Alkanes

Alkanes are saturated hydrocarbons — they contain only carbon—carbon and carbon—hydrogen single bonds.

They are among the least reactive organic compounds.

They are used as fuels and lubricants and as starting materials for a range of other compounds. This means that they are very important to industry. 
The main source of alkanes is crude oil. 

The general formula 
The general formula for all chain alkanes is C_n H _2n+2 Hydrocarbons may be unbranched chains, branched chains, or rings. 



Unbranched chains Unbranched chains are often called straight chains but the C—C—C angle is 109.5°. This means that the chains are not actually straight. In an unbranched alkane, each carbon atom has two hydrogen atoms except the end carbons which have one extra. 

For example, pentane, C₅H₁₂

 

 

Alkanes

Physical properties

POLARITY

Alkanes  are almost non polar

Why ?

Refer to subjects

Electronegativity

 

Polarity

Polar bond

 

Polar molecule

Polar compound

Examples of polar compounds

Examples of non-polar compounds although polar bond

 

 

 

Intermolecular forces between molecules are weak van der waals forces

The larger the molecule , the more atoms are there , the more electrons ,the stronger the Van der waal forces

BOILING POINTS

Increasing chain length

Increases no of atoms , increases electrons , increases VDW bond strength

Increasing energy required to break intermolecular bonds so increases boiling points of them

The short chains are gases at room temperature

C1,2,3,4

Pentane and other alkanes until 17 carbon alkanes are liquid

And alkanes with about 18 carbon atoms are solids at room temperature

The solids have a waxy feel

Alkanes with branched chains

They molcules cannot pack together closely like in unbranched chains so the attraction forces are less in between molecules and the van der waals are not very effective so the melting and boiling points are less

Solubility of alkanes

Alkanes are insoluble in water

This is because water molecules are held together by hydrogen bonds which are more strong than the van der waals forces attracting the alkanes molecules

(IN ORDER TO DISSOLVE IN WATER THE SUBSTANCE MUST HAVE STRONGER BONDS THAN THE BONDS BETWEEN THE WATER MOLECULES )

ALKANES ARE MISCIBLE WITH RELATIVELY NON-POLAR LIQUIDS

 

 

Alkanes are relatively unreactive

This is due to the strong carbon-carbon and carbon-hydrogen bonds

They do not react with acids , bases , oxidising agents and reducing agents.

However they do have two main reactions :

(1) Combustion: they burn

a) In plentiful supply of oxygen to form carbon dioxide and water

b) In restricted supply of oxygen to form carbon monoxide and water

(2) Halogenation: they react with halogens under suitable conditions

 

Fractional Distillation of crude oil

·       Fossil fuels : fuels formed over millions of years by breakdown of plant and animal remains at high temperatures and pressure deep below the Earth’s surface.

·       It is non-renewable as if it forms very slowly

·       Crude oil is a mixture of mostly alkanes

·       Different crude oils from different places will have different composition

 

·       Impurities in crude oil :

For example sulfur.

When burnt it will produce Sulfur dioxide SO₂

This will react with oxygen in the atmosphere to produce sulfur trioxide. The later will react with water to form sulfuric acid which will cause acid rain.