A-Level Chemistry AQA Notes

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Alkenes

Alkenes are a homologous series made up of unsaturated hydrocarbons containing at least one C=C double bond, with the general formula CnH2n.
C-H – these are σ-bonds formed due to the direct overlap of the electron clouds belonging to each carbon atom
C=C – these contain both a σ-bond and two π-bonds. It is an area of high electron density, so can react with electrophiles.
π-bonds – formed due to the overlap of adjacent p-orbitals above and below the carbon atoms. It restricts the rotation around the planar C=C double bond, leading to stereoisomerism in some alkenes
Alkenes have a trigonal planar shape with a bond angle of 120⁰.

Addition Reactions of Alkenes

An electrophile is an electron pair acceptor attracted to areas of high electron density
Electrophilic addition with HBr

Major and minor products form when an unsymmetrical alkene reacts. The major product will form from the most stable carbocation intermediate, with the most alkyl groups attached.
Tertiary > secondary> primary carbocations. This reduces the positive charge, due to inductive electron donation.

Electrophilic addition with Br2. A dipole is induced in the halogen molecule by the high electron density of the C=C
Bromine water acts as a test for unsaturation. An unsaturated compound will decolourise the bromine water, as a dihaloalkane will form, using up the bromine

If alcohol is added, an alcohol will reform:

Addition Polymers

Addition polymers are macromolecules made from small repeating units of monomers
They are formed from alkenes and substituted alkenes
Addition polymers are often called poly(alkenes)

Addition polymers are unreactive ss the main carbon chain is saturated and non-polar, with only weak London forced between the chains
Plastics can be modified using plasticisers, which force the polymer chains apart to reduce London forces and make the polymer more flexible.

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