2-Fluorobenzoic acid (also named o‑fluorobenzoic acid) is an aromatic carboxylic acid derived from benzoic acid by substitution of a fluorine atom at the ortho (2‑) position of the benzene ring. Its molecular formula is C₇H₅FO₂, and it possesses a molar mass of approximately 140.11 g·mol⁻¹. The compound is a crystalline solid at room temperature and is sparingly soluble in water but more soluble in organic solvents such as ethanol, acetone, and dimethyl sulfoxide.
Chemical Structure
- IUPAC name: 2‑fluorobenzoic acid
- Synonyms: ortho‑fluorobenzoic acid, o‑fluorobenzoic acid
- Structure: A benzene ring bearing a carboxylic acid (–CO₂H) substituent at carbon‑1 and a fluorine atom at carbon‑2.
Physical Properties
| Property | Value (reported) |
|---|---|
| Appearance | White to off‑white crystalline solid |
| Melting point | 166–168 °C (reported) |
| Boiling point | Decomposes before boiling |
| Solubility | ~0.28 g · 100 mL⁻¹ in water at 25 °C; miscible with many polar organic solvents |
| Density | ≈1.40 g · cm⁻³ (25 °C) |
Chemical Properties
- Acidity: The presence of the electron‑withdrawing fluorine atom increases the acidity relative to benzoic acid (pK_a ≈ 4.2). Reported pK_a values for 2‑fluorobenzoic acid lie in the range 3.0–3.3.
- Reactivity: The ortho‑fluorine substituent can influence electrophilic aromatic substitution reactions, often directing new substituents meta‑to the carboxyl group due to the combined effects of the electron‑withdrawing fluorine and the deactivating carboxyl group.
- Stability: The compound is stable under ambient conditions but may decompose upon prolonged heating.
Synthesis
Common laboratory or industrial routes to 2‑fluorobenzoic acid include:
- Oxidation of 2‑fluorotoluene: Catalytic oxidation (e.g., using KMnO₄, Na₂Cr₂O₇, or a metal‑catalyzed aerobic process) converts the methyl group to a carboxyl group.
- Halogen exchange (Halex) on benzoic acid derivatives: Treatment of 2‑chlorobenzoic acid with a fluorinating agent such as potassium fluoride or cesium fluoride under high temperature can afford the fluorinated product.
- Directed ortho‑metalation (DoM) followed by carboxylation: Lithiation of fluorobenzene at the ortho position using a strong base (e.g., n‑BuLi) and subsequent carbonation with CO₂ yields 2‑fluorobenzoic acid after acidic work‑up.
Applications
- Synthetic intermediate: Employed as a building block in the preparation of pharmaceuticals, agrochemicals, and functional materials where a fluorinated aromatic moiety is desired.
- Cross‑coupling reactions: Serves as a substrate for Suzuki, Heck, and Negishi couplings, facilitating the introduction of various aryl or alkyl groups at the ortho position.
- Polymer science: Utilized in the synthesis of fluorinated polyesters and other polymers to impart enhanced chemical resistance and modified thermal properties.
Safety and Handling
- Hazard classification: Classified as an irritant; may cause skin and eye irritation and respiratory tract irritation upon inhalation of dust or vapors.
- Protective measures: Use of gloves, goggles, and laboratory coats is recommended. Work in a well‑ventilated area or fume hood to avoid inhalation.
- Environmental impact: Fluorinated aromatic compounds can be persistent; appropriate waste disposal procedures should be followed in accordance with local regulations.
References
- CRC Handbook of Chemistry and Physics, 102nd ed., 2023.
- L. A. Bruice, Organic Chemistry, 9th ed., Pearson, 2020 – discussion of acidities of substituted benzoic acids.
- J. Org. Chem. 2021, 86, 10234‑10241 – oxidative conversion of 2‑fluorotoluene to 2‑fluorobenzoic acid.