Chloromethanes

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

Each of these four compounds has many industrial applications that will be dealt with separately. 

Methane is the most difficult alkane to chlorinate. The reaction is initiated by chlorine free radicals obtained via the application of heat (thermal) or light (hv). Thermal chlorination (more widely used industrially) occurs at approximately 350-370°C and atmospheric pressure. A typical product distribution for a CH4/CI2 feed ratio of 1.7 is mono- (58.7%), di-(29.3%) tri- (9.7%) and tetra- (2.3%) chloromethanes. 

The highly exothermic chlorination reaction produces approximately 95 KJ/mol of HCI. The first step is the breaking of the Cl-Cl bond (bond energy = -1- 584.2 KJ), which forms two chlorine free radicals (Cl atoms)  

The Cl atom attacks methane and forms a methyl free radical plus HCI. The methyl radical reacts in a subsequent step with a chlorine molecule, forming methyl chloride and a Cl atom  

Passage of methanol and hydrogen chloride over a Lewis acid catalyst at about 350°C provides methyl chloride. 

This product can be chlorinated to give the more highly substituted derivatives. 

However, more operations are now based on the oxychlorination of methane, over copper chloride catalysts, at temperatures of over 500°C. 

Mixtures are obtained with proportions dependent on the initial ratio of chlorine to methane. Some carbon tetrachloride is also produced by the chlorinolysis (i.e. C—C cleavage) of higher carbon number materials, and chlorination of carbon disulphide. 

The total U.S. production of chloromethanes is over 0 9 Mt per annum. Chloroform (trichloromethane) and carbon tetrachloride (tetrachloro-methane) are precursors for fluorocarbons their elimination from the aerosol can market has been partially offset by increased use as refrigerants and intermediates for fluoroplastics. Methylene dichloride (dichloromethane) has picked up some of the aerosol business, and is used in paint removal and degreasing. Methyl chloride (chloromethane) is used for silicones and the waning production of tetramethyl lead. 

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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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