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Petroleum and Petrol


Petroleum is a mixture of natural gas and crude oil. It is believed to be formed by the decomposition of deposits of dead marine organic matter by the action of bacteria, heat and pressure millions of years ago.


Petroleum is composed mainly of a mixture of hydrocarbons (about 90%) together with a small proportion of compounds containing sulphur, oxygen, and nitrogen (about 10%).

Fractional Distillation of Petroleum Components

The different components of petroleum are separated by fractional distillation, i.e. the difference in the boiling points of the components is used as a basis for separating them.

The table below gives the major fractions obtained by the fractional distillation of crude oil, their boiling points and uses.

 Fraction  Number of Carbon Atoms per Molecule  Boiling Point Range OC  Uses
 Natural gas  1- 4  Below 20  Fuel for heating and lighting, for the manufacture of certain compounds, such as ethyne, hydrogen, carbondisulphide, and tetrachloromethane.
 Petroleum ethers and ligroin  5 - 7  20 - 120  As organic solvent
 Petrol (gasoline)  5 - 12  40 - 205  As motor and aeroplane fuel, for heating and lighting, and as solvent for grease and paints.
 Kerosene (paraffin oils)  12 - 18  175 - 325  For heating and lighting; fuel for jet engine, and for driving tractors. Also as solvent for grease and paints.
 Gas oil and diesel oil  12 - 25  275 - 400  As fuel for diesel engine, for heating, and as raw materials in the cracking process.
 Lubricating oils, waxes, grease, and vaseline  Above 20  Above 400  Lubrication, medicine, manufacture of candle, polish, ointments, creams, and water proofing materials.
 Bitumen, asphalt, and other residue  Above 40  Above 400  For surfacing roads and air fields; other residues may be used as fuel, as protective pipe coating, and as paint. 

Cracking of Petroleum

Cracking of petroleum is the breakdown of hydrocarbon fractions (alkanes) with large number of carbon atoms into smaller molecules, or the conversion of straight chain hydrocarbons to branched-chain hydrocarbons.

There are two methods of cracking - thermal (carried out by heating to high temperatures of about 700oC and pressures of about 30 atm) and catalytic (the use of aluminium oxide and silicon(IV) oxides as catalyst).

Reason for Cracking

1. To obtain large quantity of petrol - i.e., to produce more of small molecular hydrocarbons, which are in the C5 - C12 range. Petrol is high in demand, but is produced from the fractional distillation of crude oil in small quantity.

2. To produce petrol of higher quality - i.e., to convert straight- chain petrol to branched-chain petrol. The branched-chain petrol burn more smoothly, hence they are more efficient in an internal combustion engine than the straight-chain petrol which is produced more from the fractional distillation of crude oil.

Examples of reactions that occur during cracking process:

1. Conversion of large molecular alkane to high grade petrol containing branched-chains.

         C C16H34 ®      C8H18       +        C8H16
Branched octane               Octene

2. Conversion of a straight chain alkane to a branched-chain.

C7H16  ® CH3CH(CH3)CH(CH3)CH2CH3
2,3-dimethyl Pentane

3. Conversion of a straight chain alkane to a branched chain alkane and an alkene.

2-methyl pentane
 +  C2H4

 4. Conversion of a straight chain alkane to a cycloalkane and hydrogen gas

C4H10  ®  
+ H2(g)    

Note: cracking process yields also several gases as by-products, e.g., hydrogen, ethene, benzene, ethane, butene, propene and propane, which can be used in reforming alkanes in the petrol range, and as raw materials in the petrochemical industry for the manufacture of several useful chemicals and products.

Example, hydrogen is used for the manufacture of ammonia; ethane for ethanol; benzene and propene for the manufacture of drugs, detergents, plastics, synthetic rubber, fertilizers, weed killers and synthetic fibres.

Reformation of Petrol

Hydrocarbons in the petrol range, as well as special fuels (e.g. high octane aviation spirit), can be reformed from gaseous hydrocarbons. The processes which can be used in carrying this out are:

(1) polymerization (2) alkylation (3) isomerization and (4) hydrogenation. These are summarized below:

1. Polymerization and hydrogenation

    Isobutene        Isobutene                             Iso-octene

2. alkylation

isobutane             isobutene                                   iso-octane
(methyl propane)   (methyl propene)                    (2,2,4-trimethylpentane)    

3. isomerization


Quality of Petrol and Meaning of Octane Number

The quality of petrol is a measure of how it burns in an internal combustion engine. A poor quality petrol burns rapidly and unevenly and generates an explosion which disturbs the up and down movement of the piston, resulting in a strange sound normally referred to as ‘Knocking’ - this petrol contains large proportion of straight chain alkanes.

A good quality petrol on the other hand burns smoothly and evenly - this contains a large proportion of branched alkanes.

Note:: A given petrol consists of both branched and straight chain alkanes.

The octane number of a petrol is an arbitrary number which denotes the proportion of the branched-chain to the straight chain alkanes in the petrol. A scale which ranges from zero (for n-hepane - straight chain alkane) to 100 (for 2,2,4 - trimethyl pentane - branched chain alkane) has been devised.

Hence, a petrol with an octane number of 95 consists of 95 parts of the branched alkane (2,2,4 - trimethyl pentane) and just 5 parts of the straight-chain alkane (n-heptane) - this is a very high quality petrol and is regarded as a ‘five star’ or ‘super’ petrol. The octane rating of the regular petrol is between 85 and 92.

Uses of Petrol Additives

High octane rated petrol is very expensive and might not be readily affordable, especially in the third world countries. Certain chemicals have been developed to be used as additives. These chemicals function to increase the octane rating of low grade petrol by slowing the combustion rate of the petrol - they are therefore called anti-knock.

Examples of petrol additives are tetraethyl lead(IV) (TEL), Pb(C(C2H5)4; methanol and ethanol.

Note: Tetraethyl lead(IV) is no longer in use, especially in the developed countries due to its environmental pollution effect - it introduces lead into the environment, thereby causing severe environmental and health hazards.


Hydrocarbon and Alkanes
Isomerism in Alkanes
IUPAC Nomenclature of Alkanes


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