Definition
Methylnaphthalene refers to any methyl-substituted derivative of naphthalene, most commonly the positional isomers 1‑methylnaphthalene and 2‑methylnaphthalene. These compounds have the molecular formula C₁₁H₁₀ and belong to the class of polycyclic aromatic hydrocarbons (PAHs).
Overview
Methylnaphthalenes are colorless to pale‑yellow liquids at room temperature, with characteristic aromatic odors. They are produced in petroleum refining processes, combustion of organic matter, and as by‑products of the petrochemical industry. In the field of fuel science, 1‑ and 2‑methylnaphthalene are employed as reference standards for the determination of cetane numbers in diesel fuels, owing to their well‑characterized ignition qualities. They are also used in laboratory research as model compounds to study PAH behavior, environmental fate, and toxicology.
Etymology / Origin
The name combines “methyl,” denoting the –CH₃ substituent derived from methane, and “naphthalene,” which originates from the Greek ἄναφθαλος (ánaphthalos), meaning “invisible flame,” a reference to the substance’s use in early lighting. The term therefore literally describes a naphthalene molecule bearing a methyl group.
Characteristics
| Property | Value (typical for 1‑ and 2‑methylnaphthalene) |
|---|---|
| Molecular formula | C₁₁H₁₀ |
| Molar mass | 142.20 g·mol⁻¹ |
| Appearance | Colorless to pale yellow liquid |
| Density (20 °C) | ~0.99 g·cm⁻³ |
| Boiling point | 257–258 °C |
| Melting point | –30 °C (approx.) |
| Vapor pressure (25 °C) | ~0.5 mm Hg |
| Solubility in water | Very low (≈0.03 g L⁻¹) |
| Log P (octanol/water) | ~4.0 |
| Primary uses | Cetane‑number reference fuel, PAH research, synthesis intermediate |
| Environmental behaviour | Persistent, lipophilic, bioaccumulative; degrades slowly under photolytic or microbial action |
| Toxicological note | Classified as a possible carcinogen (IARC Group 2B); exposure may cause respiratory and skin irritation |
Related Topics
- Naphthalene – the parent PAH from which methylnaphthalenes are derived.
- Polycyclic aromatic hydrocarbons (PAHs) – a class of organic compounds consisting of fused aromatic rings, many of which are environmental pollutants.
- Cetane number – a measure of the ignition quality of diesel fuel; methylnaphthalenes serve as standard reference compounds.
- Diesel fuel standards – regulatory frameworks that specify performance criteria, often referencing methylnaphthalene-derived values.
- Environmental toxicology of PAHs – the study of the ecological and health impacts of aromatic hydrocarbons, including methylnaphthalenes.
This entry summarizes established chemical and industrial information on methylnaphthalene and its principal isomers.