Chlorination a substitution reaction

Burning of hydrocarbons takes place only at high temperatures, as provided, for example, by a flame or a spark. Once started, however, the reaction gives off heat which is often sufficient to maintain the high temperature and to permit burning to continue. The quantity of heat evolved w hen one mole of a hydrocarbon is burned to carbon dioxide and water is called (he heat of combustion for methane its value is 213 kcal. 

Oxidation by halogens is of particular interest to us partly because we know more about it than the other reactions of methane—and, in one way or another, is the topic of discussion throughout the remainder of this chapter. 

Under the influence of ultraviolet light or at a temperature of 250-400 a mixture of the two gases, methane and chlorine, reacts vigorously to yield hydrogen chloride and a compound of formula CH3CI. We say that methane has undergone chlorination, and we call the product, CH Cl, chloromethane or methyl chloride (CH3 = methyl). 

Chlorination is a typical example of a broad class of organic reactions known as substitution. A chlorine atom has been substituted for a hydrogen atom of 

The methyl chloride can itself undergo further substitution to form more hydrogen chloride and CH2CI2, dichloromethane or methylene chloride (CH2 = mi thylene). 

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