Bromine is a red-brown liquid.

When it is shaken with an alkene like ethene, it loses its colour.

The reaction between ethene and bromine...

...produces 1,2-dibromoethane, which is colourless.

This is a simple test for the presence of the alkenes.

This is a model of ethene, C 2 H 4. The carbon atoms are shown as black spheres and the hydrogen atoms as white spheres.

This is a model of a bromine molecule, Br 2.

The double bond in ethene is a region with a high density of negative charge.

When a bromine molecule comes close, its bonding pair of electrons is repelled.

The bond between the two bromine atoms breaks. The free bromide ion takes away both electrons as a lone pair.

One of the bonding pairs of electrons in the ethene double bond is accepted by nearest bromine atom.

A covalent bond forms between one of the carbon atoms and the bromine atom.

The ethene molecule has become a positively charged ion, called a carbocation.

A covalent bond forms between the carbocation and the bromide ion.

The product of the reaction is 1,2-dibromoethane.

A bromine molecule comes close to an ethene molecule.

A dipole is induced in the bromine molecule.

A pair of electrons from one of the carbon-carbon bonds in ethene is attracted to the δ + bromine atom.

The bonding pair of electrons in the bromine molecule is repelled towards the δ – bromine atom.

The bond between the two bromine atoms breaks.

A covalent bond forms between one of the carbon atoms and the δ + bromine atom.

The ethene molecule has become a positively charged ion, called a carbocation.

The free bromine atom receives both bonding electrons and becomes a bromide ion.

The lone pair of electrons in the bromide ion is attracted to the positive charge on the carbocation.

The bromide ion moves towards the positively-charged carbon atom. A covalent bond forms between them.

A hydrogen bromide molecule comes close to an ethene molecule.

It has a permanent dipole because bromine is more electronegative than hydrogen.

A pair of electrons from one of the carbon-carbon bonds in ethene is attracted to the δ + hydrogen atom.

The bonding pair of electrons in the hydrogen bromide molecule is repelled towards the δ – bromine atom.

The covalent bond in hydrogen bromide breaks.

A covalent bond forms between one of the carbon atoms and the δ + hydrogen atom.

The ethene molecule has become a positively charged ion, called a carbocation.

The free bromine atom receives both bonding electrons and becomes a bromide ion.

The lone pair of electrons in the bromide ion is attracted to the positive charge on the carbocation.

The bromide ion moves towards the positively-charged carbon atom. A covalent bond forms between them.

A sulphuric acid molecule comes close to an ethene molecule.

It has a permanent dipole because oxygen is more electronegative than hydrogen.

A pair of electrons from one of the carbon-carbon bonds in ethene is attracted to the δ + hydrogen atom.

The bonding pair of electrons in the sulphuric acid molecule is repelled towards the δ – oxygen atom.

The covalent bond between hydrogen and oxygen in sulphuric acid breaks.

A covalent bond forms between one of the carbon atoms and the δ + hydrogen atom.

The ethene molecule has become a positively charged ion, called a carbocation.

A hydrogensulphate ion forms.

The lone pair of electrons in the hydrogensulphate ion is attracted to the positive charge on the carbocation.

The hydrogensulphate ion moves towards the positively-charged carbon atom. A covalent bond forms between them.

The hydrogensulphate ion moves towards the positively-charged carbon atom. A covalent bond forms between them.

Sulphuric acid was needed at the beginning. As it is also produced in the reaction, it acts as a catalyst.

Addition polymerisation involves a free radical mechanism.

In the initiation stage, radicals are made using an organic peroxide catalyst.

The O–O bond is weak and breaks to make R–O radicals (alkoxy radicals).

An alkoxy radical combines with one of the electrons from the double bond in ethene.

An alkoxy radical combines with one of the electrons from the double bond in ethene.

This forms a new covalent bond and a new radical.

The new radical reacts with another ethene molecule.

This forms another new radical, longer than before.

The chain grows longer until two radicals react with each other.

This is called the termination step.

A growing chain may curl around and attack itself by back-biting.

The radical moves to another part of the chain, and allows branched chains to grow.