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Organic Chemistry

Petroleum: A Fossil Fuel

The fossil fuels

The fossil fuels are petroleum, coal and natural gas. They are named as fossil fuels because they are the remains of plants and animals that lived millions of years ago.

Petroleum formed from the remains of dead organisms that fell to the ocean floor and were buried under thick sediment.  High pressures slowly converted them to petroleum over millions of years.

Natural gas is mainly methane. It is often found with petroleum. It is formed in the same way but high temperatures and high pressures caused the compounds to break down to gas.

Coal is the remains of lush vegetation that grew in ancient swamps. The dead vegetation was buried under thick sediment. Pressure and heat slowly converted it to coal over millions of years.

What is in petroleum?

Petroleum is a smelly mixture of hundreds of different compounds. They are organic compounds which mean that they contain carbon and usually hydrogen. In fact most are hydro carbons. They contain only carbon and hydrogen.


A formula drawn out in the above way is called a structural formula. Carbon atoms are bonded to each other to make spine of each molecule. The hydrogen atoms are bonded to the carbon atoms. In petroleum we can find hydrocarbon molecules of different shapes and sizes with different number of carbon atoms from 1 to over 70.

How we use petroleum

Around half the petroleum pumped from oil wells is used for transport. It provides the fuel for cars, trucks, planes and ships. Most of the rest is burnt for heat in factories, homes and power stations. A small percentage is used as the starting chemicals to make many other things: plastics, shampoo, paint, thread, fabric and more.

A non-renewable resource

Petroleum is still forming slowly under the oceans. But it will run out one day as we are using it much faster.

Some experts say the world’s reserves will last about 40 more years.

Refining petroleum

Refining is the separation of the compounds into groups with molecules of a similar size. Petroleum contains hundreds of different hydrocarbons. Hence it needs to be separated. It is carried out by fractional distillation.

Refining petroleum in the lab

When the petroleum is heated, the compounds start to evaporate. The ones with smaller lighter molecules go first since it takes less energy to free these from the liquid.

As the hot vapours rise, so does the thermometer reading. The vapours condense in the cool test tube.

When the thermometer reading reached 100oC, replace the first test tube with an empty one. The first fraction from distillation will be the liquid in the first test tube.

Three more fractions are collected in the same way at 150 oC, 200 oC and 300 oC.

Comparing the fractions

The above fractions are compared to check how they burn etc.

Fraction 1 catches fire easily. The flame burns high, which shows that the liquid is volatile so it evaporates easily.

Fraction 2 catches fire quite easily. The flame burns less high. So this fraction is less volatile than fraction 1.

Fraction 3 seems less volatile than fraction 2. It does not catch fire readily or burn easily. It is not so flammable.

Fraction 4 does not ignite easily. A wick is to be used to keep it burning. It is the least flammable of the four.

These implications show that the larger the molecules in a hydrocarbon:

  1. The higher its boiling point will be.
  2. The less volatile it will be.
  3. The less easily it will flow.
  4. The less easily it will burn.

Cracking hydrocarbons

After fractional distillation

Petroleum is separated into fraction by fractional distillation. The fractions all need further treatment before they can be used.

  1. They contain impurities mainly sulfur compounds. These will burn to form harmful sulfur dioxide gas if left in the fuels.
  2. Some fractions are separated further into single compounds or smaller groups of compounds.
  3. Part of fraction may be cracked.

Cracking breaks molecules down into smaller ones.

Cracking a hydrocarbon

Heating causes the hydro carbon to break down and this process is called cracking. A thermal decomposition occurs. We need to see that

  1. The reactant which is having a high boiling point and was not flammable- which means it had large molecules with long chains of carbon atoms.
  2. The product has a low boiling point and is very volatile. So it must have small molecules with short carbons.
  3. Also the product must be a hydrocarbon since nothing new was added.

Cracking is carried out in the following way in a refinery

  1. The long-chain hydrocarbon is heated to vaporize it.
  2. The vapour is usually passed over a hot catalyst.
  3. Thermal decomposition takes place.

Why cracking is important?

  1. Cracking helps you to make the best use of petroleum.
  2. Cracking always produces short-chain compounds with a carbon-carbon double bond. This bond makes the compounds reactive.

Families of organic compounds

 There are millions of organic compounds. To avoid confusion, the compounds are named in a very logical way.

