addition reaction

Chemical reaction in which the atoms of an element or compound react with a double bond or triple bond in an organic compound by opening up one of the bonds and becoming attached to it, for example:

CH₂CH₂ + HCl → CH₃CH₂Cl

Another example is the addition of hydrogen atoms to unsaturated compounds in vegetable oils to produce margarine. Addition reactions are used to make polymers from alkenes.

Polymers Organic compounds obtained from the fractional distillation of crude oil (unrefined petroleum) can be broken down into smaller fraction molecules by cracking. The smallest molecule from this process is ethene (C2H4), an unsaturated hydrocarbon of the alkene group. In ethene, there is a double bond between the two carbon atoms:

Plastics are polymers, made from small alkene molecules known as monomers reacting together to form longer chain molecules. This process is called addition polymerization and the alkene monomer molecules join together by addition reaction.

Alkenes are much more reactive than alkanes. Compounds with a double bond linking carbon atoms are unsaturated, as each of the carbons involved has less than four binding partners. In an addition reaction, one of the bonds breaks away from the ethene molecule, to form two new chemical bonds with the reactant molecule. During addition polymerization, the ethene molecules join together to form a polythene molecule:

The type of product formed from polymerization reaction depends on the reactant molecule joining with the ethene molecule. For example, chloro-ethene combines to produce polyvinyl chloride (PVC):

The properties of the PVC molecules change depending on the length of the polymer chain, so PVC may be used to manufacture products from soft imitation leather to hard plastic water pipes.

Polystyrene, a product of styrene joined with styrene, is another polymer made from an addition reaction:

Polystyrene is used to make hard plastics that are used as outer casings for computers and also foam packaging.