Dichloromethane

Description

Dichloromethane, commonly called methylene chloride, is a colorless liquid with an ethereal, but penetrating odor. Its miscibility in alcohol and ether and slight solubility in water has made it an ideal solvent and otherwise extremely versatile chemical. It has been used industrially (solvent and paint remover), as a drug (inhalation anesthetic) and as an agricultural chemical (growth regulator and fertilizer).

Chemical properties

Dichloromethane is a colorless liquid with a mild, sweet odor. It does not occur naturally in the environment. It is made from methane gas or wood alcohol. It is narcotic in high concentrations and carcinogenic. Inhalation exposure to this substance irritates the nose and throat and affects the central nervous system. Although dichloromethane is the least toxic C1 chlorohydrocarbon, it does present hazards.

The Uses of Dichloromethane

Methylene chloride is used principally as a solvent in paint removers. It is also used as an aerosol propellant, processing solvent in the manufacture of steroids, antibiotics, vitamins, and tablet coatings; as a degreasing agent; in electronics manufacturing; and as a urethane foamblowing agent.Methylene chloride is also used in metal cleaning, as a solvent in the production of polycarbonate resins and triacetate fibers, in film processing, ininkformulations, and as anextraction solvent for spice oleoresins, caffeine, and hops. It was once registered for use in the United States as an insecticide for commodity fumigation of strawberries, citrus fruits, and a variety of grains. Dichloromethane was used as an anesthetic gas but is no longer used because of the narrow therapeutic index.

Production Methods

Dichloromethane was first prepared by Regnault in 1840 by the chlorination of methyl chloride in sunlight. It became an industrial chemical of importance during the Second World War. Two commercial processes are currently used for the production of dichloromethane—hydrochlorination of methanol and direct chlorination of methane (Rossberg et al., 1986; Holbrook, 1993). The predominant method of manufacturing dichloromethane uses as a first step the reaction of hydrogen chloride and methanol to give methyl chloride. Excess methyl chloride is then mixed with chlorine and reacts to give dichloromethane, with chloroform and carbon tetrachloride as co-products. This reaction is usually carried out in the gas phase thermally but can also be performed catalytically or photolytically. At low temperature and high pressure, the liquid-phase process is capable of giving high selectivity for dichloromethane (Rossberg et al., 1986; Holbrook, 1993).

Definition

ChEBI: Dichloromethane is a member of the class of chloromethanes that is methane in which two of the hydrogens have been replaced by chlorine. A dense, non-flammible colourless liquid at room temperature (b.p. 40℃, d = 1.33) which is immiscible with water, it is widely used as a solvent, a paint stripper, and for the removal of caffeine from coffee and tea. It has a role as a polar aprotic solvent, a carcinogenic agent and a refrigerant. It is a member of chloromethanes and a volatile organic compound.

Reactions

Methylene chloride reacts violently in the presence of alkali or alkaline earth metals and will hydrolyze to formaldehyde in the presence of an aqueous base. Alkylation reactions occur at both functions, thus di-substitutions result.

Air & Water Reactions

Methylene chloride is a colourless liquid with a mild, sweet odour. Somewhat water soluble. Subject to slow hydrolysis which is accelerated by light.

Reactivity Profile

Dichloromethane reacts vigorously with active metals such as lithium, sodium and potassium, and with strong bases such as potassium tert-butoxide. Dichloromethane is incompatible with strong oxidizers, strong caustics and chemically active metals such as aluminum or magnesium powders. The liquid will attack some forms of plastic, rubber and coatings. Dichloromethane reacts with sodium-potassium alloy, (potassium hydrogen + N-methyl-N-nitrosurea), nitrogen tetraoxide and liquid oxygen. Dichloromethane also reacts with titanium. On contact with water Dichloromethane corrodes iron, some stainless steels, copper and nickel. Dichloromethane is incompatible with alkali metals. Dichloromethane is incompatible with amines, zinc and alloys of aluminum, magnesium and zinc. Dichloromethane is liable to explode when mixed with dinitrogen pentaoxide or nitric acid. Mixtures of Dichloromethane in air with methanol vapor are flammable.

Health Hazard

Dichloromethane is classified as only slightly toxic by the oral and inhalation routes. Exposure to high concentrations of dichloromethane vapor (>500 ppm for 8 h) can lead to lightheadedness, fatigue, weakness, and nausea. Contact of the compound with the eyes causes painful irritation and can lead to conjunctivitis and corneal injury if not promptly removed by washing. Dichloromethane is a mild skin irritant, and upon prolonged contact (e.g., under the cover of clothing or shoes) can cause burns after 30 to 60 min exposure. Dichloromethane is not teratogenic at levels up to 4500 ppm or embryotoxic in rats and mice at levels up to 1250 ppm.

Flammability and Explosibility

Noncombustible. Dichloromethane vapor concentrated in a confined or poorly ventilated area can be ignited with a high-energy spark, flame, or high-intensity heat source.

