Article No : a06_233

Chloromethanes

MANFRED ROSSBERG, Hoechst Aktiengesellschaft, Frankfurt/Main, Germany

WILHELM LENDLE, Hoechst Aktiengesellschaft, Frankfurt/Main, Germany

GERHARD PFLEIDERER, Hoechst Aktiengesellschaft, Frankfurt/Main, Germany

ADOLF TO¨ GEL, Hoechst Aktiengesellschaft, Frankfurt/Main, Germany

THEODORE R. TORKELSON, Dow Chemical, Midland, Michigan, United States 48674

KLAUS K. BEUTEL, Dow Chemical Europe, Horgen, Switzerland

1. Introduction...... 15 5.2. Analysis ...... 33 2. Physical Properties ...... 16 6. Storage, Transport, and Handling ...... 34 3. Chemical Properties ...... 19 7. Behavior of Chloromethanes 4. Production ...... 20 in the Environment ...... 35 4.1. Theoretical Bases...... 20 7.1. Presence in the Atmosphere...... 35 4.2. Production of Monochloromethane ...... 23 7.2. Presence in Water Sources ...... 36 4.3. Production of and 8. Uses and Economic Aspects ...... 36 Trichloromethane ...... 25 9. Toxicology ...... 37 4.4. Production of Tetrachloromethane ...... 29 References ...... 39 5. Quality Specifications...... 33 5.1. Purity of the Commercial Products and their Stabilization...... 33

1. Introduction methyl by the chlorination of occurred before World War I, with the intent of Among the halogenated hydrocarbons, the chlo- hydrolyzing it to . A commercial meth- rine derivatives of methane monochloromethane ane chlorination facility was first put into opera- (methyl chloride) [74-87-3], dichloromethane tion by the former Farbwerke Hoechst in 1923. In (methylene chloride) [75-09-2], trichloro- the meantime, however, a high-pressure metha- methane () [67-66-3], and tetrachlor- nol synthesis based on carbon monoxide and omethane () [56-23-5] play hydrogen had been developed, as a result of an important role from both industrial and eco- which the opposite process became practical – nomic standpoints. These products find broad synthesis of methyl chloride from methanol. application not only as important chemical inter- mediates, but also as solvents. Dichloromethane was prepared for the first time in 1840 by V. REGNAULT, who successfully Historical Development. Monochloro- chlorinated methyl chloride. It was for a time methane was produced for the first time in produced by the reduction of trichloromethane 1835 by J. DUMAS and E. PELIGOT by the reaction (chloroform) with zinc and in of with methanol in the presence alcohol, but the compound first acquired signifi- of . M. BERTHELOT isolated it in 1858 cance as a solvent after it was successfully pre- from the chlorination of marsh gas (methane), as pared commercially by chlorination of methane did C. GROVES in 1874 from the reaction of and monochloromethane (Hoechst AG, Dow with methanol in the presence Chemical Co., and Stauffer Chemical Co.). of zinc chloride. For a time, monochloromethane Trichloromethane was synthesized indepen- was produced commercially from betaine hydro- dently by two groups in 1831: J. VON LIEBIG chloride obtained in the course of beet sugar successfully carried out the alkaline cleavage of manufacture. The earliest attempts to produce chloral, whereas M. E. SOUBEIRAIN obtained the

2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/14356007.a06_233.pub3 16 Chloromethanes Vol. 9 compound by the action of bleach on Originally, tetrachloromethane played a role both ethanol and acetone. In 1835, J. DUMAS only in the dry cleaning industry and as a fire showed that trichloromethane contained only a extinguishing agent. Its production increased single hydrogen atom and prepared the substance dramatically, however, with the introduction of by the alkaline cleavage of trichloroacetic acid chlorofluoromethane compounds 50 years ago, and other compounds containing a terminal CCl3 these finding wide application as non-toxic group, such as b-trichloroacetoacrylic acid. In refrigerants, as propellants for aerosols, as analogy to the synthetic method of M. E. SOU- foam-blowing agents, and as specialty solvents. BEIRAIN, the use of hypochlorites was extended to include other compounds containing acetyl groups, particularly acetaldehyde. V. REGNAULT 2. Physical Properties prepared trichloromethane by chlorination of monochloromethane. Already by the middle of The most important physical properties of the the last century, chloroform was being produced four chloro derivatives of methane are presented on a commercial basis by using the J. VON LIEBIG in Table 1; Figure 1 illustrates the vapor pressure procedure, a method which retained its impor- curves of the four chlorinated methanes. tance until ca. the 1960s in places where the The following sections summarize additional preferred starting materials methane and mono- important physical properties of the individual chloromethane were in short supply. Today, tri- compounds making up the chloromethane series. chloromethane – along with dichloromethane – is prepared exclusively and on a massive scale by Monochloromethane is a colorless, flam- the chlorination of methane and/or monochlor- mable gas with a faintly sweet odor. Its solubility omethane. Trichloromethane was introduced in- in water follows Henry’s law; the temperature to the field of medicine in 1847 by J. Y. SIMPSON, dependence of the solubility at 0.1 MPa (1 bar) is: who employed it as an inhaled anaesthetic. As a result of its toxicologic properties, however, it t, C 15304560 has since been totally replaced by other com- gofCH3Cl/kg of H2O 9.0 6.52 4.36 2.64 pounds (e.g., Halothane). Tetrachloromethane was first prepared in 1839 by V. REGNAULT by the chlorination of Monochloromethane at 20 C and 0.1 MPa (1 3 trichloromethane. Shortly thereafter, J. DUMAS bar) is soluble to the extent of 4.723 cm in 100 succeeded in synthesizing it by the chlorination cm3 of benzene, 3.756 cm3 in 100 cm3 of tetra- 3 3 of marsh gas. H. KOLBE isolated tetrachloro- chloromethane, 3.679 cm in 100 cm of acetic methane in 1843 when he treated carbon disulfide acid, and 3.740 cm3 in 100 cm3 of ethanol. It with chlorine in the gas phase. The corresponding forms azeotropic mixtures with dimethyl ether, liquid phase reaction in the presence of a catalyst, 2-methylpropane, and dichlorodifluoromethane giving CCl4 and S2Cl2, was developed a short (CFC 12). time later. The key to economical practicality of this approach was the discovery in 1893 by Dichloromethane is a colorless, highly vol- Mu€LLER and DUBOIS of the reaction of S2Cl2 with atile, neutral liquid with a slightly sweet smell, CS2 to give sulfur and tetrachloromethane, there- similar to that of trichloromethane. The solubility by avoiding the production of S2Cl2. of water in dichloromethane is: Tetrachloromethane is produced on an indus- trial scale by one of two general approaches. The t, C 30 0 þ 25 first is the methane chlorination process, using gofH2O/kg of CH2Cl2 0.16 0.8 1.98 methane or mono-chloromethane as starting ma- terials. The other involves either perchlorination or chlorinolysis. Starting materials in this case The solubility of dichloromethane in water and include C1 to C3 hydrocarbons and their chlori- in aqueous hydrochloric acid is presented in nated derivatives as well as Cl-containing resi- Table 2. dues obtained in other chlorination processes Dichloromethane forms azeotropic mixtures (vinyl chloride, propylene oxide, etc.). with a number of substances (Table 3). Vol. 9 Chloromethanes 17

Table 1. Physical properties of chloromethanes

Unit Monochloromethane Dichloromethane Trichloromethane Tetrachloromethane

Formula CH3Cl CH2Cl2 CHCl3 CCl4 Mr 50.49 84.94 119.39 153.84 C 97.7 96.7 63.8 22.8 Boiling point at 0.1 MPa C 23.9 40.2 61.3 76.7 Vapor pressure at 20 C kPa 489 47.3 21.27 11.94 Density of liquid at 20 C kg/m3 920 1328.3 1489 1594.7 (0.5 MPa) Density of vapor at bp kg/m3 2.558 3.406 4.372 5.508 0 Enthalpy of formation DH298 kJ/mol 86.0 124.7 132.0 138.1 Specific heat capacity of kJ kg1 K1 1.595 1.156 0.980 0.867 liquid at 20 C Enthalpy of vaporization at bp kJ/mol 21.65 28.06 29.7 30.0 Critical temperature K 416.3 510.1 535.6 556.4 Critical pressure MPa 6.68 6.17 5.45 4.55 Cubic expansion coeff. of K1 0.0022 0.00137 0.001399 0.00116 liquid (0 – 40 C) Thermal conductivity at 20 CWK1 m1 0.1570 0.159 0.1454 0.1070 Surface tension at 20 C N/m 16.2 103 28.76 103 27.14 103 26.7 103 Viscosity of liquid at 20 CPa s 2.7 104 4.37 104 5.7 104 13.5 104 (0.5 MPa) 20 nD 1.4244 1.4467 1.4604 Ignition temperature C 618 605 – – Limits of ignition in air, lower vol% 8.1 12 – – Limits of ignition in air, upper vol% 17.2 22 – – mg=LðairÞ Partition coefficient air/water mg=LðwaterÞ 0.3 0.12 0.12 0.91 at 20 C

Dichloromethane is virtually nonflammable the definitions established in DIN 51 755 and in air, as shown in Figure 2, which illustrates the ASTM 56–70 as well as DIN 51 758 and ASTM D range of flammable mixtures with oxygen – 93–73. Thus, it is not subject to the regulations nitrogen combinations [10, 11]. Dichloromethane governing flammable liquids. As a result of the thereby constitutes the only nonflammable com- existing limits of flammability (CH2Cl2 vapor/ mercial solvent with a low boiling point. The air), it is assigned to explosion category G 1 (VDE substance possesses no flash point according to 0165). The addition of small amounts of dichlor- omethane to flammable liquids (e.g., gasoline, esters, benzene, etc.) raises their flash points; addition of 10 – 30 % dichloromethane can render such mixtures nonflammable.

