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51.7 CRACKING AND REFORMING ORGANIC COMPOUNDS

We know that the requirement of petrol or gasoline is increasing day by day and at the same time the availability of petroleum is falling short of our requirement. Therefore, in order to meet the ever-increasing demand of petrol and to overcome the crisis due to less availability of petroleum,
scientists have been successful in developing some processes to convert other fractions of petroleum into gasoline. Let us be familiar with some of these processes.

 CRACKING

 It is a process of decomposing higher hydrocarbons of the petroleum fraction into lower hydrocarbons with low boiling points by strong heating.

 The process involves the cleavage of carbon-carbon and carbon-hydrogen bonds and results in the formation of molecules of lower hydrocarbons. By this process, the higher boiling fractions of petroleum, like fuel oil, are converted into gasoline. Thus, cracking can be employed to improve the yield of gasoline. The types of hydrocarbons formed depend upon the conditions employed for cracking. In thermal cracking the higher hydrocarbons are heated to a high temperature, whereby lower hydrocarbons are formed For example,

775K

CH3 (CH2)8 CH3 → C8H18 + CH2 = CH2

Decane                          Octane                Ethylene

 

Similarly, octane can produce other hydrocarbons involving cleavage of C-C bonds as shown below:

C6H14 + C2H4

Hexane         Ethene

Hexane

C8H18

n-Octane

C4H10 + C2H4 + CH4 + C

Butane          Ethene        Methane

Cracking can also be carried out at lower temperature by the use of suitable catalysts like silica and alumina. This type of cracking is referred to as catalytic cracking.

Cracking is generally employed for the production of good quality gasoline, from heavier fractions of petroleum.

 REFORMING

Reforming is a process of conversion of straight alkanes into ring molecules-cycloalkanes and arenes.The reaction is carried out in the presence of suitable catalyst like palladium, platinum or nickel. The reaction involves dehydrogenation, cyclisation and even isomerisation in some
cases. The reaction is generally carried out at high temperature For example, the conversion of n-hexane to benzene as shown below:

REFORMING

Reforming converts low-grade gasoline to high-grade gasoline

 Isomerisation. This process is used to convert straight -chain alkanes into branched-chain alkanes. The reaction is carried out by heating with anhydrous aluminium chloride and hydrogen

chloride.

                                                    CH3

AICl3 / HCl              |

CH3(CH2)4CH3                            CH3CHCH2CH2CH2

Hexane                                                    2-Methylpentane