Halogenation of Alkanes
Chlorination of Alkanes
In contrast to methane, alkanes with longer or branched chains contain different types of hydrogen atoms: namely, primary, secondary, and tertiary. Propane, for example, contains six primary and two secondary hydrogen atoms. Which hydrogen is most likely to be substituted in a radical monochlorination? Statistically, three times more 1-chloropropane molecules should be formed than 2-chloropropane molecules.
However, the product ratio of 1-chloropropane : 2-chloropropane at a reaction temperature of 25 °C is not 3 : 1, but rather 43 : 57! Therefore, the reaction is obviously not controlled merely by simple statistics. In fact, the product ratio is determined by the relative stability of the intermediate alkyl radicals. Considering the number of various types of hydrogen atoms in propane, namely, six primary and two secondary, the reactivity ratio secondary : primary in the chlorination of propane is 28.5 : 7.2, that is about 4 : 1. In radical propane chlorination, secondary hydrogens, in comparison to primary hydrogens, are the fourth reactive. Additionally, the relative reactivity of the different types of hydrogens depends on the attacking halogen radical, the stability of the newly formed H-X bond and the reaction temperature. At low temperatures, as a result of the lower thermal energy, the different stabilities of alkyl radicals are more important in product ratio determination. Thus, the selectivity is increased. At high temperatures (600 °C), the product ratio is predominantly controlled by statistics. Tertiary C-H bonds are even more reactive than secondary bonds are. This may, for instance, be concluded by investigating the chlorination of 2-methylbutane.
In the chlorination of 2-methylbutane, the reactivity ratio of the different hydrogen types is tertiary : secondary : primary = 5 : 4 : 1 (at 25 °C).
Radical chlorination cannot be employed for the synthesis of specific alkyl halides, since the reaction always yields mixtures of single- and multiple-chlorinated products. Nevertheless, radical halogenation of alkanes is a useful reaction. In this method, non-reactive alkanes can be converted into comparatively more reactive alkyl halides. Subsequently, a wide range of alkyl compounds can be obtained from these alkyl halides through nucleophilic substitutions.