Alkenes and Alkynes continued Objectives To understand the key reactions that create alkenes and alkynes To understand the key uses made and the reactions involved. To introduce polymerisation

Preparation of alkenes Alkenes are created by elimination reactions where 2 atoms on adjacent C-C atoms are removed so forming a C=C bond An example is the high temp removal of H2 from ethane or propane to make ethene or propene. Most alkenes cannot be produced by the removal of hydrogen from alkanes Instead they are made by the dehydration of alcohols or the dehydrohalegonation of alkyl halides.

Preparation of alkenes continued  Dehydration of alcohols. The H and OH are removed from adjacent carbons.  E.g. ethanol may be heated with sulphuric acid to create ethene and water Sulphuric and phosphoric acid are the acids most commonly used in the dehydration of alcohols to form alkenes.

Branched chain alcohols These may give different alkenes dependent on where the double bond forms as there is more than one option. Usually one isomer is produced in much greater amounts than the other.

Dehydrohalogenation It is the loss of a hydrogen and a halogen from an alkyl halide One of the most useful methods of preparing alkenes The reaction is carried out in the presence of a strong base such as sodium ethoxide (NaOCH2CH3). It produces an alkene, an ethyl alcohol and sodium halide.

Addition reactions of alkenes This is why we want them!Remember polymerisationThis involves hydrogenation of the C=C bond. This process gives out energy – is Exothermic Many chemicals can be added in such as oxygen and halogens But the most important is the addition of extra alkenes to form polymers

Addition reactions contd. This is done by heating ethene at high pressure in the presence of oxygen or peroxide. This has to be done by free-radical polymerisation. This cannot be used to produce the useful long chain polymers such as polyethylene. Alkenes can be encouraged to join in the presence of acids such as sulphuric acid to produce dimers or even trimers. This is called cationic polymerization

Polymerisation.

Other important monomers  PVC is poly vinyl chloride. Vinyl chloride is the common name for chloroethene

Nylon  One of the first polymers synthesised  Made from a di-carboxylic acid and a diamine

Polyacrylamide  Poly 2 propenamide or poly(1-carbamoylethylene)  Repeating units -CH 2 CHCONH 2

Alkynes The classic example is the discovery of ethyne (called acetylene) Made by adding calcium carbide to water which produces calcium hydroxide and ethyne. Ethyne is flammable and this reaction was used to form lamps called carbide lamps. Made as follows:CaO + 3C = CaC2 + CO CaC2 + 2H2O = Ca(OH) 2 + Ethyne HC Ξ CH