Chlorine with chloromethane

Dillon, K. B. et al., J. Chem. Soc., Dalton Trans., 1976, 1479 In the attempted preparation of tetrachloromethylarsinane (MeAsCU) by interaction of chlorine with the arsine while slowly warming from — 196°C, several sealed ampoules exploded at well below 0°C, probably owing to liberation of chloromethane. 

The attack of a chlorine atom on a methane hydrogen is not expected to require a precisely oriented collision. Moreover, the interatomic repulsions should be considerably smaller than in the four-center mechanism discussed previously for the reaction of molecular chlorine with methane because only two centers have to come close together (Figure 4-8). The methyl radical resulting from the attack of atomic chlorine on a hydrogen of methane then can remove a chlorine atom from molecular chlorine and form chloromethane and a new chlorine atom ... 

Chloromethane is an important industrial chemical. Olah et al. [56] have reported the selective catalytic monochlorination of methane to chloromethane over superacidic sulfated zirconia solid catalysts, for example 804 /Zr02, Pt/ S04 7Zr02, and Fe/Mn/S04 7Zr02- The reactions were conducted in a continuous-flow reactor under atmospheric pressure. At 200 °C with 30 % chlorine the selectivity to chloromethane was > 90 %.The selectivity could be enhanced by adding platinum. The only by-product was CH2CI2. The latter is formed by the subsequent chlorination of chloromethane. No chloroform or carbon tetrachloride was formed. The authors postulated that chlorination occurs by an electrophilic insertion of an electron-deficient, metal coordinated, chlorine molecule into the C-H bond of methane. One drawback of the process was that above 225 °C, part of the metal was removed as the metal chloride [56]. Formation and subsequent loss of volatile metal chlorides is a major pitfall that should be avoided during vapor-phase chlorination over solid catalysts. 

An example of this reaction that you have already studied is the alkylation of terminal alines (Section 7.5A). Another is the reaction of hydroxide ion with chloromethane. In this reaction, chloromethane is the electrophile. Because of the electronegativity of chlorine, there is a partial positive charge on the carbon. An electrostatic potential map shows the negative electron density on HO (the nucleophile) interacting with the partial positive charge on the methyl group. 

The secondary reactions are series with respect to the chloromethane but parallel with respect to chlorine. A very large excess of methane (mole ratio of methane to chlorine on the order of 10 1) is used to suppress selectivity losses. The excess of methane has two effects. First, because it is only involved in the primary reaction, it encourages the primary reaction. Second, by diluting the product, chloromethane, it discourages the secondary reactions, which prefer a high concentration of chloromethane. 

Chlorination of methane provides approximately one third of the annual U S pro duction of chloromethane The reaction of methanol with hydrogen chloride is the major synthetic method for the preparation of chloromethane... 

Termination steps are m general less likely to occur than the propagation steps Each of the termination steps requires two free radicals to encounter each other m a medium that contains far greater quantities of other materials (methane and chlorine mol ecules) with which they can react Although some chloromethane undoubtedly arises via direct combination of methyl radicals with chlorine atoms most of it is formed by the propagation sequence shown m Figure 4 21... 

Methane, chlorine, and recycled chloromethanes are fed to a tubular reactor at a reactor temperature of 490—530°C to yield all four chlorinated methane derivatives (14). Similarly, chlorination of ethane produces ethyl chloride and higher chlorinated ethanes. The process is employed commercially to produce l,l,l-trichloroethane. l,l,l-Trichloroethane is also produced via chlorination of 1,1-dichloroethane with l,l,2-trichloroethane as a coproduct (15). Hexachlorocyclopentadiene is formed by a complex series of chlorination, cyclization, and dechlorination reactions. First, substitutive chlorination of pentanes is carried out by either photochemical or thermal methods to give a product with 6—7 atoms of chlorine per mole of pentane. The polychloropentane product mixed with excess chlorine is then passed through a porous bed of Fuller s earth or silica at 350—500°C to give hexachlorocyclopentadiene. Cyclopentadiene is another possible feedstock for the production of hexachlorocyclopentadiene. 

