GCSE OCR Gateway Organic Chemistry C6.2c Addition reactions

GCSE OCR Gateway Organic Chemistry C6.2c Addition reactions

C6.2c To be able to predict the formulae and structures of products of reactions of the first four and other given members of the homologous series of alkanes, alkenes and alcohols

Reactions to include combustion; addition of bromine and hydrogen across a double bond; oxidation of alcohols to carboxylic acids using potassium manganate(VII)

Addition reactions of the alkenes

Addition reactions in alkenes occur across the carbon—carbon double bond.

In the addition reactions, essentially what happens is that one of the two bonds in the double bond breaks since one of the two is weaker than the other.

The reactant splits in two and each atom bonds to the two carbon atoms of the double bond.

The conditions are usually sufficient to set up a reaction in which the weaker bond breaks. 

a)    Bromination

You need to use neat bromine at room temperature

CH₂=CH₂(g)    +      Br₂(l)      ➞        CH₂Br CH₂Br(l)

                                 Ethene                                            1,2–dibromoethane

There is a colour change from orange bromine to colourless product.

This reaction is used as a test for the presence of a double bond in a hydrocarbon.

The reaction looks like this if we use displayed formulae:

b)    Hydrogenation

You need to use gaseous hydrogen in the presence of a finely divided nickel (Ni) catalyst at 300^oC.

CH₂=CH₂(g)    +      H₂(l)      ➞        CH₂H CH₂H(l)

                           ethene                                                 ethane

There is a change from an unsaturated molecule to the saturated colourless ethane.

This reaction is used to turn vegetable oil into solid margarine.

The reaction looks like this if we use displayed formulae:

Now you can substitute any double bonded molecule for the ethene and the same reaction will happen. 

The next blog looks at the oxidation of alcohols.