The Alkanes

A group of organic compounds containing several homologous series.

Only contain carbon and hydrogen. 

They can be divided into Aliphatic, Alicyclics, and Arenes. 

All insoluble in water. They all burn, in sufficient oxygen, to give of CO2 and H2O. 

The Aliphatic hydrocarbons can be classified into 3 families: alkanes, alkenes and alkynes. 

A family of saturated hydrocarbons - no double, or triple bonds. 

C_nH_2n+2

They can occur as straight or branched chain molecules. 

There are considerable structural isomerism in Alkanes. 

Single most important use is fossil fuels. 

Over many years decaying matter has been trapped and has formed crude oil or petroleum. 

It is also found in natural gas, methane. 

Primary distillation is used to turn crude oil from a dark, thick liquid into useful chemicals. - An industrial version of fractional distillation used in labatories. 

The liquid collected over each range of temperatues is known as fraction:

• 1-2% refinary gas - the lightest fraction of crude oil with the lowest boiling point. 
• 15-30% gasoline - a mixture of liquid hydrocarbons. Mainly used to power cars. 
• 10-15% kerosene - mainly C11 and C12 fractions, used as fuel for aircrafts. 
• 15-20% diesel oil/gas oil - used both in diesel engines and as a fuel for industrial boilers.
• 40-50% residue - complex mixture of hydrocarbons is very viscous and has a high boiling temperature. Used as fuel for the furnace in power stations. 

Crude oil from different sources contains varying proportions of the different fractions. 

To address this imbalance scientist have developed a way of breaking down the heavier, long-chain fractions - Cracking. 

Heating long-chain alkanes to high temperatures causes the molecule to split and form shorter-chain molecules. 

Producing the high temperature is very expensive. By using a catalyst it can be carried out at lower temperatures. - Catalytic or cat cracker. 

Breaking bonds is also known as bond fission - when a covalent bond is broken two electrons are shared between two atoms. There are two ways inwhich these electrons can be shared out: 

Homolytic fission involves the equal sharing out of the electrons in this bond. 

Heterolytic fission involves an unequal sharing of the electrons of the covalent bond. - Usually seen when the covalent bond has a degree of polarity. 

Cracking (thermal decompostion)

When heated to high temperatures they split into smaller molecules. 

Combustion

When alkanes are heated in a plentiful supply of air, combustion occurs. 

Alkanes are unstable in relation to water and carbon dioxide.

Combustion of alkanes is used to generate electricity, to fuel fire in our homes, and to provide cenral heating. 

Alkanes react with Chlorine but only with an input from ultraviolet light or sunlight. 

The light breaks the hydrocarbon bonds; initiation. Chlorine atoms then substitute for the hydrogen atoms. This is an example of a substitution mechanism.

Cl-Cl bonds are easier to break than the C-H bonds. 

The splitting of molecules is homolytic fission. Forming free radicals. 

The free radicals react with the methane molecules. Combing with the H to form another free radical. 

The methyl free radical reacts with another chlorine atoms. Forming another Cl free radical.

This process is repeated hundreds of times in these propagation reactions. - Chain reactions. 

This continues until there is a termination step. A reaction between the two radicals.