The high boiling or non-volatile higher fractions obtained by fractional distillation of petroleum can be further converted into useful lower hydrocarbons by cracking. The process involves treatment of these non-volatile components under high pressure and temperature to generate the smaller hydrocarbons (thermal cracking). In hydrocracking, the reaction is carried out in presence of hydrogen and catalyst. This process requires a much lower temperature (250-450 °C) as compared to thermal cracking. Similarly, catalytic cracking (a process to improve the quality of petrol) involves treatment of high boiling petroleum fractions with a finely divided silica–alumina catalyst.

Some other methods for preparation of alkanes are discussed below. These methods are mostly used only in the laboratory as most of the industrial supply is obtained from naturally occurring petroleum.

1. Halogenation of alkenes. Alkenes can be hydrogenated by hydrogen gas in the presence of a finely divided metal catalyst. The most commonly used metals include Pt, Pd or Ni.
2. Reaction of alkyl halides with organometallic reagents . This method involves the treatment of an alkyl halide with another organometallic reagent usually an organocopper reagent (Corey-House method). For good yields, the alkyl halide should be a primary halide but there is no constraint on the alkyl group in the organocopper reagent (Scheme 2).



Scheme 2

3. Reduction of alkyl halides. The reduction of alkyl halides can be carried out in the presence of a metal and acid (Scheme 3).

Scheme 3

3.5 Reactions of Alkanes

The halogenation of alkanes to generate alkyl halides is an important reaction. However, it is seldom of any preparative value. This is because alkanes usually give a mixture of isomeric alkyl halides when they are treated with a halogen. Usually, chlorine is more reactive towards the alkane than bromine. Among the alkanes, the reactivity decrease on going from tertiary to secondary to primary alkanes. Thus, when propane is chlorinated in the presence of chlorine gas under photochemical conditions, both 1-chloropropane and 2-chloropropane are formed. An analogous result is observed for bromination of propane (Scheme 4). The selectivity observed in case of bromination is due to the lower reactivity of the bromine atoms compared to the chlorine atoms.

Scheme 4

The mechanism for the halogenation of alkanes follows the pathway of radical initiated reaction and thus it consists of the following steps:

The abstraction of the proton is the rate limiting step in this reaction.