Styrene
Styrene is an organic compound with the chemical formula C₈H₈ and the structural formula CH₂=CHC₆H₅, commonly known as vinylbenzene or ethenylbenzene.[1] It appears as a clear, colorless to yellowish oily liquid with a sweet, floral odor, has a molecular weight of 104.15 g/mol, a density of 0.906 g/cm³ at 25°C, and boils at 145.2°C.[2] Styrene is flammable, volatile, and highly reactive due to its vinyl group attached to a benzene ring, making it a key monomer in polymer chemistry.[3] The majority of styrene—approximately 90%—is produced industrially through the catalytic dehydrogenation of ethylbenzene, where ethylbenzene (C₆H₅CH₂CH₃) is heated with superheated steam at 550-650°C over iron oxide catalysts, yielding styrene and hydrogen in an endothermic, reversible reaction.[4] An alternative route is the oxidation of ethylbenzene in the propylene oxide/styrene monomer (PO/SM) process, which accounts for about 10% of production but is less common overall due to complexity.[5] Global production was approximately 29 million metric tons as of 2024, primarily from petroleum-derived feedstocks, supporting its role as a foundational chemical in the petrochemical industry.[6] Styrene's primary application is as a monomer for polymerization, with around 60% used to produce polystyrene (PS), a versatile thermoplastic found in packaging, insulation, disposable cups, and consumer goods due to its lightweight, rigid, and insulating properties.[7] It is also copolymerized to form materials like acrylonitrile butadiene styrene (ABS) for automotive parts and electronics, styrene-butadiene rubber (SBR) for tires, and unsaturated polyester resins for composites in construction and boating.[8] These applications leverage styrene's ability to form durable, impact-resistant polymers essential to modern manufacturing.[9] Exposure to styrene poses health risks, primarily affecting the central nervous system; acute inhalation causes irritation to the eyes, nose, and throat, headaches, dizziness, and fatigue, while chronic exposure may lead to neurological effects like weakness, depression, and color vision changes.[10] The International Agency for Research on Cancer (IARC) classifies styrene as possibly carcinogenic to humans (Group 2B) based on limited evidence in humans and sufficient evidence in animals showing lung and other tumors, while the U.S. National Toxicology Program (NTP) lists it as reasonably anticipated to be a human carcinogen.[11][12] Human evidence remains limited, primarily from occupational exposures. Occupational safety standards limit airborne exposure to 100 ppm over an 8-hour workday, with ongoing research into its environmental persistence and bioaccumulation.[13]Chemical and Physical Properties
Structure and Nomenclature
Styrene is an organic compound with the molecular formula C₈H₈.[1] Its molecular structure consists of a benzene ring (C₆H₅-) covalently bonded to a vinyl group (-CH=CH₂), forming phenylethene, where the carbon-carbon double bond of the vinyl moiety is directly attached to the aromatic ring.[14] This arrangement imparts characteristic properties to the molecule, with the vinyl group enabling reactivity at the unsaturated site.[1] The International Union of Pure and Applied Chemistry (IUPAC) recommends the systematic name ethenylbenzene for this compound, reflecting the ethenyl (vinyl) substituent on the benzene parent structure; alternatively, vinylbenzene is also accepted in general nomenclature.[15] The common name "styrene" is a retained IUPAC name, alongside synonyms such as styrol, cinnamene, and phenylethylene.[1] The term "styrene" originates from "styrol," derived from storax balsam, a resin from trees of the Liquidambar genus (family Altingiaceae), where the compound was first isolated in the early 19th century.[16] Key identifiers for styrene include the Chemical Abstracts Service (CAS) registry number 100-42-5, a molecular weight of 104.15 g/mol, and the simplified molecular-input line-entry system (SMILES) notation C=CC1=CC=CC=C1.[1][14]Physical Characteristics
Styrene appears as a colorless to pale yellowish oily liquid at standard room temperature and pressure, exhibiting a distinctive sweet, balsamic odor that can become sharp or disagreeable in impure samples due to the presence of aldehydes.[17] This odor threshold in air is approximately 0.1 ppm, allowing detection at low concentrations.[18] Key thermodynamic properties include a density of 0.9059 g/cm³ at 20 °C, a melting point of -30.6 °C, and a boiling point of 145.2 °C at 1 atm.[17] The flash point is 31 °C (closed cup), underscoring its classification as a flammable liquid with potential for vapor-air mixtures to ignite under ambient conditions.[19] Styrene's vapor pressure measures 5 mmHg at 20 °C, reflecting moderate volatility that facilitates evaporation and contributes to its handling requirements in industrial settings.[18] The heat of vaporization is 38.2 kJ/mol, a value relevant for processes involving phase changes such as distillation or storage. In terms of solubility, styrene shows limited affinity for water, with a solubility of 0.03 g/100 mL at 20 °C, attributable in part to its nonpolar structure.[17] However, it is fully miscible with common organic solvents, including ethanol, diethyl ether, and acetone, enabling its use in solvent-based applications.[20] The refractive index, an optical property indicative of its interaction with light, is 1.5469 (n_D^{20}), consistent with its aromatic and alkenic composition.[17]| Property | Value | Conditions | Source |
|---|---|---|---|
| Density | 0.9059 g/cm³ | 20 °C | O'Neil et al. (2001) via ATSDR[17] |
| Melting point | -30.6 °C | - | SIRC (2020)[18] |
| Boiling point | 145.2 °C | 1 atm | SIRC (2020)[18] |
| Flash point | 31 °C | Closed cup | Sigma-Aldrich SDS (2024)[19] |
| Water solubility | 0.03 g/100 mL | 20 °C | O'Neil et al. (2001) via ATSDR[17] |
| Refractive index (n_D) | 1.5469 | 20 °C | O'Neil et al. (2001) via ATSDR[17] |
| Vapor pressure | 5 mmHg | 20 °C | SIRC (2020)[18] |
| Heat of vaporization | 38.2 kJ/mol | Boiling point | CAMEO Chemicals (NOAA) |