Replacement of hydrogen by halogen in alkanes

Although exchange of hydrogen for chlorine is rarely used in the laboratory purely for the purpose of preparing chlorinated alkanes, chlorination of alkanes is of great technical importance. The large amounts of heat liberated can be removed in the gas phase by working with an excess of alkane or by admixture with an inert gas, and in the liquid phase by cooled solvents. Photochemical, catalytic, and thermal processes are all used. In all cases di- and poly-chlorina-tion occur as competitors of the monochlorination that is usually desired. 

In homogeneous catalytic chlorination in the liquid phase, a chloride of I, P, S, Sb, Fe, or Sn (for patent literature see reference 324a), or radical-formers such as tetraethyllead,325 are added to the liquid or CC14 solution of the alkane. Olefins which may be present also act as catalysts.326,327 

In a technically valuable variant the gas mixture streaming into a conical reaction vessel maintains the suspended catalyst (active charcoal) in constant motion. This has the advantage of avoiding disturbances due to separation of carbon or tarry by-products such as occur in purely thermal or catalytic chlorination on fixed bed catalysts.324e 

Thermal chlorination (400-500°) in the absence of catalysts and light is a chain reaction initiated by thermal dissociation of Cl2.328 Details of laboratory apparatus for this method of chlorination, which is particularly important for alkanes of lower molecular weight, are given in Asinger s book324d and by Hass.329 The procedure can be used to convert alkanes into monochlorides and to chlorinate the latter further. 

The amounts of the various isomers formed on monochlorination of alkanes depends on the relative rates at which the variously bound hydrogen atoms are replaced. Hydrogen atoms in CH3—, —CH2—, and CH— are replaced at 300° in the proportions 1 3.25 4.43.324f Above 300° the proportion of primary chloride increases the cause of this is that dehydrochlorination, induced by chlorine, occurs preferentially at chlorinated methylene groups.330 Catalysts and irradiation do not affect the above proportions and it follows from them that the amount of terminal chloride formed must decrease continuously with increasing chain length thus, whereas propane gives 48% of 1-chloro compound, only 8.5% is formed from w-dodecane. 

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