A-LEVEL AQA ChEMISTRY NOTES
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- 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
- Electrophilic addition with H2SO4
If alcohol is added, an alcohol will reform:
- 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.