Carbon from chloromethanes

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 chloride anion is a major species in the oceans and plays an essential role in biochemistry. Compounds containing carbon-chlorine bonds occur much less frequently in nature. Volcanos emit some halocarbons, and marine algae generate chloromethane. Other marine species produce toxic organohalogen molecules that protect them from predators. Nevertheless, organic chlorine compounds are uncommon, and consequently there are few mechanisms that degrade them. 

Coulter C, JTG Hamilton, WC McRoberts, L Kulakov, MJ Larkin, DB Harper (1999) Halomethane bisulfite/halide ion methyltransferase, an unusual corrinoid enzyme of environmental significance isolated from an aerobic methylotroph using chloromethane as the sole carbon source. Appl Environ Microbiol 65 4301-4312. 

A sample of redistilled material in a screw capped bottle exploded 15 weeks after capping. This was attributed to development of internal pressure (probably of hydrogen chloride arising from hydrolysis, and perhaps also of chloromethane from scission). (The editor has also known methoxyacetic mixed anhydrides to decarbonylate, releasing carbon monoxide)... 

Relatively soon after the discovery that aqueous solutions containing PtCl - and PtClg- can functionalize methane to form chloromethane and methanol, a mechanistic scheme for this conversion was proposed (16,17). As shown in Scheme 4, a methylplatinum(II) intermediate is formed (step I), and this intermediate is oxidized to a methylplatinum(IV) complex (step II). Either reductive elimination involving the Pt(IV) methyl group and coordinated water or chloride or, alternatively, nucleophilic attack at the carbon by an external nucleophile (H20 or Cl-) was proposed to generate the functionalized product and reduce the Pt center back to Pt(II) (step III) (17). This general mechanism has received convincing support over the last two decades (comprehensive reviews can be found in Refs. (2,14,15)). Carbon-heteroatom bond formation from Pt(IV) (step III) has been shown to occur via nucleophilic attack at a Pt-bonded methyl, as discussed in detail below (Section V. A). 

There are two simple ways in which the SN2 reaction of methyl chloride could occur with hydroxide ion. These differ in the direction of approach of the reagents (Figure 8-1). The hydroxide ion could attack chloromethane at the front side of the carbon where the chlorine is attached or, alternatively, the hydroxide ion could approach the carbon on the side opposite from the chlorine in what is called the back-side approach. In either case, the making of the C-O bond is essentially simultaneous with the breaking of the C-Cl bond. The difference is that for the back-side mechanism the carbon and the attached hydrogens become planar in the transition state. 

It s relatively easy to measure dipole moments experimentally, and values for some common substances are given in Table 10.1. Once the dipole moment is known, it s then possible to get an idea of the amount of charge separation in a molecule. In chloromethane, for example, the experimentally measured dipole moment is /x = 1.87 D. If we assume that the contributions of the nonpolar C-H bonds are small, then most of the chloromethane dipole moment is due to the C-Cl bond. Since the C-Cl bond distance is 178 pm, we can calculate that the dipole moment of chloromethane would be 1.78 X 4.80 D = 8.54 D if the C-Cl bond were ionic (that is, if a full negative charge on chlorine were separated from a full positive charge on carbon by a distance of 178 pm). But because the measured dipole moment of chloromethane is only 1.87 D, we can conclude that the C-Cl bond is only about (1.87/8.54)(100%) = 22% ionic. Thus, the chlorine atom in chloromethane has an excess of about 0.2 electron, and the carbon atom has a deficiency of about 0.2 electron. 

Coulter C, Hamilton JTG, McRoberts WC, Kulakov L, Larkin MJ, Harper DB (1999) Halomethane Bisulfide/Halide Ion Methyltransferase, an Unusual Corrinoid Enzyme of Environmental Significance Isolated from an Aerobic Methyltroph Using Chloromethane as the Sole Carbon Source. Appl Environ Microbiol 65 4301... 

Figure 1.5 provides examples of o for some simple molecules. For molecules which are conical (hydrogen cyanide and chloromethane), cylindrical (carbon dioxide and ethane), or spherical (neon, methane), empirically generated a values of 10, 20, and 100 are assigned. For flexible molecules, the value of o is unity because they cannot be rigidly rotated. Note that the o values for spherical, cylindrical, and conical molecules are different from the values used by crystallographers. 

Radical chlorination is a difficult reaction to control. As the reaction proceeds and the initial product, chloromethane, accumulates, it can also undergo hydrogen abstraction by a chlorine atom, resulting in the formation of dichloromethane. Chloroform is formed from dichloromethane and carbon tetrachloride from chloroform in a similar manner. The reaction of a 1 1 ratio of methane and chlorine at 440°C (at this high temperature. homolytic fission of the chlorine-chlorine bond occurs without light) results in the product mixture shown in the following equation ... 

Kotsinaris et al. 1998), while on Ag and Pt cathodes methane was the main product. Rondinini et al. (2004) observed the preferential formation of methane from poly-chloromethane on silver in ACN, DMF and aquo-organic solvents. Trichloroethene and trichloromethane were hydrodehalogenated in aqueous solution at different pHs by Chen et al. (2003) on platinized and palladized ceramic (ebonex) supports. Main reaction products were ethane, ethene and HC1 for the former substrate, and methane and HC1 for the latter. In a preceding study (Chen et al. 1999) in oxidative conditions, they observed the parallel degradation to carbon-containing products (mainly CO2) and, in neutral or alkaline pH, Cr/CI()3 as the only chloro-containing products. 

Figure 4.1. The C3 axis of chloromethane. The circle represents the central carbon atom. One hydrogen is starred to follow its position during rotation. It is actually indistinguishable from either of the other hydrogens.

Chloromethane contains a polar C-CI bond. In the electrostatic potential map on the right, the charge distribution is shown using colors ranging from red (negative) to blue (positive). Thus, the chlorine atom is electron-rich, and the carbon and hydrogen atoms are electron-poor. 

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