Safety Profile

Confirmed carcinogen with experimental carcinogenic and tumorigenic data. Poison by intravenous route. Moderately toxic by ingestion, subcutaneous, and intraperitoneal routes. Mildly toxic by inhalation. Human systemic effects by ingestion and inhalation: paresthesia, somnolence, altered sleep time, convulsions, euphoria, and change in cardlac rate. An experimental teratogen. Experimental reproductive effects. An eye and severe skin irritant. Human mutation data reported. It is flammable in the range of 12-19% in air but ignition is difficult. It will not form explosive mixtures with air at ordinary temperatures. Mixtures in air with methanol vapor are flammable. It will form explosive mixtures with an atmosphere having a high oxygen content, in liquid O2, N2O4, K, Na, NaK. Explosive in the form of vapor when exposed to heat or flame. Reacts violently with Li, NaK, potassiumtert- butoxide, (KOH + N-methyl-Nnitrosourea). It can be decomposed by contact with hot surfaces and open flame, and then yield toxic fumes that are irritating and give warning of their presence. When heated to decomposition it emits highly toxic fumes of phosgene and Cl-.

Potential Exposure

Methylene chloride is used mainly as a low-temperature extractant of substances which are adversely affected by high temperature. It can be used as a solvent for oil, fats, waxes, bitumen, cellulose acetate; and esters. It is also used as a paint remover; as a degreaser; and in aerosol propellants

First aid

If this chemical gets into the eyes, remove anycontact Tenses at once and irrigate immediately for at least5 min, occasionally lifing upper and lower lids. Seek medi-cal attention immediately. If this chemical contacts the skin,remove contaminated clothing and wash immediately withsoap and water. Seek medical attention immediately. If thisChemical has been inhaled, remove from exposure, begin res-cue breathing (using universal precautions, including resusci-ation mask) if breathing has stopped and CPR if heart actionhas stopped. Transfer promptly to a medical facility. Whenthis chemical has been swallowed, get medical attention.Give large quantities of water and induce vomiting. Do notmake an unconscious person vomit. Medical observation isrecommended for 24- 48 h after breathing overexposure, aspulmonary edema may be delayed. As first aid for pulmonaryedema, a doctor or authorized paramedic may consideradministering a corticosteroid spray.

Carcinogenicity

Dichloromethane is reasonably anticipated to be a human carcinogen based on sufficient evidence of carcinogenicity from studies in experimental animals.

Environmental Fate

Biological. Complete microbial degradation to carbon dioxide was reported under anaerobic conditions by mixed or pure cultures. Under enzymatic conditions formaldehyde was the only product reported (Vogel et al., 1987). In a static-culture-flask screening test, methylene chloride (5 and 10 mg/L) was statically incubated in the dark at 25 °C with yeast extract and settled domestic wastewater inoculum. After 7 d, 100% biodegradation with rapid adaptation was observed (Tabak et al., 1981).
Under aerobic conditions with sewage seed or activated sludge, complete biodegradation was observed between 6 h to 1 wk (Rittman and McCarty, 1980).
Soil. Methylene chloride undergoes biodegradation in soil under aerobic and anaerobic conditions. Under aerobic conditions, the following half-lives were reported: 54.8 d in sand (500 ppb); 1.3, 9.4, and 191.4 d at concentrations of 160, 500, and 5,000 ppb, respectively, in sandy loam soil; 12.7 d (500 ppb) in sandy clay loam soil; 7.2 d (500 ppb) following a 50-d lag time. Under anaerobic conditions, the half-life of methylene chloride in clay following a 70-d lag time is 21.5 d (Davis and Madsen, 1991). The estimated volatilization half-life of methylene chloride in soil is 100 d (Jury et al., 1990).
Photolytic. Reported photooxidation products via OH radicals include carbon dioxide, carbon monoxide, formyl chloride, and phosgene (Spence et al., 1976). In the presence of water, phosgene hydrolyzes to HCl and carbon dioxide, whereas formyl chloride hydrolyzes to hydrogen chloride and carbon monoxide (Morrison and Boyd, 1971).
Chemical/Physical. Under laboratory conditions, methylene chloride hydrolyzed with subsequent oxidation and reduction to produce methyl chloride, methanol, formic acid, and formaldehyde (Smith and Dragun, 1984). The experimental half-life for hydrolysis in water at 25 °C is approximately 18 months (Dilling et al., 1975).

