Synthesis process of o-chlorobenzoyl chloride

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Synthesis process of o-chlorobenzoyl chloride The synthesis of o-chlorobenzoyl chloride (2-chlorobenzoyl chloride) primarily relies on the carboxylic acid acylation method, with other approaches such as the chlorination and hydrolysis of a toluene side chain also existing. Below are five core processes commonly used in industry and laboratories:
1. Thionyl chloride (SOCl₂) method (most widely used in industry)
Reaction equation: o-Chlorobenzoic acid + SOCl₂ → o-Chlorobenzoyl chloride + SO₂↑ + HCl↑
Key process points:
Raw materials: o-Chlorobenzoic acid, thionyl chloride (in 10%–30% excess)
Catalyst: Trace amounts of DMF (N,N-dimethylformamide)
Conditions: Reflux at 70–80°C for 2–4 hours
Post-treatment: Distillation to remove excess SOCl₂ → Atmospheric or vacuum distillation → Final product
Yield: 85%–90%
Advantages: Mild reaction conditions; by-products are gaseous and easy to separate; high product purity
Disadvantages: High cost of SOCl₂; corrosive to equipment; sulfur-containing waste gases require treatment

2. Phosphorus trichloride (PCl₃) method
Reaction equation: 3 o-Chlorobenzoic acids + PCl₃ → 3 o-Chlorobenzoyl chlorides + H₃PO₃
Key process points:
Raw materials: o-Chlorobenzoic acid, PCl₃ (molar ratio 1:0.35–0.40)
Conditions: Reaction at 60–90°C for 3–5 hours
Post-treatment: Layer separation to remove phosphorous acid (lower layer) → Distillation
Yield: 80%–85%
Advantages: PCl₃ is inexpensive; suitable for synthesizing acyl chlorides with low boiling points
Disadvantages: Phosphorous acid by-product is difficult to handle; generates phosphorus-containing wastewater

3. Phosgene (COCl₂) method (used in large-scale industrial production)
Reaction equation: o-Chlorobenzoic acid + COCl₂ → o-Chlorobenzoyl chloride + HCl↑ + CO₂↑
Key process points:
Raw materials: o-Chlorobenzoic acid, phosgene (in excess)
Catalyst: DMF, pyridine, or quaternary ammonium salts
Conditions: 40–70°C under atmospheric or reduced pressure
Yield: 90%–95%
Advantages: Low cost; all by-products are gaseous; high product purity
Disadvantages: Phosgene is highly toxic; requires stringent equipment sealing; strict safety measures and exhaust gas treatment are necessary

4. Phosphorus pentachloride (PCl₅) method
Reaction equation: o-Chlorobenzoic acid + PCl₅ → o-Chlorobenzoyl chloride + POCl₃ + HCl↑
Key process points:
Raw materials: o-Chlorobenzoic acid, PCl₅ (molar ratio 1:1.05–1.10)
Conditions: Reaction at 50–80°C for 2–3 hours
Post-treatment: Distillation to remove phosphoryl chloride (POCl₃) → Vacuum distillation
Yield: 85%–90%
Advantages: High reactivity; rapid reaction rate
Disadvantages: PCl₅ is hygroscopic; POCl₃ by-product must be recovered; highly corrosive

5. Hydrolysis of α,α,α,2-tetrachlorotoluene (side-chain chlorination route)
Reaction equation: o-ClC₆H₄CCl₃ + H₂O → o-ClC₆H₄COCl + 2HCl
(using FeCl₃/ZnCl₂ as catalyst)
Key process points:
Raw materials: o-Chlorotoluene subjected to side-chain trichlorination to produce α,α,α,2-tetrachlorotoluene
Catalyst: Lewis acids (FeCl₃, ZnCl₂)
Conditions: Hydrolysis at 100–140°C
Yield: 80%–88%
Advantages: Raw material (o-chlorotoluene) is inexpensive; suitable for co-production processes
Disadvantages: Poor selectivity in side-chain chlorination; generates large volumes of chlorine-containing wastewater; severe equipment corrosion

6. Mainstream choices
Domestic industry: Thionyl chloride method is preferred (mature, safe, and easy to control).
Large foreign enterprises: Phosgene method is more economical (due to economies of scale and well-established exhaust gas treatment systems).
Laboratories: SOCl₂/DMF or oxalyl chloride/DMF methods are most commonly used