Halogenation of Higher Alkanes

Higher alkanes react with halogens by the same kind of chain mechanism as those that we have just seen. Ethane, for example, reacts with chlorine to produce chloroethane (ethyl chloride). The mechanism is as follows  

Calculate AH° for this step using data from Table 10.1, and draw a fully labeled potential energy diagram, similar to that shown in Fig. 10.3a. 

When ethane is chlorinated, 1,1-dichloroethane and 1,2-dichloroethane, as well as more highly chlorinated ethanes, are formed in the mixture (see Section 10.3A). Write chain reaction mechanisms accounting for the formation of 1,1-dichloroethane and 1,2-dichloroethane. 

Chlorination of most alkanes whose molecules contain more than two carbon atoms gives a mixture of isomeric monochloro products (as well as more highly chlorinated compounds). 

Several examples follow. The percentages given are based on the total amount of mono-chloro products formed in each reaction. 

Like the chlorination of methane, chlorination of ethane is carried out on an industrial scale as a high-temperature gas-phase reaction. 

Photochemical energy is indicated by writing light or hv above the arrow. The symbol hv is equal to the energy of a light photon and will be discussed in more detail in Section 13.1. 

As in the chlorination of methane, it is often difficult to limit the reaction to monochlorination, and derivatives having more than one chlorine atom are also formed. 

PROBLEM 4.18 Chlorination of ethane yields, in addition to ethyl chloride, a mixture of two isomeric dichlorides. What are the structures of these two dichlorides  

In the laboratory it is more convenient to use light, either visible or ultraviolet, as the source of energy to initiate the reaction. Reactions that occur when light energy is absorbed by a molecule are called photochemical reactions. Photochemical techniques permit the reaction of alkanes with chlorine to be performed at room temperature. 

Which constitutional isomer of C6H14 gives only two monochlorination products  

Termination steps are, in general, less likely to occur than the propagation steps. Each of the termination steps requires two free radicals to encounter each other in a medium that contains far greater quantities of other materials (methane and chlorine molecules) 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 in Mechanism 4.3. 

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