Introduction to Hydrocarbons


Welcome to class! 

In today’s class, we will be talking about the introduction to hydrocarbons. Enjoy the class!




  1. Sources of hydrocarbon
  2. Classification (of aliphatic hydrocarbon)
  3. Petroleum and natural gases
  • Refining of Petroleum by fractional distillation
  • Uses of different fractions
  • Cracking of petroleum (thermal and catalytic)
  • Antiknock and Octane rating



Hydrocarbons are very simple organic compound composed mainly of hydrogen and carbon only. The sources of hydrocarbons are coal, natural gases and petroleum.

The hydrocarbon can be divided into two main classes:

  1. Aliphatic hydrocarbon
  2. Aromatic hydrocarbon

They are further divided into three groups: Alkanes, Alkenes and Alkynes. The Aliphatic may be Acyclic or Cyclic. The acyclic hydrocarbons are the straight or branched chain hydrocarbon while the cyclic hydrocarbons consist of closed ring chain such as cycloalkane e.g. cyclopropane.


Contain ring structure having non-localized orbital e.g. C6H6. Aromatic hydrocarbons are all cyclic hydrocarbons. The basic cyclic structure is the benzene ring.


Petroleum is the chief source of aliphatic hydrocarbon. It is a dark viscous liquid which is usually trapped or found under the ground or sea beds in a certain part of the world e.g. Nigeria, Saudi Arabia, Iran, USA, Iraq and Russia. Petroleum is a mixture of Alkanes, alkenes, cycloalkanes and aromatic hydrocarbons together with about 1 – 6% impurities consisting mainly of compounds of sulphur and minute quantity of H2 and O2 compounds. Natural gas consists mainly of methane.

Origin of Crude oil and Natural gas

Natural gas and petroleum are formed by the decomposition of vast quantities of organic material, undoubtedly of marine origin, buried in sediment. When these tiny aquatic organisms died, their remains gradually settled on the sea beds. Over the years, the remains became covered by mud, silt and other sediments. As the sediment piled up, their mass exerted great pressure on the lower layers, changing them to hard sedimentary rocks. During this process, bacterial activity, heat and pressure probably changed the plants and animal remains into crude oil and natural gas. The oil and gas so formed slowly moved to other areas through the tiny holes or pores in the porous rocks around them. Since oil and gas are not dense, they tend to seep upwards until they meet a non- porous layer or rocks and are trapped under it, thus forming an oil trap.


The process of petroleum refining is basically that of converting crude oil into a range of products required to meet economic market demand. How is this achieved? Crude oil consists of a very complex mixture of hydrocarbons, which individually, they exist as gas, liquid or solid at normal temperatures and pressures. The crude oil can be separated into fractions by comparatively simple distillation and for every given variety of crude oil; their relative proportions and properties are fixed. Modern competitive marketing conditions, however, demand that these fractions from crude oil are of such a quality that simple distillation is not enough.

Fractional distillation, necessitating more advanced refinery techniques is now adopted.

Crude oil (petroleum) is composed mainly of a complex mixture of hydrocarbons. By using fractional distillation, crude oil can be separated into fractions or groups of similar compounds. Each fraction contains several compounds all of which fall within a certain range of boiling points. These fractions can be differentiated from one another by their different volatility, odour texture and their relative rate of ignition and burning. The fractional distillation is carried out in a fractionating column of towers. The crude oil is passed into a fractionating column with a temperature ranging between 4000C at the bottom of the column of the steel pipe and 400C at the top part of the column.

The fractionating column is divided into several compartments by perforated plates called trays, each of which is maintained within a specified range of temperature. The crude oil is first heated to about 400oC so that all the components are vaporised. The vapour enters the bottom of the fractionating column. They rise up the column and cool. Those with high boiling points will soon condense to liquids and will not move far up the column, whereas those with low boiling points will have to cool considerably before they condense and so will move towards the top of the column. This means that substances with higher boiling points separated out in the trays on the lower part of the column, while those with lower boiling points separate out in the trays on the upper part of the column. The fractions are collected in horizontal trays at different heights on the column, redistilled to improve the purity and then further treated to obtain different liquid fuels and petrol chemicals.

The petroleum fractions are gases, petrol, kerosene, diesel oil, lubricating oil and bitumen.

  1. Natural gas: The gas fractions consist mainly of a hydrocarbon containing 1 – 4 carbon atoms per molecule and distilling around 40oC. These are methane, ethane, ethane, propane and butane. Methane and ethane are usually burnt as fuel. The propane and butane are liquefied and distributed in high-pressure gas cylinder or tank to the public for lighting and heating purposes in homes. They are also used for synthesising a large number of compound e.g. methanol, butadiene etc., They are also used for the manufacture of products like hydrogen, carbon(iv)sulphide, tetrachloromethane and ethyne.
  2. Petrol or Motor gasoline: Petrol is the most important product derived from petroleum because of the rapid increase of the use of motor vehicles. Petrol is a complex mixture of volatile hydrocarbons containing C6 – C10 carbon atoms per molecule.(such as hexane, heptane and octane) distilling off between 500C – 2000C. Petrol is used as a fuel for aeroplanes and vehicles. It is also a good solvent for paints, grease and stains etc. It is a volatile liquid. Since straight chains hydrocarbons making up the petrol fraction of petroleum usually cause engine knock and engine wear, they have to be reformed to branched-chain hydrocarbons which are not prone to knocking.
  3. Kerosene or paraffin oil: This is a mixture of hydrocarbons containing C10 – C16 carbon atoms per molecule and boiling between 1700C – 2500C. It is a fairly volatile liquid and is used as a fuel for lighting and heating. It is also used as major fuel in jet engines, aeroplanes and tractors and gas turbines. It is a good solvent for grease and paints.
  4. Gas Oil or Diesel oil: This is a mixture of hydrocarbons containing C14 – C18 carbon atoms per molecule and boiling between 3000C – 3600C.It is used in the internal combustion of diesel engines of trains, lorries and tractors etc., They are also used as raw materials in the cracking process.
  5. Lubricating oil, Grease and wax: It is a mixture of long-chain hydrocarbons with more than 20 carbons atoms per molecule which distil over in the temperature range of 3500C – 5000C.They are viscous liquids used as a lubricant for moving parts of engines and machines and also for making Vaseline or petroleum jelly. Paraffin wax is used for making candles, waterproof materials, polish, grease ointment and cream
  6. Bitumen or Asphalt: It is a complex mixture of non-soluble solids made of polycyclic hydrocarbons. It is used as a biding agent for roofing materials and in road surfacing as a protective coating.
  Name of fraction Boiling Point

Range in 0C

Carbon atoms

in molecules



1. Petroleum gas below 40 1 – 4 Fuel and manufacture of other
2. Petrol 40 – 200 4 – 12 Fuel in aeroplanes and motor vehicles
3. Kerosene 200 – 250 12 – 18 Fuel for lighting, heating and jet engine
4. Gas oil  250-350 12-25 Fuel for heating and diesel engines. Diesel oil

Raw materials for cracking


5. Lubricating Oil 350 – 500 More than 20 Lubricating moving parts of

machines, Making candles, creams & hair care products

6. Bitumen Above 500 more than 35 Surfacing roads


In our next class, we will be talking about Cracking and Reforming.  We hope you enjoyed the class.

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