Trichloromethane is a colorless, highly volatile, neutral liquid with a characteristic sweet odor. Trichloromethane vapors form no explo- sive mixtures with air [11]. Trichloromethane has excellent solvent properties for many organic

Table 2. Solubility of dichloromethane in water and aqueous hydro- chloric acid (in wt %)

Solvent Temperature, C

15 30 45 60

Water 2.50 1.56 0.88 0.53 10 % HCl 2.94 1.85 1.25 0.60 20 % HCl – 2.45 1.20 0.65 Figure 1. Vapor pressure curves of chloromethanes 18 Chloromethanes Vol. 9

Table 3. Azeotropic mixtures of dichloromethane Table 4. Azeotropic mixtures of trichloromethane

Azeotropic boiling Azeotropic boiling point, in C, at point, in C, at wt % Compound 101.3 kPa wt % Compound 101.3 kPa

30.0 acetone 57.6 15.0 formic acid 59.2 11.5 ethanol 54.6 20.5 acetone 64.5 94.8 1,3-butadiene 5.0 6.8 ethanol 59.3 6.0 tert-butanol 57.1 13.0 ethyl formate 62.7 30.0 cyclopentane 38.0 96.0 2-butanone 79.7 55.0 diethylamine 52.0 2.8 n-hexane 60.0 30.0 diethyl ether 40.80 4.5 2-propanol 60.8 8.0 2-propanol 56.6 12.5 methanol 53.4 7.3 methanol 37.8 23.0 methyl acetate 64.8 51.0 pentane 35.5 2.8 water 56.1 23.0 propylene oxide 40.6 39.0 carbon disulfide 37.0 1.5 water 38.1 55.5 C, 4 mol% ethanol þ 3.5 mol% H2O), methanol – acetone, and methanol – hexane. materials, including alkaloids, fats, oils, resins, waxes, gums, rubber, paraffins, etc. As a result of Tetrachloromethane is a colorless neutral its toxicity, it is increasingly being replaced as a liquid with a high refractive index and a strong, solvent by dichloromethane, whose properties in bitter odor. It possesses good solubility proper- this general context are otherwise similar. In ties for many organic substances, but due to its addition, trichloromethane is a good solvent for high toxicity it is no longer employed (e.g., as a iodine and sulfur, and it is completely miscible spot remover or in the dry cleaning of textiles). It with many organic solvents. The solubility should be noted that it does continue to find of trichloromethane in water at 25 Cis application as a solvent for chlorine in certain industrial processes. 3.81 g/kg of H2O, whereas 0.8 g of H2O is soluble in 1 kg of CHCl . Tetrachloromethane is soluble in water at 25 3 Important azeotropic mixtures of chloroform C to the extent of 0.8 g of CCl4/kg of H2O, the with other compounds are listed in Table 4. solubility of water in tetrachloromethane being Ternary azeotropes also exist between tri- 0.13 g of H2O/kg of CCl4. chloromethane and ethanol – water (boiling point Tetrachloromethane forms constant-boiling azeotropic mixtures with a variety of substances; corresponding data are given in Table 5.

Table 5. Azeotropic mixtures of tetrachloromethane

Azeotropic boiling point, in C, at wt % Compound 101.3 kPa

88.5 acetone 56.4 17.0 acetonitrile 71.0 11.5 allyl alcohol 72.3 81.5 formic acid 66.65 43.0 ethyl acetate 74.8 15.85 ethanol 61.1 71.0 2-butanone 73.8 2.5 butanol 76.6 21.0 1,2-dichloroethane 75.6 12.0 2-propanol 69.0 20.56 methanol 55.7 11.5 propanol 73.1 Figure 2. Range of flammability of mixtures of CH2Cl2 with 4.1 water 66.0 O2 and N2 [10] Vol. 9 Chloromethanes 19 3. Chemical Properties used in Friedel–Crafts reactions for the produc- tion of alkylbenzenes. Monochloromethane as compared to other Monochloromethane has acquired particular- aliphatic chlorine compounds, is thermally quite ly great significance as a methylating agent: stable. Thermal decomposition is observed only examples include its reaction with hydroxyl at temperatures in excess of 400 C, even in the groups to give the corresponding ethers (meth- presence of metals (excluding the alkali and ylcellulose from , various methyl ethers alkaline-earth metals). The principal products of from phenolates), and its use in the preparation of photooxidation of monochloromethane are car- methyl-substituted amino compounds (quaterna- bon dioxide and phosgene. ry methylammonium compounds for tensides). Monochloromethane forms with water or All of the various methylamines result from its water vapor a snowlike gas hydrate with the reaction with . Treatment of CH3Cl composition CH3Cl 6H2O, the latter decom- with sodium hydrogensulfide under pressure and posing into its components at þ 7.5 C and 0.1 at elevated temperature gives methyl mercaptan. MPa (1 bar). To the extent that monochloro- methane still finds application in the Dichloromethane is thermally stable to industry, its water content must be temperatures above 140 C and stable in the kept below 50 ppm. This specification is neces- presence of oxygen to 120 C. Its photooxidation sary to prevent potential failure of refrigeration produces carbon dioxide, hydrogen chloride, and equipment pressure release valves caused by a small amount of phosgene [12]. Thermal reac- hydrate formation. tion with nitrogen dioxide gives carbon monox- Monochloromethane is hydrolyzed by water ide, nitrogen monoxide, and hydrogen chloride at an elevated temperature. The hydrolysis (to [13]. In respect to most industrial metals (e.g., methanol and the corresponding chloride) is iron, copper, tin), dichloromethane is stable, ex- greatly accelerated by the presence of alkali. ceptions being aluminum, magnesium, and their Mineral acids show no influence on the alloys; traces of phosgene first arise above 80 C. compound’s hydrolytic tendencies. Dichloromethane forms a hydrate with water, Monochloromethane is converted in the CH2Cl2 17 H2O, which decomposes at 1.6 C presence of alkali or alkaline-earth metals, as well and 21.3 kPa (213 mbar). as by zinc and aluminum, into the corresponding No detectable hydrolysis occurs during the organometallic compounds (e.g., CH3MgCl, Al evaporation of dichloromethane from extracts or (CH3)3 AlCl3). These have come to play a role extraction residues. Only on prolonged action of both in preparative organic chemistry and as steam at 140 – 170 C under pressure are formal- catalysts in the production of plastics. dehyde and hydrogen chloride produced. Reaction of monochloromethane with a sodi- Dichloromethane can be further chlorinated um – lead amalgam leads to tetramethyllead, an either thermally or photochemically. Halogen antiknocking additive to gasoline intended for exchange leading to chlorobromomethane or use in internal combustion engines. The use of dibromomethane can be carried out by using the compound is declining, however, as a result bromine and aluminum or aluminum bromide. of ecological considerations. In the presence of aluminum at 220 C and 90 A very significant reaction is that between MPa (900 bar), it reacts with carbon monoxide to monochloromethane and silicon to produce the give chloroacetyl chloride [14]. Warming to corresponding methylchlorosilanes (the Rochow 125 C with alcoholic ammonia solution produces synthesis), e.g.: hexamethylenetetramine. Reaction with pheno- lates leads to the same products as are obtained in

2CH3ClþSi!SiCl2ðCH3Þ2 the reaction of formaldehyde and phenols.

The latter, through their subsequent conver- Trichloromethane is nonflammable, al- sion to siloxanes, serve as important starting though it does decompose in a flame or in contact points for the production of . with hot surfaces to produce phosgene. In the Monochloromethane is employed as a com- presence of oxygen, it is cleaved photochemical- ponent in the Wurtz–Fittig reaction; it is also ly by way of peroxides to phosgene and hydrogen 20 Chloromethanes Vol. 9 chloride [15, 16]. The oxidation is catalyzed in 2CCl4 C2Cl4þ2Cl2 ð1Þ the dark by iron [17]. The autoxidation and acid generation can be slowed or prevented by stabi- which is shifted significantly to the right above lizers such as methanol, ethanol, or amylene. 700 C and 0.1 MPa (1 bar) pressure. At 900 C and 0.1 MPa (1 bar), the equilibrium conversion Trichloromethane forms a hydrate, CHCl3 17 of CCl is > 70 % (see ! Chlorethanes and H2O, whose critical decomposition point is 4 þ 1.6 C and 8.0 kPa (80 mbar). Chloroethylenes, Section 2.5.). Upon heating with aqueous alkali, trichloro- Tetrachloromethane forms shock-sensitive, methane is hydrolyzed to formic acid, orthofor- explosive mixtures with the alkali and alka- mate esters being formed with alcoholates. With line-earth metals. With water it forms a hydrate- primary amines in an alkaline medium the iso- like addition compound which decomposes at nitrile reaction occurs, a result which also finds þ 1.45 C. use in analytical determinations. The interaction The telomerization of ethylene and vinyl of trichloromethane with phenolates to give derivatives with tetrachloromethane under pres- salicylaldehydes is well-known as the Reimer- sureandinthepresenceofperoxideshasacquireda Thiemann reaction. Treatment with benzene certain preparative significance [22–24]: under Friedel-Crafts conditions results in triphenylmethane. CH2 ¼ CH2þCCl4!CCl3CH2CH2Cl The most important reaction of trichloro- methane is that with hydrogen fluoride in the The most important industrial reactions of presence of antimony pentahalides to give mono- tetrachloromethane are its liquid-phase conver- chlorodifluoromethane (CFC 22), a precursor in sion with anhydrous hydrogen fluoride in the the production of polytetrafluoroethylene (Tef- presence of antimony (III/V) fluorides or its lon, Hostaflon, PTFE). gas-phase reaction over aluminum or chromium When treated with salicylic anhydride, tri- fluoride catalysts, both of which give the widely chloromethane produces a crystalline addition used and important compounds trichloromono- compound containing 2 mol of trichloromethane. fluoromethane (CFC 11), dichlorodifluorometh- This result finds application in the preparation of ane (CFC 12), and monochlorotrifluoromethane trichloromethane of the highest purity. Under (CFC 13). certain conditions, explosive and shocksensitive products can result from the combination of trichloromethane with alkali metals and certain 4. Production other light metals [18]. 4.1. Theoretical Bases Tetrachloromethane is nonflammable and relatively stable even in the presence of light and The industrial preparation of chloromethane de- air at room temperature. When heated in air in the rivatives is based almost exclusively on the presence of metals (iron), phosgene is produced treatment of methane and/or monochloro- in large quantities, the reaction starting at ca. 300 methane with chlorine, whereby the chlorination C [19]. Photochemical oxidation also leads to products are obtained as a mixture of the indi- phosgene. Hydrolysis to carbon dioxide and hy- vidual stages of chlorination: drogen chloride is the principal result in a moist CH þCl !CH ClþHCl H ¼103:5kJ=mol ð2Þ atmosphere [20]. Liquid tetrachloromethane has 4 2 3 D only a very minimal tendency to hydrolyze in CH ClþCl !CH Cl þHCl H ¼102:5kJ=mol ð3Þ water at room temperature (half-life ca. 70 000 3 2 2 2 D years) [21]. Thermal decomposition of dry tetrachloro- CH2Cl2þCl2!CHCl3þHClDH ¼99:2kJ=mol ð4Þ methane occurs relatively slowly at 400 C even in the presence of the common industrial metals CHCl3þCl2!CCl4þHClDH ¼94:8kJ=mol ð5Þ (with the exception of aluminum and other light metals). Above 500 – 600 C an equilibrium Thermodynamic equilibrium lies entirely on reaction sets in the side of the chlorination products, so that the Vol. 9 Chloromethanes 21 distribution of the individual products is essen- It has further been shown that traces of oxygen tially determined by kinetic parameters. strongly inhibit the reaction. Controlling the high Monochloromethane can be used in place of heat of reaction in the gas phase (which averages methane as the starting material, where this in ca. 4200 kJ per m3 of converted chlorine) at STP is turn can be prepared from methanol by using a decisive factor in successfully carrying out the hydrogen chloride generated in the previous process. In industrial reactors, chlorine conver- processes. The corresponding reaction is: sion first becomes apparent above 250 to 270 C, but it increases exponentially with increasing CH3OHþHCl!CH3ClþH2ODH ¼33kJ=mol ð6Þ temperature [31], and in the region of commercial interest – 350 to 550 C – the reaction proceeds In this way, the unavoidable accumulation of very rapidly. As a result, it is necessary to initiate hydrogen chloride (hydrochloric acid) can be the process at a temperature which permits the substantially reduced and the overall process can reaction to proceed by itself, but also to maintain be flexibly tailored to favor the production of the reaction under adiabatic conditions at the individual chlorination products. Moreover, giv- requisite temperature level of 320 – 550 C dic- en the ease with which it can be transported and tated by both chemical and technical considera- stored, methanol is a better starting material for tions. If a certain critical temperature is exceeded the chloro derivatives than methane, a substance in the reaction mixture (ca. 550 – 700 C, depen- whose availability is tied to natural gas resources dent both on the residence time in the hot zone and or appropriate petrochemical facilities. There has on the materials making up the reactor), decom- been a distinct trend in recent years toward position of the metastable methane chlorination replacing methane as a carbon base with products occurs. In that event, the chlorination methanol. leads to formation of undesirable byproducts, including highly chlorinated or high molecular Methane Chlorination. The chlorination of masscompounds(tetrachloroethene,hexachloro- methane and monochloromethane is carried out ethane, etc.). Alternatively, the reaction with industrially by using thermal, photochemical, or chlorinecangetcompletelyoutofcontrol,leading catalytic methods [25]. The thermal chlorination to the separation of soot and evolution of HCl method is preferred, and it is also the one on (thermodynamicallythemoststableendproduct). which the most theoretical and scientific inves- Once such carbon formation begins it acts auto- tigations have been carried out. catalytically, resulting in a progressively heavier Thermal chlorination of methane and its chlo- buildup of soot, which can only be halted by rine derivatives is a radical chain reaction initi- immediate shutdown of the reaction. ated by chlorine atoms. These result from thermal Proper temperature control of this virtually dissociation at 300 – 350 C, and they lead to adiabatic chlorination is achieved by working successive substitution of the four hydrogen with a high methane : chlorine ratio in the range atoms of methane: of 6 – 4 : 1. Thus, a recycling system is employed in which a certain percentage of inert gas is maintained (nitrogen, recycled HCl, or even materials such as monochloromethane or tetra- chloromethane derived from methane chlorina- tion). In this way, the explosive limits of methane and chlorine are moved into a more favorable region and it becomes possible to prepare the more highly substituted chloromethanes with The conversion to the higher stages of chlori- lower CH :Cl ratios. nation follows the same scheme [26–30]. The 4 2 Figure 3 showstheexplosionrangeofmethane thermal reaction of methane and its chlorination and chlorine and how it can be limited through the products has been determined to be a second- use of diluents, using the examples of nitrogen, order process: hydrogen chloride, and tetrachloromethane. The composition and distribution of the pro- dnðCl2Þ=dt ¼ kpðCl2ÞpðCH4Þ ducts resulting from chlorination is a definite 22 Chloromethanes Vol. 9

