Chemistry: Alkenes and Electrophilic Addition

Study Notes

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.

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 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.