Purification Methods

Shake it with portions of conc H2SO4 until the acid layer remains colourless, then wash with water, aqueous 5% Na2CO3, NaHCO3 or NaOH, then water again. Pre-dry with CaCl2, and distil it from CaSO4, CaH2 or P2O5. Store it away from bright light in a brown bottle with Linde type 4A molecular sieves, in an atmosphere of dry N2. Other purification steps include washing with aqueous Na2S2O3, passage through a column of silica gel, and removal of carbonyl-containing impurities as described under Chloroform. It has also been purified by treatment with basic alumina, distillation, and stored over molecular sieves under nitrogen [Puchot et al. J Am Chem Soc 108 2353 1986]. Dichloromethane from Japanese sources contained MeOH as stabiliser which is not removed by distillation. It can, however, be removed by standing over activated 3A Molecular Sieves (note that 4A Sieves cause the development of pressure in bottles), passed through activated Al2O3 and distilled [Gao et al. J Am Chem Soc 109 5771 1987]. It has been fractionated through a platinum spinning band column, degassed, and distilled onto degassed molecular sieves Linde 4A (heated under high vacuum at over 450o until the pressure readings reached the low values of 10-6 mm, ~1-2hours ). Stabilise it with 0.02% of 2,6-di-tert-butyl-p-cresol [Mohammad & Kosower J Am Chem Soc 93 2713 1971]. [Beilstein 1 IV 35.] Rapid purification: Reflux over CaH2 (5% w/v) and distil it. Store it over 4A molecular sieves.

Toxicity evaluation

Dichloromethane is usually released to the atmosphere. It can react withhydroxyl radicals with a half-life of about a fewmonths. Dichloromethane released to water can be evaporated to atmosphere with a half-life of 35.6 h at moderate mixing conditions. Some of dichloromethane in water can be biodegraded completely within several hours and a few days. Small part of dichloromethane released to water can be degraded by hydrolysis. However, hydrolysis is not an important process under natural condition and may take 18 months or more to degrade completely. Dichloromethane released to soil will go to the soil surface and then the atmosphere. Some part of dichloromethane in soil will leak to the groundwater and water cycle.
DCM’s production and use as solvent, chemical intermediate, grain fumigant, paint stripper and remover,metal degreaser, and refrigerant may result in its release to the environment through various waste streams. Vapor-phase DCM is expected to be degraded in the atmosphere by reaction with photochemically produced hydroxyl radicals; the half-life for this reaction in air is estimated to be approximately 119 days (in the absence of direct photolysis). If released to soil,DCMis expected to have very high mobility based on an estimated Koc of 24. Volatilization from moist soil surfaces is expected to be an important fate process based on an estimated Henry’s law constant of 3.25×10^-3 atm-m³ mol^-1. DCM may volatilize from dry soil surfaces based on its vapor pressure. Biodegradation in soil may occur. DCM, when released into water, is not expected to adsorb to suspended solids and sediment in water based on the estimated Koc. Biodegradation is possible in natural waters but will probably be very slow compared with evaporation.

Incompatibilities

Incompatible with strong oxidizers, caustics; chemically active metals, such as aluminum, magnesium powders; potassium, lithium, and sodium; concentrated nitric acid causing fire and explosion hazard. Contact with hot surfaces or flames causes decomposition producing fumes of hydrogen chloride and phosgene gas. Attacks some forms of plastics, rubber and coatings. Attacks metals in the presence of moisture.

Waste Disposal

Consult with environmental regulatory agencies for guidance on acceptable disposal practices. Generators of waste containing this contaminant (≥100 kg/mo) must conform to EPA regulations governing storage, transportation, treatment, and waste disposal. Incineration, preferably after mixing with another combustible fuel; care must be exercised to assure complete combustion to prevent the formation of phosgene; an acid scrubber is necessary to remove the halo acids produced.

Regulations

Several jurisdictions have acted to reduce the use and release of various volatile organic compounds, including dichloromethane. The California Air Resources Board was one of the first jurisdictions to regulate dichloromethane; in 1995, it limited the levels of total volatile organic compounds (VOCs) contained in aerosol coating products. Subsequent regulations prevented manufacture, sale, supply, or application of any aerosol coating product containing dichloromethane (Air Resources Board, 2001). California has also prohibited the manufacture, sale, or use of automotive cleaning and degreasing products containing dichloromethane.
In Japan, the environmental quality standards for dichloromethane state that outdoor air levels shall not exceed 0.15 mg/m3 (Ministry of the Environment Government of Japan, 2014).
A guideline value of 3 mg/m3 for 24-hour exposure is recommended by WHO. In addition, the weekly average concentration should not exceed one seventh (0.45 mg/m3) of this 24-hour guideline (WHO, 2000).
In the European Union, the VOC Solvent Emissions Directive (Directive 1999/13/EC) was implemented for new and existing installations on 31 October 2007 (European Commission,1999). The Directive aims to reduce industrial emissions of VOCs from solvent-using activities, such as printing, surface cleaning, vehicle coating, dry cleaning, and manufacture of footwear and pharmaceutical products. Installations conducting such activities are required to comply either with emission limit values or with a reduction scheme. Reduction schemes allow the operator to reduce emissions by alternative means, such as by substituting products with a lower solvent content or changing to solvent-free production processes. The Solvents Directive was implemented in 2010 into the Industrial Emission Directive 2010/75/EU (IED).