Figure 3. Explosive range of CH4 –Cl2 mixtures containing N2, HCl, and CCl4 Test conditions: pressure 100 kPa; tem- perature 50 C; ignition by 1-mm spark function of the starting ratio of chlorine to meth- Figure 5. Product distribution in methane chlorination, ideal ane, as can be seen from Figure 4 and Figure 5. mixing reactor a) Methane; b) Monochloromethane; c) Dichloromethane; These relationships have been investigated d) Trichloromethane; e) Tetrachloromethane frequently [32, 33]. The composition of the reaction product has been shown to be in excel- lent agreement with that predicted by calcula- tions employing experimental relative reaction rate constants [34–37]. The products arising from thermal chlorination of monochloromethane and from the pyrolysis of primary products can also be predicted quantitatively [38]. The relation- ships among the rate constants are nearly inde- pendent of temperature in the region of technical interest. If one designates as k1 through k4 the successive rate constants in the chlorination process, then the following values can be as- signed to the relative constants for the individual stages:

k1 ¼ 1 (methane) k2 ¼ 2:91 (monochloromethane) k3 ¼ 2:0 (dichloromethane) k4 ¼ 0:72 (trichloromethane)

With this set of values, the selectivity of the chlorination can be effectively established with respect to optimal product distribution for reac- tors of various residence time (stream type or mixing type, cf. Fig. 4 and Fig. 5). Additional recycling into the reaction of partially chlorinat- Figure 4. Product distribution in methane chlorination, plug ed products (e.g., monochloromethane) permits streamreactor a) Methane; b) Monochloromethane; c) Dichloromethane; further control over the ratios of the individual d) Trichloromethane; e) Tetrachloromethane components [39, 40]. Vol. 9 Chloromethanes 23

It has been recognized that the yield of par- occurs at a temperature as low as 600 C. As a tially chlorinated products (e.g., dichloro- result of the influence of pressure and by the use methane and trichloromethane) is diminished by of a larger excess of chlorine, the equilibrium can recycling. This factor has to be taken into account be shifted essentially 100 % to the side of tetra- in the design of reactors for those methane chlor- chloromethane. These circumstances are utilized inations which are intended to lead exclusively to in the Hoechst high-pressure chlorinolysis pro- these products. If the emphasis is to lie more on cedure (see below) [41, 42]. the side of trichloro- and tetrachloromethane, then mixing within the reactor plays virtually no Methanol Hydrochlorination. Studies role, particularly since less-chlorinated materials have been conducted for purposes of reactor can always be partially or wholly recycled. De- design [43] on the kinetics of the gas-phase tails of reactor construction will be discussed reaction of hydrogen chloride with methanol in below in the context of each of the various the presence of aluminum oxide as catalyst to processes. give monochloromethane. Aging of the catalyst has also been investigated. The reaction is first Chlorinolysis. The technique for the pro- order in respect to hydrogen chloride, but nearly duction of tetrachloromethane is based on what independent of the partial pressure of methanol. is known as perchlorination, a method in which an The rate constant is proportional to the specific excess of chlorine is used and C1-toC3-hydro- surface of the catalyst, whereby at higher tem- carbons and their chlorinated derivatives are peratures (350 – 400 C) an inhibition due to pore employed as carbon sources. In this process, diffusion becomes apparent. tetrachloroethene is generated along with tetrachloromethane, the relationship between the two being consistent with Eq. 1 in page 13 and 4.2. Production of dependent on pressure and temperature (cf. also Monochloromethane Fig. 6). It will be noted that at low pressure (0.1 to 1 Monochloromethane is produced commercially MPa, 1 to 10 bar) and temperatures above 700 C, by two methods: by the hydrochlorination (es- conditions under which the reaction takes place terification) of methanol using hydrogen chlo- at an acceptable rate, a significant amount of ride, and by chlorination of methane. Methanol tetrachloroethene arises. For additional details hydrochlorination has become increasingly see ! Chlorethanes and Chloroethylenes, Sec- important in recent years, whereas methane chlo- tion 2.5.. Under conditions of high pressure – rination as the route to monochloromethane as greater than 10 MPa (100 bar) – the reaction final product has declined. The former approach has the advantage that it utilizes, rather than generating, hydrogen chloride, a product whose disposal – generally as hydrochloric acid – has become increasingly difficult for chlorinated hydrocarbon producers. Moreover, this method leads to a single target product, monochloro- methane, in contrast to methane chlorination (cf. Figs. 4 and 5). As a result of the ready and low- cost availability of methanol (via the low pres- sure methanol synthesis technique) and its facile transport and storage, the method also offers the advantage of avoiding the need for placing pro- duction facilities in the vicinity of a methane supply. Since in the chlorination of methane each substitution of a chlorine atom leads to genera- Thermodynamic equilibrium 2 CCl C Cl þ 2 Figure 6. 4 2 4 tion of an equimolar amount of hydrogen chlo- Cl2 a) 0.1 MPa; b) 1 MPa; c) 10 MPa ride – cf. Eqs. 2 – 5 – a combination of the two 24 Chloromethanes Vol. 9 methods permits a mixture of chlorinated filled with aluminum oxide. Removal of heat methanes to be produced without creating large generated by the reaction (33 kJ/mol) is accom- amounts of hydrogen chloride at the same time; plished by using a heat conduction system. A hot cf. Eq. 6. spot forms in the catalyst layer as a result of the Monochloromethane production from metha- exothermic nature of the reaction, and this nol and hydrogen chloride is carried out catalyti- migrates through the catalyst packing, reaching cally in the gas phase at 0.3 – 0.6 MPa (3 – 6 bar) the end as the latter’s useful life expires. and temperatures of 280 – 350 C. The usual The reaction products exiting the reactor are catalyst is activated aluminum oxide. Excess cooled with recycled hydrochloric acid (> 30 %) hydrogen chloride is introduced in order to pro- in a subsequent quench system, resulting in sep- vide a more favorable equilibrium point (located aration of byproduct water, removed as ca. 20 % 96 – 99 % on the side of products at 280 – 350 C) hydrochloric acid containing small amounts of and to reduce the formation of dimethyl ether as a methanol. Passage through a heat exchanger side product (0.2 to 1 %). effects further cooling and condensation of more The raw materials must be of high purity in water, as well as removal of most of the excess order to prolong catalyst life as much as possible. HCl. The quenching fluid is recovered and sub- Technically pure (99.9 %) methanol is em- sequently returned to the quench circulation sys- ployed, along with very clean hydrogen chloride. tem. The gaseous crude product is led from the In the event that the latter is obtained from separator into a 96 % sulfuric acid column, where hydrochloric acid, it must be subjected to special dimethyl ether and residual water (present in a purification (stripping) in order to remove inter- quantity reflective of its partial vapor pressure) fering chlorinated hydrocarbons. are removed, the concentration of the acid dimin- ishing to ca. 80 % during its passage through the Process Description. In a typical produc- column. In this step, dimethyl ether reacts with tion plant (Fig. 7), the two raw material streams, sulfuric acid to form ‘‘onium salts’’ and methyl hydrogen chloride and methanol, are warmed sulfate. It can be driven out later by further over heat exchangers and led, after mixing and dilution with water. It is advantageous to use the additional preheating, into the reactor, where recovered sulfuric acid in the production of fer- conversion takes place at 280 – 350 C and ca. tilizers (superphosphates) or to direct it to a 0.5 MPa (5 bar). sulfuric acid cleavage facility. The reactor itself consists of a large number of Dry, crude monochloromethane is subse- relatively thin nickel tubes bundled together and quently condensed and worked up in a high-

