Alkenes

C_nH_2n

• A few alkenes are found in petroleum
• but most are obtained by the cracking of alkanes

• The names of Alkenes end in -ene
• As the alkenes are in a homologous series, the formula of one alkene differs from the formula of the next by a -CH₂ group.
• Thus C₂H₄ differs from C₃H₆ by CH₂.

• Their physical properties change gradually as the number of carbon atoms increase.
• Their mp, bp, densities and viscosities increase whereas the flammability decreases.

• All alkenes contain a C=C double bond.
• Organic compounds with one or more double bonds are said to be unsaturated.
• This is because NOT all the carbon atoms are bonded to their maximum number of four other atoms.

Ethene is the MOST IMPORTANT ALKENE

It is used to make many useful industrial chemicals and products. These include the manufacture of:-

•    -ethanol (used as solvent & fuel)
•    -ethanoic acid (used in vinegar)
•    -poly(ethene) (an important plasic)
•    -chemicals for making other plastics

• Because alkenes have the same functional group C=C, they have similar chemical properties.
• The presence of the double bond makes Alkenes more reactive than Alkanes

Alkenes burn in air like alkanes. An example is the combustion of ethene.

• ethene(g) + oxygen(g) ---> carbon dioxide(g) + water(l)
• C₂H₄(g) + 3O₂(g) ---> 2CO₂(g) + 2H₂O(l)

• Some carbon monoxide and soot (unburnt carbon particles) are produced if the combustion is incomplete.
• Alkenes produce more soot than alkanes when they burn in air.

The C=C group in Alkenes reacts with chemicals in addition reactions.

• ADDITION OF HYDROGEN
• Alkenes react with hydrogen to produce Alkanes.
• This reaction is called hydrogenation. A nickel (Ni) catalyst must be used.
• An example is the reaction of ethene with hydrogen.

• C₂H₄(g) + H₂(g) ---> C₂H₆(g)
• ethene(g) + hydrogen(g) ---> ethane(g)

Hydrogenation is used to change vegetable oils into margarine.

• ADDITION OF BROMINE
• Bromine reacts with ethene in an adition reaction. The bromine molecule adds t the C=C double bond of the ethene molecules.

• ADDITION OF WATER
• Alkenes react wth water to produce alcohols.
• A molecule of H₂O adds to the C=C double bonds.
• This is done by passing a mixture of alkene and steam over a catalyst.
• An example is the addition of water to ethene, producing ethanol.
• C₂H₄(g) + H₂O(l) ---> C₂H₅OH(l)
• ethene + water ---ethanol
• Alcohols are important solvents. They are also used to make other chemicals.

• Thousands of identical alkene molecules can join together to make one big molecule, called a polymer.
• For example, thousands of ethene molecule can be joined together to make the polymer poly(ethene)

• Alkanes and other unsaturated compounds react with bromine in addition reactions.
• The reddish-brown colour of the bromine quickly disappears.
• We say the bromine has been decolourised.