Families of organic compounds
Organic chemistry is the chemistry of organic carbon compounds other than CO, CO2 and carbonates.
Carbon atoms form the spine of hydrocarbon molecules. When they are joined by single covalent carbon-carbon bonds only, we say the hydrocarbons are saturated – they can’t take part in addition reactions.
They are known as ALKANES
Addition reactions : involve 2 or more molecules reacting to form 1 molecule
A homologous series : is a family of compounds with a general formula, similar chemical properties, and a regular graduation (trend) in physical properties (e.g. density, boiling point, etc.)
| Alkane | Formula | Boiling point / °C |
| Methane | CH4 | -162.0 |
| Ethane | C2H6 | -88.6 |
| Propane | C3H8 | -44.2 |
| Butane | C4H10 | -0.5 |
| Pentane | C5H12 | 36.3 |
| Hexane | C6H14 | 68.7 |
We see that the general formula is CnH2n+2 and that the chain length increases by a CH2 unit each time.
Addition polymerisation is where 2 or more molecules react to form one long chain molecule with a repeating unit.
Alkane hydrocarbons are said to be SATURATED, i.e. they can’t take part in addition reactions. Alene hydrocarbons form a homologous series with general formula CnH2n and they are UNSATURATED because they can take part in addition reactions.
Here are the first 2 alkenes :
Alkenes are more reactive than alkanes because of their C=C double bond
The alkenes undergo addition reactions in which one of the C=C double dons breaks, allowing each carbon atom to form another hydrocarbon bond. One bond breaks, 2 bonds are formed.
Test for unsaturation
C2H4 + Br2 à C2H4Br2
When bromine is added, the solution will become coloured (red-brown). As the unsaturated molecules react, the colour will go, and the solution will become decolourised.
N.B. We remember that dilute solutions of bromine are yellow
Combustion of organic compounds
Alkane hydrocarbons and alcohols have a widespread use as fuels - they burn easily to produce energy economically. Alkenes will also burn, but they have valuable uses as industrial raw materials so they don’t tend to be used as fuels.
When any compound burns, the products are the oxides of the elements in the compound - combustion is a reaction with the oxygen in the air, accompanied by heat, light and often a flame.
When organic compounds are burnt in a plentiful supply of air, the carbon in them is oxidised to carbon dioxide and the hydrocarbon is oxidised to water. This is called COMPLETE COMBUSTION.
In a limited supply of air, incomplete combustion occurs, carbon monoxide and/or carbon (soot) and/or unburnt organic compound. Carbon monoxide is poisonous because it reduces the capacity of blood to carry oxygen.
Examples
C8H18 + 25O2 à 8CO2 + 9H2O
Octane
C2H4 + 3O2 à 2CO2 + 2H2O
Ethene
Alkenes have a high proportion of carbon relative to hydrogen in their molecules and so when they burn, they often produce CO (evidence - yellow flame) and CC (smoky flame) by incomplete combustion. We have seen that alkenes are too valuable to be used as fuels (i.e. plastics)
CH3CH2OH(l) + 31/2 O2 à 2CO2 + 3H2O