Figure 7. Production of monochloromethane by methanol hydrochlorination a) Heat exchangers; b) Heater; c) Multiple-tube reactor; d) Quench system; e) Quench gas cooler; f) Quenching fluid tank; g) Sulfuric acid column; h) CH3Cl condensation; i) Intermediate tank; j) CH3Cl distillation column Vol. 9 Chloromethanes 25

pressure (2 MPa, 20 bar) distillation column to ane reacts with phosgene at 400 C to give CH3Cl give pure liquid monochloromethane. The gas- [51]. The methyl acetate – methanol mixture that eous product emerging from the head of this arises during polyvinyl alcohol synthesis can be column (CH3Cl þ HCl), along with the liquid converted to monochloromethane with HCl at distillation residue – together making up ca. 100 C in the presence of catalysts [52]. It has 5 – 15 % of the monochloromethane product also been suggested that monochloromethane mixture – can be recovered for introduction into could be made by the reaction of methanol with an associated methane chlorination facility. The the ammonium chloride that arises during sodi- overall yield of the process, calculated on the um carbonate production [53]. basis of methanol, is ca. 99 %. The dimethyl ether which results from meth- The commonly used catalyst for vapor-phase ylcellulose manufacture can be reacted with hydrochlorination of methanol is g-aluminum hydrochloric acid to give monochloromethane oxide with an active surface area of ca. 200 [54]. The process is carried out at 80 – 240 C m2/g. Catalysts based on silicates have not under sufficient pressure so that water remains as achieved any technical significance. Catalyst a liquid. Similarly, cleavage of dimethyl ether aging can be ascribed largely to carbon deposi- with antimony trichloride also leads to mono- tion. Byproduct formation can be minimized chloromethane [55]. and catalyst life considerably prolonged by In methanolysis reactions for the manufacture doping the catalyst with various components of silicones, monochloromethane is recovered and by introduction of specific gases (O2) into and then reintroduced into the process of silane the reaction components [44]. The life of the formation [56]: catalyst in a production facility ranges from SiCl ðCH Þ þ2CH OH!SiðOHÞ ðCH Þ þ2CH Cl ð10Þ about 1 to 2 years. 2 3 2 3 2 3 2 3

Siþ2CH3Cl!SiCl2ðCH3Þ2 ð11Þ Liquid-Phase Hydrochlorination. The once common liquid-phase hydrochlorination of methanol using 70 % zinc chloride solution at 130 – 150 C and modest pressure is currently 4.3. Production of Dichloromethane of lesser significance. Instead, new production and Trichloromethane techniques involving treatment of methanol with hydrogen chloride in the liquid phase The industrial synthesis of dichloromethane also without the addition of catalysts are becoming leads to trichloromethane and small amounts of preeminent. The advantage of these methods, tetrachloromethane, as shown in Figure 4 and apart from circumventing the need to handle Figure 5. Consequently, di- and trichloromethane the troublesome zinc chloride solutions, is that are prepared commercially in the same facilities. they utilize aqueous hydrochloric acid, thus In order to achieve an optimal yield of these obviating the need for an energy-intensive products and to ensure reliable temperature hydrochloric acid distillation. The disadvantage control, it is necessary to work with a large of the process, which is conducted at 120 – methane and/or monochloromethane excess rel- 160 C, is its relatively low yield on a space – ative to chlorine. Conducting the process in this time basis, resulting in the need for large reaction way also enables the residual concentration of volumes [45–47]. chlorine to be kept in the fully reacted product at an exceptionally low level (< 0.01 vol%), which Other Processes. Other techniques for pro- in turn simplifies workup. Because of the large ducing monochloromethane are of theoretical excess of carbon-containing components, the significance, but are not applied commercially. operation is customarily accomplished in a re- Monochloromethane is formed when a mix- cycle mode. ture of methane and oxygen is passed into the electrolytes of an alkali chloride electrolysis Process Description. One of the oldest pro- [48]. Treatment of dimethyl sulfate with alumi- duction methods is that of Hoechst, a recycle num chloride [49] or sodium chloride [50] results chlorination which was introduced as early as in the formation of monochloromethane. Meth- 1923 and which, apart from modifications reflect- 26 Chloromethanes Vol. 9

Figure 8. Methane chlorination by the Hoechst method (production of dichloromethane and trichloromethane) a) Loop reactor; b) Process gas cooler; c) HCl absorption; d) Neutralization system; e) Compressor; f) First condensation step (water); g) Gas drying system; h) Second condensation system and crude product storage vessel (brine); i) Distillation columns for CH3Cl, CH2Cl2, and CHCl3 ing state-of-the-art technology, continues essen- mately 70 wt % dichloromethane, is tially unchanged, retaining its original impor- approximately 27 wt % trichloromethane, and tance. The process is shown in Figure 8. 3 wt % tetrachloromethane. The gas which is circulated consists of a Methane chlorination is carried out in a simi- mixture of methane and monochloromethane. lar way by Chemische Werke Huls€ AG, whose To this is added fresh methane and, as appropri- work-up process employs prior separation of ate, monochloromethane obtained from metha- hydrogen chloride by means of an adiabatic nol hydrochlorination. Chlorine is then intro- absorption system. After the product gas has duced and the mixture is passed into the reactor. been washed to neutrality with sodium hydrox- The latter is a loop reactor coated with nickel or ide, it is dried with sulfuric acid and compressed highalloy steel in which internal gas circulation is to ca. 0.8 MPa (8 bar), whereby the majority of constantly maintained by means of a coaxial inlet the resulting chloromethanes can be condensed tube and a valve system. The reaction is con- with relatively little cooling (at approximately ducted adiabatically, the necessary temperature 12 to 15 C). Monochloromethane is recycled of 350 – 450 C being achieved and maintained to the chlorination reactor. The subsequent work- by proper choice of the chlorine to starting up to pure products is essentially analogous to material (CH4 þ CH3Cl) ratio and/or by pre- that employed in the Hoechst process. warming the mixture [57]. The fully reacted gas Other techniques, e.g., those of Montecatini mixture is cooled in a heat exchanger and passed and Asahi Glass, function similarly with respect through an absorber cascade in which dilute to drying and distillation of the products. hydrochloric acid and water wash out the result- The loop reactor used by these and other ing hydrogen chloride in the form of 31 % hydro- manufacturers (e.g., Stauffer Chem. Co.) [58] chloric acid. The last traces of acid and chlorine has been found to give safe and trouble-free are removed by washing with sodium hydroxide, service, primarily because the internal circula- after which the gases are compressed, dried, and tion in the reactor causes the inlet gases to be cooled and the reaction products largely con- brought quickly to the initiation temperature, densed. Any uncondensed gas – methane and to thereby excluding the possibility of formation some extent monochloromethane – is returned to of explosive mixtures. This benefit is achieved at the reactor. The liquified condensate is separated the expense of reduced selectivity in the conduct by distillation under pressure into its pure com- of the reaction, however (cf. Figs. 4 and 5). In ponents, monochloromethane, dichloromethane, contrast, the use of an empty tube reactor with trichloromethane (the latter two being the prin- minimal axial mixing has unquestionable advan- cipal products), and small amounts of tetrachlor- tages for the selective preparation of dichloro- omethane. The product composition is approxi- methane [59, 60]. The operation of such a reactor Vol. 9 Chloromethanes 27 is considerably more complex, however, espe- boiling components, the low-boiling materials cially from the standpoint of measurement and from the first reactor, including hydrogen chlo- control technology, since the starting gases need ride, are further treated with chlorine in a second to be brought up separately to the ignition tem- reactor. Reactors of this kind must be constructed perature and then, after onset of the reaction with of special materials with high resistance to both its high enthalpy, heat must be removed by means erosion and corrosion. Special steps are required of a cooling system. By contrast, maintenance of (e.g., washing with liquid chloromethanes) to constant temperature in a loop reactor is relative- remove from the reaction gas dust derived from ly simple because of the high rate of gas circula- the fluidized-bed solids. tion. A system operated by Frontier Chem. Co. employs a tube reactor incorporating recycled Raw Materials. Very high purity standards tetrachloromethane for the purpose of tempera- must be applied to methane which is to be ture control [61]. chlorinated. Some of this methane is derived from petrochemical facilities in the course of Reactor Design. Various types of reactors naphtha cleavage to ethylene and propene, are in use, with characteristics ranging between whereas some comes from low-temperature dis- those of fully mixing reactors (e.g., the loop tillation of natural gas (the Linde process). Com- reactor) and tubular reactors. Chem. Werke Huls€ ponents such as ethane, ethylene, and higher operates a reactor that permits partial mixing, hydrocarbons must be reduced to a minimum. thereby allowing continuous operation with little Otherwise, these would also react under the or no preheating. conditions of methane chlorination to give the Instead of having the gas circulation take corresponding chlorinated hydrocarbons, which place within the reactor, an external loop can would in turn cause major problems in the puri- also be used for temperature control, as, e.g., in fication of the chloromethanes. For this reason, the process described by Montecatini [62] and every effort is made to maintain the level of used in a facility operated by Allied Chemical higher hydrocarbons below 100 mL/m3. Inert Corp. In this case, chlorine is added to the reacted gases such as nitrogen and carbon dioxide (but gases outside of the chlorination reactor, neces- excluding oxygen) have no significant detrimen- sary preheating is undertaken, and only then is tal effect on the thermal chlorination reaction, the gas mixture led into the reactor. apart from the fact that their presence in exces- The space – time yield and the selectivity of sive amounts results in the need to eliminate the chlorination reaction can be increased by considerable quantities of off-gas from the re- operating two reactors in series, these being cycling system, thus causing a reduction in prod- separated by a condensation unit to remove uct yield calculated on the basis of methane high-boiling chloromethanes [63]. introduced. Solvay [64] has described an alternative Chlorine with a purity of ca. 97 % (residue: means of optimizing the process in respect to hydrogen, carbon dioxide, and oxygen) is com- selectivity, whereby methane and monochloro- pressed and utilized just as it emerges from methane are separately chlorinated in reactors electrolysis. Newer chlorination procedures are driven in parallel. The monochloromethane pro- designed to utilize gaseous chlorine of higher duced in the methane chlorination reactor is purity, obtained by evaporization of previously isolated and introduced into the reactor for chlo- liquified material. rination of monochloromethane, which is also Similarly, monochloromethane destined for supplied with raw material from a methanol further chlorination is a highly purified product hydrochlorination system. The reaction is carried of methanol hydrochlorination, special proce- out at a pressure of 1.5 MPa (15 bar) in order to dures being used to reduce the dimethyl ether simplify the workup and separation of products. content, for example, to less than 50 mL/m3. Because of its effective heat exchange char- Depending on the level of impurities present acteristics, a fluidized-bed reactor is used by in the starting materials, commercial processes Asahi Glass Co. for methane chlorination [65]. incorporating recycling can lead to product The reaction system consists of two reactors yields of 95 – 99 % based on chlorine or 70 – connected in series. After separation of higher 85 % based on methane. The relatively low 28 Chloromethanes Vol. 9 methane-based yield is a consequence of the need hydrogen chloride can only be isolated by distil- for removal of inert gases, although the majority lation to the point of azeotrope formation (20 % of this exhaust gas can be subjected to further HCl). recovery measures in the context of some asso- Newer technologies have as their goal workup ciated facility. of the chlorination off-gas by dry methods. These permit use of less complicated construction ma- Off-Gas Workup. The workup of off-gas terials. Apart from the reactors, in which nickel from thermal methane chlorination is relatively and nickel alloys are normally used, all other complicated as a consequence of the methane apparatus and components can be constructed of excess employed. Older technologies accom- either ordinary steel or stainless steel. plished the separation of the hydrogen chloride Hydrogen chloride can be removed from the produced in the reaction through its absorption in off-gas by an absorption – desorption system de- water or azeotropic hydrochloric acid, leading to veloped by Hoechst AG and utilizing a wash with ordinary commercial 30 – 31 % hydrochloric monochloromethane, in which hydrogen chlo- acid. This kind of workup requires a major outlay ride is very soluble [66]. A similar procedure for materials of various sorts: on the one hand, involving HCl removal by a wash with trichlor- coatings must be acid-resistant but at the same omethane and tetrachloromethane has been de- time, materials which are stable against attack by scribed by Solvay [64]. chlorinated hydrocarbons are required. A further disadvantage frequently plagues Other Processes. The relatively complicat- these ‘‘wet’’ processes is the need to find a use ed removal of hydrogen chloride from methane for the inevitable concentrated hydrochloric acid, can be avoided by adopting processes that begin particularly given that the market for hydrochlo- with methanol as raw material. An integrated ric acid is in many cases limited. Hydrogen chlorination/hydrochlorination facility (Fig. 9) chloride can be recovered from the aqueous has been developed for this purpose and brought hydrochloric acid by distillation under pressure, on stream on a commercial scale by Stauffer permitting its use in methanol hydrochlorination; Chem. Co. [67]. alternatively, it can be utilized for oxychlorina- Monochloromethane is caused to react with tion of ethylene to 1,2-dichloroethane. Disadvan- chlorine under a pressure of 0.8 – 1.5 MPa (8 – 15 tages of this approach, however, are the relatively bar) at elevated temperature (350 – 400 C) with high energy requirement and the fact that the subsequent cooling occurring outside of the

