74-87-3 Chloromethane

Chloromethane is the most abundant organohalogen - anthropogenic or natural - in the atmosphere. The myriad natural sources of CH3C1 dwarf the anthropogenic contribution (Table 3.1). Subsequent to the previous survey (7) a number of new natural sources of CH3C1 have been identified, and other reviews have appeared (42, 246, 247). 

Gribble, Naturally Occurring Organohalogen Compounds — A Comprehensive Update, Progress in the Chemistry of Organic Natural Products, Vol. 91, 

Salicornia sp., Batis maritima, and Frankenia grandifolia) produce large amounts of CH3C1, perhaps constituting the largest natural terrestrial source (Table 3.1) (252). 

It is proposed that the CH3C1 missing source (vide supra) may be the abiotic methylation of chloride in plants and soils (280, 281). This methylation by plant pectin in senescent and dead leaves efficiently produces CH3C1 and shows a positive correlation with temperature. Plants studied include Norway maple, horse chestnut, cherry, oak, beech, a Eucalyptus sp., and a salt marsh (Batis maritima) (281). This important study complements that of Myneni (172-174) and Keppler et al. (175,176), cited earlier, and Oberg (298, 299). 

Biomass combustion is a major global source and may even be the major source of atmospheric CH3C1 (223, 282-288). These studies indicate that CH3C1 production is maximized in low intensity fires, from incomplete combustion, and by increased chloride concentration. It is estimated that six billion tons of biomass are consumed 

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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. 

FIGURE 4 2 Electro static potential maps of methanol and chloro methane The electrostatic potential is most negative near oxygen in methanol and near chlorine in chloromethane The most positive region is near the O—H proton in methanol and near the methyl group in chloromethane... 

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

The gas phase chlorination of methane is a reaction of industrial importance and leads to a mixture of chloromethane (CH3CI) dichloromethane (CH2CI2) trichloromethane (CHCI3) and tetrachloromethane (CCI4) by sequential substitution of hydrogens... 

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... 

One of the chief uses of chloromethane is as a starting material from which sili cone polymers are made Dichloromethane is widely used as a paint stripper Trichloromethane was once used as an inhalation anesthetic but its toxicity caused it to be replaced by safer materials many years ago Tetrachloromethane is the starting mate rial for the preparation of several chlorofluorocarbons (CFCs) at one time widely used as refrigerant gases Most of the world s industrialized nations have agreed to phase out all uses of CFCs because these compounds have been implicated m atmospheric processes that degrade the Earth s ozone layer... 

Dichloromethane trichloromethane and tetra chloromethane are widely known by their common names methylene chloride chloroform and carbon tetrachloride respectively... 

Write equations for the initiation and propagation steps for the formation of dichloromethane by free radical chlorination 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... 

Consider the chlorination of methane to chloromethane The heats of formation of the reac tants and products appear beneath the equation These heats of formation for the chemical com pounds are taken from published tabulations the heat of formation of chlorine as it is for all elements IS zero... 

Methane reacts with CI2 to give chloromethane dichloromethane trichloromethane and tetrachloromethane... 

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