  1. If the name ends in –ane the compound belongs to the alkanes. Eg. Ethane, C2H6
  2. If the name ends in –ene the compound belongs to the alkenes. Eg. Ethene, C2H4
  3. If the name ends in –ol the compound belongs to the alcohols. Eg. Ethanol, C2H5OH
  4. If the name ends in –oic acid the compound belongs to the carboxylic acids. Eg. Ethanoic acid CH3COOH
  5. The name meth means there are 1 carbon atoms, methane, CH4
  6. The name eth means there are 2 carbon atoms, ethane, C2H6
  7. The name prop means there are 3 carbon atoms, propane, C3H8
  8. The name but means there are 4 carbon atoms, methane, C4H10
  9. The name pent means there are 5 carbon atoms, methane, C5H12
  10. The name hex means there are 6 carbon atoms, methane, C6H14

Functional groups

A functional group is defined as that part of a molecule that largely dictates how the molecule will react.

Homologous series

In a homologous series:

  1. All the compounds have the same general formula.
  2. The chain length increases by 1 each time
  3. As the chain gets longer, the compounds show a gradual change in properties.

The alkanes

Key points about alkanes

  1. The alkanes are the simplest family of organic compounds.
  2. They are hydrocarbons. They contain only carbon and hydrogen.
  3. Their carbon-carbon bonds are all single bonds.
  4. They form a homologous series with the general formula CnH2n+2
  5. They are found in petroleum and natural gas.
  6. The alkanes are called saturated as their carbon-carbon bonds are single bonds.
  7. They alkanes are quite unreactive.
  8. Alkanes do burn well in an excellent supply of oxygen and forms carbon dioxide and water vapour. It also gives out plenty of heat. So they are used as fuels.
  9. The alkanes undergo incomplete combustion if there is not enough oxygen. It produces poisonous carbon monoxide.


Though some alkane molecules have the same formula they have different structures.

Isomers are compounds with the same formula but different structures. For example, there are 75 isomers with the formula C10H22.

The alkenes

Key points about alkenes

  1. The alkenes are hydrocarbons
  2. They form a homologous series with the general formula CnH2n.
  3. They contain the C=C double bond.
  4. They are unsaturated as they contain double bond.
  5. The alkenes are made from alkanes by cracking.
  6. Alkenes are much more reactive than alkanes because the double bond can break to add on other atoms. This reaction is called addition reactions. An addition reaction turns an unsaturated alkene into a saturated compound.


Alkene molecules undergo addition reaction in which they add on each other to form compounds with very long carbon chains. The alkene molecules are called monomers. The long chain compounds that form are called polymers. The reaction is called polymerisation.

The alcohols

What are alcohols?

The alcohols are the family of organic compounds that contain the OH group.

Key points about alcohols

  1. They form a homologous series with the general formula CnH2n+1OH.
  2. Their OH functional group means they all will react in a similar way.

Ethanol, an important alcohol

  1. Ethanol is the  vital alcohol in alcoholic drinks
  2. It is a good solvent. It dissolves many substances that do not dissolve in water.
  3. It evaporates easily. It is volatile.
  4. Ethanol burns well in oxygen giving out plenty of heat. Hence it is used as fuel in car engines.
  5. It can be made quite cheaply from waste plant material.
  6. Ethanol is an attractive option in many countries which have no petroleum and have to buy it from other countries.

Ethanol is made by two processes namely fermentation and the hydration of Ethene.

Ethanol also produces carbon dioxide when it burns like other fossil fuels. This is a greenhouse gas, linked to global warming. But ethanol has less impact on carbon dioxide levels in the atmosphere.

The carboxylic acids

Key points about carboxylic acids

  1. The family forms a homologous series with the general formula CnH2nO2.
  2. The functional group COOH is also called the carboxyl group.

Ethanoic acid is a major acid in carboxylic acids. It is made by fermentation and by using oxidizing agents.


Ethanoic acid reacts with alcohols to give compounds called esters.

Some points to be noted:

  1. Two molecules have joined to make a larger molecule, with the loss of a small molecule, water. So this is called a condensation reaction.
  2.  The reaction is reversible and sulfuric acid acts as a catalyst.
  3. The alcohol part comes first in the name but second in the formula.
  4. Propyl ethanoate (CH3COOCH2CH3) smells of pears. Many esters have attractive smell and tastes. So they are added to shampoos and soaps for their smells and also for ice creams as flavourings.