1. Chemical Structure and Properties
Molecular Formula: CH₂Cl₂
Structural Formula:
A chlorinated hydrocarbon with two chlorine atoms bonded to a central carbon atom and two hydrogen atoms.
Physical Properties:
Appearance: Clear, colorless liquid with a sweet, chloroform-like odor.
Boiling Point: 39.6°C; Density: 1.33 g/cm³; Vapor Pressure: 350 mmHg at 20°C.
Solubility: Slightly soluble in water (13 g/L at 25°C); miscible with most organic solvents (ethanol, ether).
Chemical Properties:
Stability: Non-flammable but decomposes at >120°C to release phosgene (COCl₂), HCl, and chlorine gas.
Reactivity: Reacts with strong bases (e.g., NaOH) to form dichlorocarbene (CCl₂).
Acts as a polar aprotic solvent in organic synthesis.
2. Industrial Applications
Solvent Industry:
Paint Stripping: Dominates industrial paint removal due to high solvency for resins and polymers.
Pharmaceuticals: Extraction solvent for antibiotics (e.g., penicillin) and caffeine decaffeination.
Chemical Synthesis:
Reaction Medium: Used in SN₂ reactions, Grignard reagent preparation, and peptide synthesis.
Foam Blowing:
Polyurethane Foams: Blowing agent for insulation materials (phased out in many regions).
Metal Cleaning:
Vapor Degreasing: Removes oils and greases from precision machinery parts.
3. Safety and Toxicology
Health Hazards:
Acute Exposure:
Inhalation (≥500 ppm): CNS depression, dizziness, and asphyxia (TLV-TWA: 50 ppm).
Skin Contact: Causes defatting and dermatitis (rabbit skin LD₅₀: >2,000 mg/kg).
Ingestion: Highly toxic (oral LD₅₀ rat: 1,600 mg/kg); liver/kidney damage.
Chronic Effects:
Carcinogenicity: Classified as Group 2A (probable human carcinogen) by IARC due to liver/lung cancer risks.
Neurotoxicity: Peripheral neuropathy and cognitive impairment in occupational settings.
Protection Measures:
PPE: Chemical-resistant gloves (e.g., neoprene), organic vapor respirators, and ventilation.
Storage: Sealed containers in cool, ventilated areas away from alkaline materials.
4. Environmental and Regulatory Compliance
Environmental Impact:
Groundwater Contamination: Persistent (half-life >6 months) and a common Superfund site pollutant.
Ozone Depletion Potential (ODP): Zero (no ozone layer harm).
Global Warming Potential (GWP): 8.7 (100-year horizon).
Regulatory Frameworks:
EU:
REACH: Restricted in consumer products (Annex XVII); banned in paint strippers since 2020.
CLP: Classified as Carc. 1B (H350), STOT SE 1 (H372).
USA:
EPA: Banned in consumer paint strippers (2019); regulated under Clean Air Act (HAP).
OSHA: Permissible Exposure Limit (PEL): 25 ppm (8-hour TWA).
China:
GB 13690-2009: Classified as Hazardous Chemical (Class 3.2).
Waste Management:
Incineration: High-temperature (>1,200°C) with HCl scrubbing.
Biodegradation: Anaerobic microbial degradation to ethene (pilot-scale).
5. Case Studies and Application Insights
Case 1: Transition to Safer Paint Strippers (EU, 2020):
Challenge: Replace DCM in consumer paint strippers due to fatal exposure incidents.
Solution: Adoption of benzyl alcohol-based and lactic acid formulations.
Result: 80% reduction in DCM-related poisoning cases post-ban.
Case 2: Green Chemistry in Pharmaceuticals (Pfizer, 2021):
Process: Replaced DCM with cyclopentyl methyl ether (CPME) in API synthesis.
Outcome: Reduced solvent toxicity by 90% and improved process safety (PASS criteria).
Comparative Analysis:
DCM vs. Ethyl Acetate:
Pros: Higher solvency for polar/non-polar mixtures; lower cost.
Cons: Ethyl acetate is biodegradable and non-carcinogenic.
DCM vs. Liquid CO₂ Cleaning:
Pros: CO₂ is non-toxic and recyclable; DCM offers faster degreasing.
Specifications：
Methylene Chloride (Dichloromethane) is a colorless, volatile solvent with high purity, excellent solvency, and low flammability, widely used in paint stripping, pharmaceuticals, and chemical processing, available from China Amines Co.