Reactions involving alkanes

What are three important reactions involving alkanes?

1. Combustion: The chemical reaction of a substance with oxygen.
2. Halogenation: The substitution of hydrogen with a halogen.
   • This occurs in three steps:
     1. Initiation: Splitting of a diatomic halogen, e.g. F$_{2}$, creating a free radical represented with a $F^{*}$:
        
        $F_{2} + energy \rightarrow F^{*} + F^{*}$
        
        
        
     2. Propagation: Propagation of free radicals:
        
        $CH_{4} + F^{*} \rightarrow CH_{3}^{*} + HF$
        $CH_{3}^{*} + F_{2}\rightarrow CH_{3}Cl + F^{*}$
        
        
        
        

     3. Termination: When two radicals form a bond (this can lead to impurities when the pairing up of radicals is not the desired end-product).
        
        $F^{*} + F^{*} \rightarrow F_{2}$
        $CH_{3}^{*} + F^{*} \rightarrow CH_{3}F$
        $CH_{3}^{*} + CH_{3}^{*} \rightarrow C_{2}H_{6}$
        
        
        
        

3. Cracking: Breaking larger hydrocarbons into smaller ones, including alkanes. This is commonly accomplished with heat, pressure and catalysts.

What pattern exists for the relative stabilities of free radicals?

$Methyl < Primary < Secondary < Tertiary$

Figure 42.4: Relative stabilities of free radicals.