting primarily of saturated aliphatic hydrocarbons, exhibits low chemical reactivity under standard laboratory conditions, remaining inert toward most common reagents, including acids and bases.[29] It demonstrates chemical stability when stored properly, with no significant decomposition occurring at ambient temperatures and pressures.[30] However, it may react with strong oxidizing agents, such as nitric acid or chlorine, potentially leading to combustion or other exothermic reactions.[29] As a highly flammable substance, petroleum ether has an autoignition temperature of approximately 280–288 °C, igniting spontaneously above this threshold in the presence of air.[31] Upon combustion, it undergoes complete oxidation to produce carbon dioxide and water, following the general reaction for alkanes: \mathrm{C_nH_{2n+2} + \left(n + \frac{n+1}{2}\right) O_2 \rightarrow n CO_2 + (n+1) H_2O} This behavior underscores its role as a solvent where controlled flammability is managed through safety protocols.[1] High-purity grades of petroleum ether are characterized by low sulfur content (typically ≤0.02% as S) and minimal aromatic hydrocarbons (often <1%), which minimize the risk of unwanted side reactions, such as sulfur-induced corrosion or aromatic-initiated polymerization, in sensitive applications.[32] Its volatility facilitates efficient extractions in laboratory settings, as detailed in relevant usage sections.Standards and Specifications
Regulatory Standards
In the United States, the Occupational Safety and Health Administration (OSHA) regulates petroleum ether as a hazardous flammable liquid under 29 CFR 1910.106, which governs the handling, storage, and use of such substances with flash points below 200°F (93°C).[33] The Environmental Protection Agency (EPA) classifies petroleum ether under the Resource Conservation and Recovery Act (RCRA) as a characteristic hazardous waste if it exhibits ignitability, designated as waste code D001 for materials with a flash point less than 60°C.[34] For occupational exposure, OSHA sets a permissible exposure limit (PEL) for petroleum distillates, including naphtha-like fractions akin to petroleum ether, at 100 ppm (400 mg/m³) as an 8-hour time-weighted average (TWA).[35] Internationally, petroleum ether, as a mixture of hydrocarbons, falls under the European Union's REACH Regulation (EC) No. 1907/2006, requiring registration, evaluation, and authorization for substances manufactured or imported in quantities exceeding 1 tonne per year. REACH Annex XVII imposes restrictions on benzene content in hydrocarbon mixtures, exempting certain classifications as carcinogens if benzene levels are below 0.1% w/w; this limit applies to consumer products and articles to mitigate health risks.[36] Regulatory evolution for petroleum ether intensified in the 1980s amid growing evidence of n-hexane neurotoxicity, a key component in some grades, prompting shifts toward safer formulations. The American Conference of Governmental Industrial Hygienists (ACGIH) reduced the threshold limit value (TLV) for n-hexane from 100 ppm to 50 ppm in 1981, influencing OSHA's subsequent lowering of the PEL from 500 ppm to 50 ppm by 1989 to address peripheral neuropathy risks from chronic exposure. These changes spurred industry adoption of low-n-hexane petroleum ether grades, with n-hexane content limited to under 0.1% in many commercial solvents by the late 1980s to comply with updated occupational health standards.[37]Quality Specifications
Petroleum ether quality specifications ensure consistency in its use as a solvent, focusing on purity, volatility, and absence of contaminants that could affect performance or safety. These specifications are primarily governed by the American Chemical Society (ACS) reagent grade standards, which define limits for key physical and chemical properties to verify suitability for laboratory and industrial applications.[38] The boiling range is a critical parameter, typically specified as 35–60 °C for standard grades, with at least 90% of the material distilling within this interval and a tolerance of ±5 °C to account for minor variations in production. Residue on evaporation, measured after complete volatilization, must not exceed 0.001% (10 ppm), ensuring minimal non-volatile impurities that could leave contaminants in extractions or reactions; this is tested per ASTM D1353. Acidity is required to be neutral, passing tests such as ASTM D1093, with limits often set at ≤0.0003 meq/g to prevent interference in sensitive chemical processes.[38][39][40] Impurity limits are stringent to minimize health risks and ensure compatibility. Benzene content is capped at ≤2 ppm via gas chromatography (GC), as higher levels could pose carcinogenic hazards. Total aromatics are limited to <1% for industrial grades but often <0.02% in laboratory versions to avoid UV absorption issues. Sulfur compounds, measured as S, must be <5 ppm per ASTM D5453, reducing corrosion potential. Color is specified as ≤10 APHA (Platinum-Cobalt units, per ISO 6271) or equivalent to a minimum Saybolt +30, indicating clarity and absence of colored impurities via ASTM D1209 or D6045.[41][39][40][42]| Property | Specification Limit | Test Method |
|---|---|---|
| Boiling Range | 35–60 °C (≥90% vol.) | ASTM D86 |
| Residue on Evaporation | ≤0.001% | ASTM D1353 |
| Acidity | Neutral (≤0.0003 meq/g) | ASTM D1093 |
| Benzene | ≤2 ppm | GC (ASTM D6229) |
| Total Aromatics | <0.02% (lab); <1% (industrial) | GC or UV |
| Sulfur (as S) | <5 ppm | ASTM D5453 |
| Color | ≤10 APHA or +30 Saybolt min. | ISO 6271 / ASTM D1209 |