Figure 9. Chlorination of monochloromethane by the Stauffer process [68] a) Chlorination reactor; b) Quench system; c) Multistage condensation; d) Crude product storage vessel; e) Drying; f) Distillation and purification of CH2Cl2 and CHCl3; g) Hydrochlorination reactor; h) Quench system; i) H2SO4 drying column; j) Compressor Vol. 9 Chloromethanes 29 reactor. The crude reaction products are separat- unacceptably high loss of methane through ed in a multistage condensation unit and then combustion. worked up by distillation to give the individual In this context, the ‘‘Transcat’’ process of the pure components. Monochloromethane is re- Lummus Co. is of commercial interest [70]. In turned to the reactor. After condensation, gas- this process, methane is chlorinated and oxy- eous hydrogen chloride containing small chlorinated in two steps in a molten salt mixture amounts of monochloromethane is reacted with comprised of copper(II) chloride and potassium methanol in a hydrochlorination system corre- chloride. The starting materials are chlorine, air, sponding to that illustrated in Figure 7 for the and methane. The process leaves virtually no production of monochloromethane. Following residue since all of its byproducts can be its compression, monochloromethane is returned recycled. to the chlorination reactor. This process is distin- Experiments involving treatment of methane guished by the fact that only a minimal amount with other chlorinating agents (e.g., phosgene, of the hydrogen chloride evolved during the nitrosyl chloride, or sulfuryl chloride) have failed synthesis of dichloromethane and trichloro- to yield useful results. The fluidized-bed reaction methane is recovered in the form of aqueous of methane with tetrachloromethane at 350 to hydrochloric acid. 450 C has also been suggested [71]. As a substitute for thermal chlorination at high The classical synthetic route to trichloro- temperature, processes have also been developed methane proceeded from the reaction of chlorine which occur by a photochemically-initiated rad- with ethanol or acetaldehyde to give chloral, ical pathway. According to one patent [68], which can be cleaved with calcium hydroxide monochloromethane can be chlorinated selec- to trichloromethane and calcium formate [72]. tively to dichloromethane at 20 C by irradia- Trichloromethane and calcium acetate can also tion with a UV lamp, the trichloromethane con- be produced from acetone using an aqueous tent being only 2 – 3 %. A corresponding reaction solution of chlorine bleach at 60 – 65 C. A with methane is not possible. description of these archaic processes can be Liquid-phase chlorination of monochloro- found in [73]. methane in the presence of radical-producing agents such as azodiisobutyronitrile has been achieved by the Tokuyama Soda Co. The reac- 4.4. Production of tion occurs at 60 – 100 C and high pressure [69]. The advantage of this low-temperature reaction Tetrachloromethane is that it avoids the buildup of side products Chlorination of Carbon Disulfide, The common in thermal chlorination (e.g., chlorinat- chlorination of carbon disulfide was, until the ed C2-compounds such as 1,1-dichloroethane, late 1950s, the principal means of producing 1,2-dichloroethene, and trichloroethene). Heat tetrachloromethane, according to the following generated in the reaction is removed by evapo- overall reaction: ration of the liquid phase, which is subsequently condensed. Hydrogen chloride produced during CS2þ2Cl2!CCl4þ2S ð12Þ the chlorination is used for gas-phase hydro- The resulting sulfur is recycled to a reactor for chlorination of methanol to give mono- conversion with coal or methane (natural gas) to chloromethane, which is in turn recycled for carbon disulfide. A detailed look at the reaction chlorination. shows that it proceeds in stages corresponding to It is tempting to try to avoid the inevitable the following equations: production of hydrogen chloride by carrying out the reaction in the presence of oxygen, as in the 2CS2þ6Cl2!2CCl4þ2S2Cl2 ð13Þ oxychlorination of ethylene or ethane. Despite intensive investigations into the prospects, how- CS2þ2S2Cl2 CCl4þ6S ð14Þ ever, no commercially feasible applications have The process developed at the Bitterfeld resulted. The low reactivity of methane requires plant of I.G. Farben before World War II was the use of a high reaction temperature, but this in improved by a number of firms in the United turn leads to undesirable side products and an States, including FMC and the Stauffer Chem. 30 Chloromethanes Vol. 9

Co. [74–76], particularly with respect to purifi- processes, such as those derived from methane cation of the tetrachloromethane and the result- chlorination, vinyl chloride production (via ei- ing sulfur. ther direct chlorination or oxychlorination In a first step, carbon disulfide dissolved in of ethylene), allyl chloride preparation, etc. tetrachloromethane is induced to react with chlo- The course of the reaction is governed by the rine at temperatures of 30 – 100 C. Either iron or position of equilibrium between tetrachloro- iron(III) chloride is added as catalyst. The con- methane and tetrachloroethene, as illustrated version of carbon disulfide exceeds 99 % in this earlier in Figure 6, whereby the latter always step. In a subsequent distillation, crude tetra- arises as a byproduct. In general, these processes chloromethane is separated at the still head. The are employed for the production of tetrachlor- disulfur dichloride recovered from the still pot is oethylene (see ! and Chlor- transferred to a second stage of the process where oethylenes, Section 2.5.3. and [77]), in which it is consumed by reaction with excess carbon case tetrachloromethane is the byproduct. Most disulfide at ca. 60 C. The resulting sulfur is production facilities are sufficiently flexible such separated (with cooling) as a solid, which has that up to 70 wt % tetrachloromethane can be the effect of shifting the equilibrium in the reac- achieved in the final product [78]. The product tions largely to the side of tetrachloromethane. yield can be largely forced to the side of tetra- Tetrachloromethane and excess carbon disulfide chloromethane by recycling tetrachloroethylene are withdrawn at the head of a distillation appa- into the chlorination reaction, although the re- ratus and returned to the chlorination unit. A quired energy expenditure is significant. Higher considerable effort is required to purify the tetra- pressure [79] and the use of hydrocarbons con- chloromethane and sulfur, entailing hydrolysis of taining an odd number of carbon atoms increases sulfur compounds with dilute alkali and subse- the yield of tetrachloromethane. When the reac- quent azeotropic drying and removal from the tion is carried out on an industrial scale, a tem- molten sulfur by air stripping of residual disulfur perature of 500 to 700 C and an excess of dichloride. Yields lie near 90 % of the theoretical chlorine are used. The corresponding reactors value based on carbon disulfide and about 80 % either can be of the tube type, operated adiabati- based on chlorine. The losses, which must be cally by using a recycled coolant (N2, HCl, CCl4, recovered in appropriate cleanup facilities, result or C2Cl4) [80–82], or else they can be fluidized- from gaseous emissions from the chlorination bed systems operated isothermally [83, 84]. By- reaction, from the purification systems (hydroly- products under these reaction conditions include sis), and from the molten sulfur processing. ca. 1 – 7 % perchlorinated compounds (hexachlo- The carbon disulfide method is still employed roethane, hexachlorobutadiene, hexachloroben- in isolated plants in the United States, Italy, and zene), the removal of which requires an addition- Spain. Its advantage is that, in contrast to chlorine al expenditure of effort. substitution on methane or chlorinating cleavage Pyrolytic introduction of chlorine into chlori- reactions, no accumulation of hydrogen chloride nated hydrocarbons has become increasingly or hydrochloric acid byproduct occurs. important due to its potential for consuming chlorinated hydrocarbon wastes and residues Perchlorination (Chlorinolysis). Early in from other processes. Even the relatively high the 1950s commercial production of tetrachlor- production of hydrogen chloride can be tolerated, omethane based on high-temperature chlorina- provided that reactors are used which operate at tion of methane and chlorinating cleavage reac- high pressure and which can be coupled with tions of hydrocarbons ( C3) and their chlorinat- other processes that consume hydrogen chloride. ed derivatives was introduced. In processes of Another advantage of the method is that it can be this sort, known as perchlorinations or chlorino- used for making both tetrachloromethane and lyses, substitution reactions are accompanied by tetrachloroethylene. The decrease in demand for rupture of C – C bonds. Starting materials, in tetrachloromethane in the late 1970s and early addition to ethylene, include propane, propene, 1980s, a consequence of restrictions (related to dichloroethane, and dichloropropane. Increasing the ozone hypothesis) on the use of chlorofluor- use has been made of chlorine-containing by- ocarbons prepared from it, has led to stagnation products and the residues from other chlorination in the development of new production capacity. Vol. 9 Chloromethanes 31

Hoechst High-Pressure Chlorinolysis. chlorination facilities and high-boiling residues The high-pressure chlorinolysis method devel- from vinyl chloride production). oped and put in operation by Hoechst AG has the A number of serious technical problems had to same goals as the process just described. It can be be overcome in the development of this process, seen in Figure 6 that under the reaction condi- including perfection of the nickel-lined high- tions of this process – 620 C and 10 to 15 MPa pressure reactor, which required the design of (100 to 150 bar) – the equilibrium special flange connections and armatures.

