Chemistry: Alkenes and Electrophilic Addition

Alkenes

• Alkenes are unsaturated hydrocarbons, meaning they contain a carbon-carbon double bond.
• This double bond is an area of high electron density, making it susceptible to attack by electrophiles.
• An electrophile is an electron acceptor, attracted to areas of high electron density.
• Bromine water is used to identify unsaturated compounds.
• The solution turns from orange-brown to colourless if a double bond is present.

Electrophilic Addition

• Alkenes undergo electrophilic addition reactions which occur at the double bond.
• Electrophiles include HBr, Br₂ and H₂SO₄.
• They can be used to form alkyl hydrogensulphates or halogenoalkanes from alkenes.
• The reaction mechanism involves the electrophile attacking the double bond, leading to the formation of a carbocation.
• A carbocation is a carbon atom with only three bonds and carries a positive charge.
• The stability of carbocations varies, with tertiary being the most stable and primary the least stable.
• More stable carbocations are more likely to form.
• The major product of an electrophilic addition reaction will always be the most stable carbocation.

Addition Polymers

• Addition polymers are formed by breaking the double bond in alkenes to form a repeating unit.
• The repeating unit is shown with extended bonds through brackets to show that it bonds to other repeating units.
• Different reaction conditions can give the plastics produced different properties.
• High temperatures and pressures produce branched chain polymers with weak intermolecular forces.
• Lower temperatures and pressures produce straight chain polymers with strong intermolecular forces.
• Polymers have multiple strong non-polar covalent bonds, making them unreactive.
• This property means they are useful for manufacturing everyday plastic products.
• However, their unreactive nature also means they are not biodegradable.

Poly(chloroethene) (PVC)

• PVC is an addition polymer with waterproof properties.
• These properties are gained by the addition of plasticisers during the reaction.