Carbon tetrachloride [56-23-5] (tetrachloromethane), CCl, at ordinary temperature and pressure is a heavy, colorless Hquid with a characteristic nonirritant odor it is nonflammable. Carbon tetrachloride contains 92 wt % chlorine. When in contact with a flame or very hot surface, the vapor decomposes to give toxic products, such as phosgene. It is the most toxic of the chloromethanes and the most unstable upon thermal oxidation. The commercial product frequendy contains added stabilizers. Carbon tetrachloride is miscible with many common organic Hquids and is a powerhil solvent for asphalt, benzyl resin (polymerized benzyl chloride), bitumens, chlorinated mbber, ethylceUulose, fats, gums, rosin, and waxes. 

Problem 1.8 concerned the charge distribution in methane (CH4), chloromethane (CH3CI), and methyllithium (CH3Li). Inspect molecular models of each of these compounds, and compare them with respect to how charge is distributed among the various atoms (carbon, hydrogen, chlorine, and lithium). Compare their electrostatic potential maps. 

The successive substitution of methane hydrogens with chlorine produces a mixture of four chloromethanes ... 

An alternative way to produce methyl chloride (monochloromethane) is the reaction of methanol with HCl (see later in this chapter, Chemicals from Methanol ). Methyl chloride could he further chlorinated to give a mixture of chloromethanes (dichloromethane, chloroform, and carhon tetrachloride). 

Zinc chloride is also a catalyst for a liquid-phase process using concentrated hydrochloric acid at 100-150°C. Hydrochloric acid may be generated in situ by reacting sodium chloride with sulfuric acid. As mentioned earlier, methyl chloride may also be produced directly from methane with other chloromethanes. However, methyl chloride from methanol may be further chlorinated to produce dichloromethane, chloroform, and carbon tetrachloride. 

The mechanism(s) by which these photocatalyzed oxidations are initiated remain uncertain. Early proposals have included involvement of either the photo-produced holes (h+) arising directly from semiconductor photo-excitation, or the (presumed) derivative hydroxyl radical (OH) which was argued to arise from the hole oxidation of adsorbed hydroxyls (h+ + OH-—> OH ). Recent subambient studies [4] with physisorbed chloromethane and oxygen suggest the dioxygen anion (02 ) as a key active species, and the photocatalytic high efficiency chain destruction of TCE is argued to be initiated by chlorine radicals (Cl) [5]. The chlorine-enhanced photocatalytic destruction of air contaminants has been proposed [1, 2, 6] to depend upon reactions initiated by chlorine radicals. 

Chlorine Trifluoride Tech. Bull. , Morristown, Baker Adamson, 1970 Incandescence is caused by contact with bromine, iodine, arsenic, antimony (even at -10°C) powdered molybdenum, niobium, tantalum, titanium, vanadium boron, carbon, phosphorus or sulfur [1], Carbon tetraiodide, chloromethane, benzene or ether ignite or explode on contact, as do organic materials generally. Silicon also ignites [2],... 

As the reaction progresses, the concentration of chloromethane in the mixture increases and a second substitution reaction begins to occur => Chloromethane reacts with chlorine to produce dichloromethane. 

Propagation. In this step, the intermediate reacts with a stable molecule to produce another reactive intermediate and a product molecule. The propagation step yields a new electrophilic species, the methyl radical, which has an unpaired electron. In a second propagation step, the methyl radical abstracts a chlorine atom from a chloromethane molecule, and generates a chlorine radical. 

Alkanes appear to react with platinum(IV) in an identical manner to benzene (34, 84) chloromethane and chloroethane can be detected as the reaction products from methane and ethane, respectively. When propane, butane, or hexane is the reactant, the terminal chloro isomers predominate over the internal isomers. This was interpreted to mean that primary C—H bonds are the most reactive (34), but a more detailed study has shown that this conclusion does not necessarily follow from the experimental results (84). When cyclohexane is the reactant, dehydrogenation (or chlorination and then dehydrohalogenation) occurs to give benzene as one of the reaction products (29, 34, 84). 

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