2CCl C Cl þ2Cl 4 2 4 2 Multistep Chlorination Process. Despite lies almost exclusively on the side of tetra- the fact that its stoichiometry results in high chloromethane, especially in the presence of an yields of hydrogen chloride or hydrochloric acid, excess of chlorine [41, 42, 85]. This method thermal chlorination of methane to tetrachloro- utilizes chlorine-containing residues from other methane has retained its decisive importance. processes (e.g., methane chlorination and vinyl Recent developments have assured that the re- chloride) as raw material, although these must be sulting hydrogen chloride can be fed into other free of sulfur and cannot contain solid or poly- processes which utilize it. In principle, tetra- merized components. chloromethane can be obtained as the major The conversion of these materials is carried product simply by repeatedly returning all of the out in a specially constructed high-pressure tube lower boiling chloromethanes to the reactor. It is reactor which is equipped with a pure nickel liner not possible to employ a 1 : 4 mixture of the to prevent corrosion. Chlorine is introduced in reactants methane and chlorine at the outset. This excess in order to prevent the formation of by- is true not only because of the risk of explosion, products and in order maintain the final reaction but also because of the impossibility of dealing temperature (620 C) of this adiabatically con- with the extremely high heat of reaction. Unfor- ducted reaction. If hydrogen-deficient starting tunately, the simple recycling approach is also materials are to be employed, hydrogen-rich uneconomical because it necessitates the avail- components must be added to increase the en- ability of a very large workup facility. Therefore, thalpy of the reaction. In this way, even chlorine- it is most advantageous to employ several reac- containing residues containing modest amounts tors coupled in series, the exit gases of each being of aromatics can be utilized. Hexachloroben- cooled, enriched with more chlorine, and then zene, for example, can be converted (albeit rela- passed into the next reactor [86]. Processes em- tively slowly) at the usual temperature of this ploying supplementary circulation of an inert gas process and in the presence of excess chlorine to (e.g., nitrogen) have also been suggested [87]. tetrachloromethane according to the equilibrium The stepwise chlorination of methane and/or reaction: monochloromethane to tetrachloromethane is based on a process developed in the late 1950s

C6Cl6þ9Cl2 6CCl4 and still used by Hoechst AG (Fig. 10) [88]. The first reactor in a six-stage reactor cascade The mixture exiting the reactor is comprised is charged with the full amount of methane and/or of tetrachloromethane, the excess chlorine, hy- monochloromethane required for the entire pro- drogen chloride, and small amounts of hexa- duction batch. Nearly quantitative chlorine con- chlorobenzene, the latter being recycled. This version is achieved in the first reactor at 400 C, mixture is quenched with cold tetrachloro- using only a portion of the necessary overall methane, its pressure is reduced, and it is subse- amount of chlorine. The gas mixture leaving quently separated into crude tetrachloromethane the first reactor is cooled and introduced into the and chlorine and hydrogen chloride. The crude second reactor along with additional chlorine, the product is purified by distillation to give tetra- mixture again being cooled after all of the added chloromethane meeting the required specifica- chlorine has been consumed. This stepwise ad- tions. This process is advantageous in those dition of chlorine with intermittent cooling is situations in which chlorine-containing residues continued until in the last reactor the component accumulate which would otherwise be difficult to ratio CH4 :Cl2 ¼ 1 : 4 is reached. The reactors deal with (e.g., hexachloroethane from methane themselves are loop reactors with internal circu- 32 Chloromethanes Vol. 9

Figure 10. Production of tetrachloromethane by stepwise chlorination of methane (Hoechst process) a) Reactor; b) Cooling; c) First condensation (air); d) Second condensation (brine); e) Crude product storage vessel; f) Degassing/dewatering column; g) Intermediate tank; h) Light-end column; i) Column for pure CCl4; j) Heavy-end column; k) HCl stream for hydrochlorination; l) Adiabatic HCl absorption; m) Vapor condensation; n) Cooling and phase separation; o) Off-gas cooler lation, a design which, because of its efficient the quench system is directed to a stripping mixing, effectively shifts the product distribution column where it is purified prior to being dis- toward more highly chlorinated materials. The carded. Residual off-gas is largely freed from gas mixture leaving the reactors is cooled in two remaining traces of halogen compounds by low- stages to 20 C, in the course of which the temperature cooling and are subsequently passed majority of the tetrachloromethane is liquified, through an off-gas purification system (activated along with the less chlorinated methane deriva- charcoal) before being released into the atmo- tives (amounting to ca. 3 % of the tetrachloro- sphere, by which point the gas consists mainly of methane content). This liquid mixture is then nitrogen along with traces of methane. accumulated in a crude product storage vessel. The liquids which have been collected in the The residual gas stream is comprised largely crude product containment vessel are freed of of hydrogen chloride but contains small amounts gaseous components – Cl2, HCl, CH3Cl – by of less highly chlorinated materials. This is sub- passage through a degassing/dehydrating col- jected to adiabatic absorption of HCl using either umn, traces of water being removed by distilla- water or azeotropic (20 %) hydrochloric acid, tion. Volatile components are returned to the whereby technical grade 31 % hydrochloric acid reaction system prior to HCl absorption. The is produced. Alternatively, dry hydrogen chlo- crude product is then worked up to pure carbon ride can be withdrawn prior to the absorption tetrachloride in a multistage distillation facility. step, which makes it available for use in other Foreruns (light ends) removed in the first column processes which consume hydrogen chloride are returned to the appropriate stage of the reactor (e.g., methanol hydrochlorination). The steam cascade. The residue in the final column (heavy which arises during the adiabatic absorption is ends), which constitutes 2 – 3 wt % of the withdrawn from the head of the absorption col- tetrachloromethane production, is made up of umn and condensed in a quench system. The hexachloroethane, tetrachloroethylene, trichlo- majority of the chloromethanes contained in this roethylene, etc. This material can be converted outflow can be separated by subsequent cooling advantageously to tetrachloromethane in a high- and phase separation. Wastewater exiting from pressure chlorinolysis unit. Vol. 9 Chloromethanes 33

Overall yields in the process are ca. 95 % vent with the broadest spectrum of applications, based on methane and > 98 % based on chlorine. is also distributed in an especially pure form (> 99.99 wt %) for such special applications as Other Processes. Oxychlorination as a way the extraction of natural products. of producing tetrachloromethane (as well as par- Monochloromethane and tetrachloromethane tially chlorinated compounds) has repeatedly do not require the presence of any stabilizer. been the subject of patent documents [89–91], Dichloromethane and trichloromethane, on the particularly since it leads to complete utilization other hand, are normally protected from adverse of chlorine without any HCl byproduct. Pilot- influences of air and moisture by the addition of plant studies using fluidized-bed technology small amounts of efficient stabilizers. The fol- have not succeeded in solving the problem lowing substances in the listed concentration of the high rate of combustion of methane. ranges are the preferred additives: On the other hand the Transcat process, a two- stage approach mentioned in page 13 and em- bodying fused copper salts, can be viewed more Ethanol 0.1 – 0.2 wt % Methanol 0.1 – 0.2 wt % positively. Cyclohexane 0.01 – 0.03 wt % Direct chlorination of carbon to tetrachloro- Amylene 0.001 – 0.01 wt % methane is thermodynamically possible at atmo- spheric pressure below 1100 K, but the rate of the Other substances have also been described as reaction is very low because of the high activa- being effective stabilizers, including phenols, tion energy (lattice energy of graphite). Sulfur amines, nitroalkanes, aliphatic and cyclic ethers, compounds have been introduced as catalysts in epoxides, esters, and nitriles. these experiments. Charcoal can be chlorinated Trichloromethane of a quality corresponding to tetrachloromethane in the absence of catalyst to that specified in the Deutsche Arzneibuch, 8th with a yield of 17 % in one pass at 900 to 1100 K edition (D.A.B. 8), is stabilized with 0.6 – 1 wt % and 0.3 – 2.0 MPa (3 – 20 bar) pressure. None of ethanol, the same specifications as appear in the these suggested processes has been successfully British Pharmacopoeia (B.P. 80). Trichloro- introduced on an industrial scale. A review of methane is no longer included as a substance in direct chlorination of carbon is found in [92]. the U.S. Pharmacopoeia, it being listed only in In this context it is worth mentioning the the reagent index and there without any dismutation of phosgene specifications.

2COCl2!CCl4þCO2 5.2. Analysis another approach which avoids the formation of hydrogen chloride. This reaction has been stud- Table 6 lists those classical methods for testing ied by Hoechst [93] and occurs in the presence of the purity and identity of the chloromethanes that 10 mol% tungsten hexachloride and activated are most important to both producers and con- charcoal at 370 to 430 C and a pressure of 0.8 sumers. Since the majority of these are methods MPa. The process has not acquired commercial with universal applicability, the corresponding significance because the recovery of the WCl6 is very expensive. Table 6. Analytical testing methods for chloromethanes

Method 5. Quality Specifications Parameter DIN ASTM 5.1. Purity of the Commercial Boiling range 51 751 D 1078 Density 51 757 D 2111 Products and their Stabilization Refraction index 53 491 D 1218 Evaporation residue 53 172 D 2109 The standard commercial grades of all of the Color index (Hazen) 53 409 D 1209 chloromethanes are distinguished by their high Water content (K. Fischer) 51 777 D 1744 purity (> 99.9 wt %). Dichloromethane, the sol- pH value in aqueous extract – D 2110 34 Chloromethanes Vol. 9

