Organic Chemistry IGCSE
Hydrocarbons are compounds contain carbon and hydrogen only.
There are two types of hydrocarbons: Saturated and unsaturated hydrocarbons
1. If all bonds between carbon atoms are single, the compounds are saturated.
Saturated hydrocarbons are called Alkanes. Following are the important alkanes:
Methane (1 carbon atom) CH4
Ethane (2 carbon atoms) C2H6
Propane (3 carbon atoms) C3H8
Butane (4 carbon atoms) C4H10
Saturated compounds (alkanes) are not reactive because all four valencies of carbon atoms are
full.
2. If a double bond is present between two carbon atoms, the compounds are unsaturated.
Unsaturated hydrocarbons are Alkenes. Following are the important alkenes:
Ethene (2 carbon atoms) C2H4
Propene (3 carbon atoms) C3H6
Butene (4 carbon atoms) C4H8
Unsaturated compounds (alkenes) are very reactive because all four valencies of carbon atoms
are not full.
Petroleum (crude oil)
Petroleum is a natural resource for all hydrocarbons. It is formed from the dead bodies of
animals and plants. The dead bodies are covered with mud and soil. Due to changes in
temperature and pressure, the dead bodies of animals and plants are changed in to petroleum
and the soil and mud to sedimentary rock.
Crude oil contains many useful substances. These substances are separated from crude oil by
fractional distillation. Fractional distillation is a process depending on the boiling points. Light
fractions with low boiling points are collected at the top of the fractionating column while heavy
fractions with high boiling points are collected at the bottom.
Cracking
This is a process of breaking large hydrocarbons in to small molecules. These small molecules
are more useful than those large molecules. Cracking is done using catalyst (catalytic
cracking) or heat (thermal cracking). Alkene (ethane) will always be a by-product during
cracking. This alkenes is useful for making plastics or other substances.
For example:
Test for unsaturation
Mix the compound with Bromine water. Bromine water changes the colour from red-brown to
colourless if the compound is unsaturated. No change in colour if the compound is saturated.
Homologous series
Group of compounds having:




A general formula
Similar chemical properties
Gradual change in physical properties
Successive member differ by a –CH2 group
is called a homologous series. Alkanes, alkenes, alcohols and carboxylic acid are
homologous series.
Functional group:
A double bond or atoms which determine the function of a compound is called a functional
group.
=, -OH, -Cl, -COOH are functional groups.
Alcohols
Alcohols are organic compounds with a –OH group as the functional group. Examples are:
Methanol
Ethanol
CH3OH
C2H5OH
Propanol
C3H7OH
Butanol
C4H9OH
Making alcohol. There are two methods: Batch process and continuous process
Batch process:
Sugar (glucose) is mixed with yeast and keep it at 30-40oC for 48 hours. The enzymes in yeast
convert sugar in to alcohol and carbon dioxide. This process is called fermentation. Alcohol is
separated from the mixture by distillation.
This process is slow but environmental friendly (no pollution). The row materials are renewable.
Continuous process
Ethene is mixed with steam and passed over a catalyst. Water is added to ethene to produce
ethanol.
The process can pollute air because fuel is burnt for energy. The row material ethene is not
renewable. But the process is quick.
Hydrocarbons and alcohols are good fuels. They all produce carbon dioxide and water when
burn in air.
When air/oxygen is limited, hydrocarbons produce carbon monoxide together with water.
Carboxylic acids
Organic acids have functional group –COOH
Examples are:
Methanoic acid:
Ethanoic acid
HCOOH
CH3COOH
Propanoic acid
C2H5 COOH
Esterification Reaction
Carboxylic acid and alcohol react in presence of con sulphuric acid to produce ester and water.
Esters are sweet smelling substance and so esters are used to make perfumes and food
flavourings and artificial fruit juices.
Carboxylic acids are weak acids.
Addition reactions of alkenes
Atoms can be added to the double bond. The product will have single bond and only one
product will be formed. Such a reaction is called addition reaction.
1. Addition of hydrogen
This is done in presence of a catalyst.
Ethene + hydrogen  Ethane
Addition of a halogen
Ethene + Chlorine  Dichloro ethane
Addition of water
Ethene + water (steam)  ethanol (catalyst)
Isomerism
Isomerism is compounds having same molecular formula but different structural formula
Polymerisation
When small molecules combine to form a large molecule, the process is called polymerisation.
Small molecules involved are called monomers and large molecule formed is called polymer.
There are two types of polymerisation: Addition polymerisation and condensation
polymerisation.
Addition polymerisation: Mainly plastics are made. Polymer is the only product.
Poly ethane
Monomer: Ethene
OR
Poly propene
Monomer: Propene
OR
Teflon (Poly tetra fluoro ethane)
Monomer: tetrafluoro ethane
OR
Condensation polymerisation:
In condensation polymerisation, functional groups react
and small molecules like water is also formed.
Examples for IGCSE
Terylene (poly ester)
Nylon (Poly amide)
Natural polymers: Starch, Proteins and Lipids (oils)
Starch is made up of glucose molecules (monomers) by condensation reaction of –OH groups
Water is removed
Proteins are made up of amino acid monomers by condensation reaction.
Lipids are made up of glycerol and fatty acid molecules
Glycerol + fatty acid  Lipid + water
If water is added to starch, proteins or lips, reverse reaction occurs and the monomers will be
formed (glucose, amino acids, fatty acids etc). This process is called hydrolysis of natural
polymers. In human body, this is done by digestive enzymes and so it is also called digestion.
Starch  Glucose (hydrolysis or digestion)
Proteins  amino acids (hydrolysis or digestion)
Lipids (fats)  fatty acids + glycerol (hydrolysis or digestion)