Deutsche Industrie Norm (DIN) and American structed of iron or steel. The most suitable mate- Society for the Testing of Materials (ASTM) rial for use with products of very high purity is recommendations are also cited in the Table. stainless steel (material no. 1.4 571). The use in Apart from these test methods, gas chroma- storage and transport vessels of aluminum and tography is also employed for quality control other light metals or their alloys is prevented by both in the production and shipment of chloro- virtue of their reactivity with respect to the methanes. Gas chromatography is especially chloromethanes. applicable to chloromethanes due to their low Storage vessels must be protected against the boiling point. Even a relatively simple chromato- incursion of moisture. This can be accomplished graph equipped only with a thermal conductivity by incorporating in their pressure release systems (TC) detector can be highly effective at detecting containers filled with drying agents such as silica impurities, usually with a sensitivity limit of a gel, aluminum oxide, or calcium chloride. Alter- few parts per million (mg/kg). natively, the liquids can be stored under a dry, inert gas. Because of its very low boiling point, dichloromethane is sometimes stored in 6. Storage, Transport, and Handling containers provided either with external water cooling or with internal cooling units installed in Dry monochloromethane is inert with respect to their pressure release systems. most metals, thus permitting their presence dur- Strict specifications with respect to safety ing its handling. Exceptions to this generaliza- considerations are applied to the storage and tion, however, are aluminum, zinc, and magne- transfer of chlorinated hydrocarbons in order to sium, as well as their alloys, rendering these prevent spillage and overfilling. Illustrative is the unsuitable for use. Thus most vessels for the document entitled ‘‘Rules Governing Facilities storage and transport of monochloromethane are for the Storage, Transfer, and Preparation for preferentially constructed of iron and steel. Shipment of Materials Hazardous to Water Sup- Since it is normally handled as a compressed plies’’ (‘‘Verordnung fur€ Anlagen zum Lagern, gas, monochloromethane must, in the Federal Abfullen€ und Umschlagen wassergef€ahrdender Republic of Germany, be stored in accord with Stoffe’’, VAwS). Facilities for this purpose Accident Prevention Regulation (Unfall- must be equipped with the means for safely verhutungsvorschrift,€ UVV) numbers 61 and recovering and disposing of any material which 62 bearing the title ‘‘Gases Which Are Com- escapes [103]. pressed, Liquified, or Dissolved Under Pressure’’ Shipment of solvents normally entails the use (‘‘Verdichtete, verflussigte,€ oder unter Druck of one-way containers (drums, barrels) made of geloste€ Gase’’) and issued by the Trade Federa- steel and if necessary coated with protective tion of the Chemical Industry (Verband der paint. Where product quality standards are un- Berufsgenossenschaften der chemischen Indus- usually high, especially as regards minimal resi- trie). Additional guidelines are provided by gen- due on evaporation, stainless steel is the material eral regulations governing high-pressure storage of choice. containers. Stored quantities in excess of 500 t Larger quantities are shipped in containers, also fall within the jurisdiction of the Emergency railroad tank cars, tank trucks, and tankers of both Regulations (Storfallverordnung)€ of the German the transoceanic and inland-waterway variety. So Federal law governing emission protection. that product specifications may be met for mate- Gas cylinders with a capacity of 40, 60, 300, or rial long in transit, it is important during initial 700 kg are suitable for the transport of smaller transfer to ensure high standards of purity and the quantities of monochloromethane. Shut-off absence of moisture. valves on such cylinders must be left-threaded. Rules for transport by all of the various stan- Larger quantities are shipped in containers, rail- dard modes have been established on an interna- road tank cars, and tank trucks, these generally tional basis in the form of the following agree- being licensed for a working pressure of 1.3 MPa ments: RID, ADR, GGVSee, GGVBinSch, (13 bar). IATA-DGR. The appropriate identification num- The three liquid chloromethanes are also nor- bers and warning symbols for labeling as haz- mally stored and transported in vessels con- ardous substances are collected in Table 7. Vol. 9 Chloromethanes 35

Table 7. Identification number and hazard symbols of chloromethanes national guidelines related to water quality pro- tection [94, 95]. Identification Product number Hazard symbol There are fundamental reasons for needing to restrict chlorocarbon emissions to an absolute Monochloromethane UN 1063 H (harmful) minimum. Proven methods for removal of chlor- IG (inflammable gas) Dichloromethane UN 1593 H (harmful) omethanes from wastewater, off-gas, and resi- Trichloromethane UN 1888 H (harmful) dues are Tetrachloromethane UN 1846 P (poison) Vapor stripping with recycling The use and handling of chloromethanes – Adsorption on activated charcoal and recycling both by producers and by consumers of the Recovery by distillation substances and mixtures containing them – are Reintroduction into chlorination processes [96] governed in the Federal Republic of Germany by Combustion in facilities equipped with offgas regulations collected in the February 11, 1982 cleanup version of the ‘‘Rules Respecting Working Ma- terials’’ (‘‘Arbeitsstoff-Verordnung’’). To some 7.1. Presence in the Atmosphere extent, at least, these have their analogy in other European countries as well. Included are stipula- All four chloromethanes are emitted to the atmo- tions regarding the labeling of the pure sub- sphere from anthropogenic sources. In addition, stances themselves as well as of preparations large quantities of monochloromethane are re- containing chloromethane solvents. The central leased into the atmosphere by the combustion authorities of the various industrial trade orga- of plant residues and through the action of nizations issue informational and safety bro- sunlight on algae in the oceans. Estimates of chures for chlorinated hydrocarbons, and these the extent of nonindustrial generation of mono- should be studied with care. chloromethane range from 5 106 t/a [97] to The standard guidelines for handling mono- 28 106t/a [98]. chloromethane as a compressed gas are the Natural sources have also been considered for ‘‘Pressure Vessel Regulation’’ (‘‘Druckbeh€al- trichloromethane [99] and tetrachloromethane ter-Verordnung’’) of February 27, 1980, with the [100] on the basis of concentration measure- related ‘‘Technical Rules for Gases’’ (‘‘Tech- ments in the air and in seawater (Table 8). nische Regeln Gase’’, TRG) and the ‘‘Technical The emission of chloromethanes from indus- Rules for Containers’’ (‘‘Technische Regeln try is the subject of legal restrictions in many Beh€alter’’, TRB), as well as ‘‘Accident Preven- countries. The applicable regulations in the Fed- tion Guideline 29 – Gases’’ (‘‘Unfallver- eral Republic of Germany are those of the TA hutungsvorschrift€ [UVV] 29, Gase’’). Luft [101]. For MAK values, TLV values, and considera- The most important sink for many volatile tions concerning toxicology and ecotoxicology organic compounds is their reaction in the of the chloromethanes see Chapter 139. lower atmosphere with photochemically generated OH radicals. The reactivity of mono- chloromethane, dichloromethane, and trichloro- 7. Behavior of Chloromethanes methane with OH radicals is so high that in in the Environment the troposphere these substances are relatively rapidly destroyed. Chloromethanes are introduced into the environ- ment from both natural and anthropogenic Table 8. Atmospheric concentration of chloromethanes (in 1010vol. sources. They are found in the lower atmosphere, %) [99] and tetrachloromethane can even reach into the stratosphere. Trichloromethane and tetrachloro- Compound Continents Oceans Urban areas methane can be detected in many water supplies. CH3Cl 530 ...1040 1140 ...1260 834 The chloromethanes, like other halogenated CH2Cl2 36 35 <20 ...144 hydrocarbons, are viewed as water contaminants. CHCl3 9 ...25 8 ...40 6 ...15 000 CCl 20 ...133 111 ...128 120 ...18 000 Thus, they are found in both national and inter- 4 36 Chloromethanes Vol. 9

Table 9. Velocity of decomposition of chloromethanes in the atmo- weakly alkaline medium to cleavage with the sphere [97] elimination of HCl. Reaction velocity The microbiological degradability of dichlor- with OH radicals omethane has been established [106–112]. This 12 3 kOH10 cm Half-life, is understood to be the reason for the absence or Compound molecule1 s1 weeks only very low concentrations of dichloromethane

CH3Cl 0.14 12 in the aquatic environment [103]. CH2Cl2 0.1 15 Since trichloromethane and tetrachloro- CHCl3 0.1 15 CCl <0.001 >1000 methane are stable compounds with respect to 4 both biotic and abiotic processes, their disap- By contrast, the residence time in the tropo- pearance is thought to be largely a consequence sphere of tetrachloromethane is very long, with of transfer into the atmosphere by natural strip- the result that it can pass into the stratosphere, ping phenomena. where it is subjected to photolysis from hard Treatment with chlorine is a widespread tech- ultraviolet radiation. The Cl atoms released in nique for disinfecting drinking water. In the this process play a role in the ozone degradation process, result, largely trichlor- which is presumed to occur in the stratosphere omethane as a result of the reaction of chlorine (Table 9) [102]. with traces of organic material. A level of 25 mg/ L of trihalomethanes is regarded in the Federal Republic of Germany as the maximum accept- 7.2. Presence in Water Sources able annual median concentration in drinking water [113]. Seawater has been found to contain relatively high concentrations of monochloromethane (5.9 – 21109 mL of gas/mL of water) [98], in 8. Uses and Economic Aspects addition to both trichloromethane (8.3 – 14 109 g/L) [99] and tetrachloromethane (0.17 – As a result of very incomplete statistical records 0.72109 g/L) [99]. Dichloromethane, on the detailing production and foreign trade by indi- other hand, could not be detected [97]. vidual countries, it is very difficult to describe Chloromethanes can penetrate both surface precisely the world market for chloromethanes. and groundwater through the occurrence of ac- The information which follows is based largely cidents or as a result of improper handling during on systematic evaluation of the estimates of production, transportation, storage, or use (Table experts, coupled with data found in the secondary 10). Groundwater contamination by rain which literature, as well as personal investigations and has washed chlorinated hydrocarbons out of the calculations. air is not thought to be significant on the basis of The Western World includes about 40 produ- current knowledge. One frequent additional cers who produce at least one of the chlorinated cause of diffuse groundwater contamination that C1 hydrocarbons. No authoritative information is can be cited is defective equipment (especially available concerning either the production ca- leaky tanks and wastewater lines) [103]. pacity or the extent of its utilization in the The chloromethanes are relatively resistant to Comecon nations or in the People’s Republic of hydrolysis. Only in the case of monochloro- China. It can be assumed, however, that a large methane in seawater is abiotic degradation of part of the domestic requirements in these coun- significance, this compound being subject in tries is met by imports. In reference to production capacity, see [114]. Table 10. Chloromethane concentration in the Rhine river (mg/L) [104, 105] In comparing the reported individual capaci- ties it is important to realize that a great many Compound Date Mean value Max. value facilities are also capable of producing other chlorinated hydrocarbons. This situation is a CH2Cl2 1980 not detected CHCl3 1980 4.5 result of the opportunities for flexibility both in CHCl3 1982 0.4 ...12.5 50.0 the product spectrum (cf. Section 4.1) and in the CCl 1982 <0.1 ...3.3 44.4 4 various manufacturing techniques (e.g., tetra- Vol. 9 Chloromethanes 37 chloromethane/tetrachloroethene, cf. Section The most important use of dichloromethane, 4.4). If one ignores captive use for further chlo- representing ca. 40 – 45 % of the total market, is rination (especially of monochloromethane), it as a cleaning agent and paint remover. An addi- can be concluded that the largest portion of the tional 20 – 25 % finds application as a pressure world use of chloromethanes (ca. 34 %) can be mediator in aerosols. One further use of dichlor- attributed to tetrachloromethane. The most omethane is in extraction technology (decaffei- important market, accounting for over 90 % of nation of coffee, extraction of hops, paraffin the material produced, is that associated with the extraction, and the recovery of specialty production of the fluorochlorocarbons CFC 11 pharmaceuticals). (trichloromonofluoromethane) and CFC 12 In all of these applications, especially those (dichlorodifluoromethane). These fluorochloro- related to the food and drug industries, the purity carbons possess outstanding properties, such as level requirements for dichloromethane are ex- nonflammability and toxicological safety, and ceedingly high (> 99.99 wt %). are employed as refrigerants, foaming agents, aerosol propellants, and special solvents. Trichloromethane holds the smallest market The production level of tetrachloromethane is share of the chloromethane family: 16 %. Its directly determined by the market for its fluori- principal application, amounting to more than nated reaction products CFC 11 and CFC 12. The 90 % of the total production, is in the production appearance of the so-called ozone theory, which of monochlorodifluoromethane (CFC 22), a asserts that the ozone layer in the stratosphere is compound important on the one hand as a refrig- affected by these compounds, has resulted since erant, but also a key intermediate in the prepara- 1976 in a trend toward reduced production of tion of tetrafluoroethene. The latter can be poly- tetrachloromethane. This has been especially merized to give materials with exceptional ther- true since certain countries (United States, mal and chemical properties, including PTFE, Canada, Sweden) have imposed a ban on Hostaflon, Teflon, etc. aerosol use of fully halogenated fluorochlorocar- Chloroform is still used to a limited extent as bons. However, since 1982/1983 there has been an extractant for pharmaceutical products. Due to a weak recovery in demand for tetrachloro- its toxicological properties, its use as an inhala- methane in the production of fluorine-containing tory anaesthetic is no longer significant. Small compounds. amounts are employed in the synthesis of ortho- Outside Europe, a smaller amount of tetra- formic esters. chloromethane finds use as a disinfectant and as a Table 12 provides an overview of the fungicide for grain. structure of the markets for the various chlori- nated C1 compounds, subdivided according to Monochloromethane and dichloromethane region. each account for about 25 % of the world market for chloromethanes (Table 11). The demand for monochloromethane can be attributed largely 9. Toxicology (60 – 80 %) to the production of silicones. Its use as a starting material for the production of the Monochloromethane. Chloromethane [74- gasoline anti-knock additive tetramethyllead is 87-3], methyl chloride, is an odorless gas and, in steep decline. except for freezing the skin or eyes due to

Table 11. Production capacities of chloromethanes 1000 t/year [139]

Western Europe (FRG) United States Japan

1981 1993 1981 1993 1981 1993

Monochloromethane 265 295 (100) 300 274 70 106 Dichloromethane 410 237 (170) 370 161 65 86 Trichloromethane 140 247 (60) 210 226 65 53 Tetrachloromethane 250 182 (150) 380 140 70 40 38 Chloromethanes Vol. 9

Table 12. Demand and use pattern of chloromethanes (1983) the chlorinated methanes. It is moderate in tox- icity by ingestion, but the liquid is quite painful to Western United Europe States Japan the eyes and skin, particularly if confined on the skin [132–134]. Absorption through the skin is Monochloromethane 230 000 t 250 000 t 50 000 t probably of minor consequence if exposure is 52 % 60 % 83 % controlled to avoid irritation. Tetramethyllead 12 % 15 % – Inhalation is the major route of toxic expo- Methylcellulose 15 % 5 % 1 % sure. The principal effects of exposure to high Other methylation concentrations (greater than 1000 ppm) are reactions, e.g., tensides, pharmaceuticals ca. 21 % ca. 20 % ca. 16 % anesthesia and incoordination. Exposure to methylene chloride results in the formation of Dichloromethane 210 000 t 270 000 t 35 000 t carboxyhemoglobin (COHb) caused by its me-

Degreasing and paint 46 % 47 % 54 % tabolism to carbon monoxide. This COHb is as remover toxic as that derived from carbon monoxide Aerosols 18 % 24 % 19 % itself. However, at acceptable levels of expo- Foam-blowing agent 9 % 4 % 11 % sure to methylene chloride, any probable ad- Extraction and 27 % 25 % 16 % verse effects of COHb will be limited to per- other uses sons with pronounced cardiovascular or Trichloromethane 90 000 t 190 000 t 45 000 t respiratory problems. Other possible toxic ef- fects of carbon monoxide itself would not be CFC 22 production 78 % 90 % 90 % Other uses, 22 % 10 % 10 % expected. e.g., pharmaceuticals, Methylene chloride is not teratogenic in ani- intermediate mals [135] and has only limited mutagenic activity in Salmonella bacteria. It does not appear Tetrachloromethane 250 000 t 250 000 t 75 000 t to be genotoxic in other species. Available re- CFC 11/12 production 94 % 92 % 90 % ports of lifetime studies at high concentrations Special solvent for 6 % 8 % 10 % have produced inconsistent results in hamsters, chemical reactions rats, and mice. No tumors, benign or malignant, were increased in hamsters; rats developed only a dose related increase in commonly occurring nonmalignant mammary tumors; white mice, evaporation, inhalation is the only significant both sexes, had a large increase in cancers of route of exposure. It acts mainly on the central the livers and lungs. Available epidemiological nervous system with well documented cases of data do not indicate an increase in cancer in excessive human exposure, leading to injury and humans; they do indicate that the current occu- even death [132]. The symptoms of overexposure pational standards are protective of employee are similar to inebriation with alcohol (a shuffling health [136, 137]. gait, incoordination, disorientation, and change in personality), but last much longer, possibly Trichloromethane. Trichloromethane [67- permanent in severe exposures. According to 66-3], chloroform, is only moderately toxic from experimental results, excessive exposure to single exposure, but repeated exposure can result methyl chloride was carcinogenic in mice and in rather severe effects [132–134]. Its use as a also affected the testes of male rats and fetuses of surgical anesthetic has become obsolete, primar- pregnant female rats [138]. It is mutagenic in ily because of delayed liver toxicity and the certain in vitro test systems. Available references development of anesthetics with a greater margin indicate that methyl chloride may increase the of safety. rate of kidney tumors in mice in conjunction with Ingestion is not likely to be a problem unless repeated injury to this organ. The TLV and the large quantities are swallowed accidentally or MAK (1985) are both 50 ppm (105 mg/m3). deliberately. Chloroform has a definite solvent action on the skin and eyes and may be absorbed Dichloromethane. Dichloromethane [75- if exposure is excessive or repeated. Its recog- 09-2], methylene chloride, is the least toxic of nized high chronic toxicity requires procedures Vol. 9 Chloromethanes 39 and practices to control ingestion, skin, and References eye contact, as well as inhalation exposure if liver and kidney injury, the most likely conse- General References quence of excessive exposure, is to be 1 Ullmann, 4th ed., 9, 404–420. prevented. 2 Winnacker-Kuchler€ , 4th ed., vol. 6; ‘‘Organische Tech- In animals, chloroform is fetotoxic (toxic to nologie II’’, pp. 2–11. the fetus of a pregnant animal) but only weakly 3 Kirk-Othmer, 5, 668–714. teratogenic if at all [135]. It does not appear to 4 J. J. McKetta, W. Cunningham, Encyl. Chem. Process. (1979) 214–270. be mutagenic by common test procedures, but Des. 8 5 C. R. Pearson: ‘‘C1 and C2-’’, in: The Hand- increases the tumor incidence in certain rats book of Environmental Chemistry, vol. 3, Springer Ver- and mice. There is considerable evidence that lag, Berlin 1982, pp. 69–88. the tumors in rat kidneys and mice livers are 6 Chloroform, Carbon Tetrachloride and other Halo- theresultofrepeatedinjurytotheseorgansand methanes: An Environmental Assessment, National that limiting exposure to levels that do not Academy of Sciences Washington, D.C., 1978. cause organ injury will also prevent cancer. It 7 A. Wasselle: C1 Chlorinated Hydrocarbons, Private is, therefore, very important that human report by the Process Economic Program SRI Interna- tional, Report No. 126. exposure be carefully controlled to prevent 8 CEFIC-B.I.T. Solvants Chlores ‘‘Methylene chloride– injury. Use in industrial applications,’’ 1983. 9 J. Schulze, M. Weiser: ‘‘Vermeidungs- und Verwer- Tetrachloromethane. Tetrachloromethane tungsmoglichkeiten€ von Ruckst€ €anden bei der Herstel- [56-23-5], carbon tetrachloride, was once recom- lung chloroorganischer Produkte,’’ Umwelt- mended as a ‘‘safety solvent.’’ Misuse and its forschungsplan des Bundesministers des Innern–Ab- rather high liver toxicity, as well as the ready fallwirtschaft. Forschungsbericht 103 01 304, UBA-FB availability of alternate safe solvents, have elim- 82–128 (1985). inated its application as a solvent. Single expo- Specific References sures are not markedly injurious to the eyes and 10 I. Mellan: Industrial Solvents Handbook, Noyes Data skin or toxic when small quantities are ingested. Corp., Park Ridge, N.J., 1970, p. 73. However, repeated exposure must be carefully 11 B. Kaesche-Krischer, Chem. Ind. Tech. 35 (1963) 856– controlled to avoid systemic toxicity, particularly 860. H.-J. Heinrich, Chem. Ing. Tech. 41 (1969) 655. to the liver and kidneys [132–134]. In humans, 12 H. J. Schumacher, Z. Elektrochem. 42 (1936) 522. injury to the kidney appears to be the principal 13 D. V. E. George, J. H. Thomas, Trans Faraday Soc. 58 cause of death. (1962) 262. 14 Du Pont, US 2 378 048, 1944 (C. W. Theobald). Inhalation can produce anesthesia at high 15 H. J. Schumacher, Z. Angew. Chem. 49 (1936) 613. concentrations, but transient liver as well as 16 A. T. Chapmann, J. Am. Chem. Soc. 57 (1935) 416. kidney injury result at much lower concentra- 17 R. Neu, Pharmazie 3 (1948) 251. tions than those required to cause incoordina- 18 F. Lenze, L. Metz, Chem. Ztg. 56 (1932) 921. tion. There appears to be individual suscepti- 19 W. B. Grummet, V. A. Stenger, Ind. Eng. Chem. 48 bility to carbon tetrachloride, with some hu- (1956) 434. mans becoming nauseated at concentrations 20 E. H. Lyons, R. G. Dickinson, J. Am. Chem. Soc. 57 that others willingly tolerate. Ingestion of (1935) 443. 21 R. Johns, Mitre Corp. Tech. Rep. MTR 7144 (1976) Mc alcohol is reported to enhance the toxicity of Lean, Va.; Mitre Corp. carbon tetrachloride. Such responses should not 22 Celanese Corp., US 2 770 661, 1953 (T. Horlenko, F. B. occur, however, if exposures are properly con- Marcotte, O. V. Luke). trolled to the recommended occupational 23 United States Rubber Co., GB 627 993, 1946. standards. 24 R. M. Joyce, W. E. Hanford, J. Am. Chem. Soc. 70 (1948) Carbon tetrachloride is not teratogenic in 2529. animals [135] nor mutagenic in common test 25 W. L. Faith, D. B. Keyes, R. L. Clark: Industrial systems, but does increase liver tumors in Chemicals, 3rd ed., J. Wiley & Sons, New York 1965, pp. 507–513. mice, probably as a result of repeated injury to 26 E. T. McBee et al., Ind. Eng. Chem. 41 (1949) 799–803. that organ. Therefore, it is very important that 27 R. N. Pease et al., J. Am. Chem. Soc. 53 (1931) 3728– humanexposurebecarefullycontrolledtoprevent 3737. liver injury. 28 W. E. Vaughan et al., J. Org. Chem. 5 (1940) 449–471. 40 Chloromethanes Vol. 9

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