Summary notes for AQA A-Level Chemistry Unit 3.3.4 - Alkenes
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Unit 3.3.4 - Alkenes
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AQA A Level Chemistry Student Book 1
Summary notes for AQA A-Level Chemistry Unit 3.3.4 - Alkenes by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: Structure, Bonding and Reactivity, Addition Reactions of Alkenes, Addition Polymers
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Unit 3.3.4 - Alkenes
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Section 3: Organic Chemistry
Alkenes
Structure, Bonding and Reactivity
Alkenes: unsaturated hydrocarbons which have at least one carbon-carbon double covalent bond in
their structure.
• General formula: CnH2n (except cyclic alkenes) ∴ members of a homologous series.
• If more than one double bond, then -diene or -triene are used.
• The arrangement of bonds around the >C=C< is planar + the H-C-H bond angles are approx. 120°
Bonding in alkenes involves a double covalent bond, a centre of high electron density.
• C=C consists of one sigma (σ) bond and one pi (π) bond
π bond = overlapping p-orbitals
• π bonds are exposed and have high electron density. σ bond = overlapping s-orbitals
• They are ∴ vulnerable to attack by electrophiles.
• π bonds are also a lot weaker than σ bonds. ∴ alkenes are much more reactive as
less energy required to break π bonds compared to breaking σ bonds in alkanes.
Addition Reactions of Alkenes
Electrophile: Atom (or group of atoms) attracted which must be capable of accepting a pair of
electrons to form a new covalent bond (e.g. Na+ would not be an electrophile).
- Features of electrophiles: can be +ve charged ions (e.g. H+) or a molecule containing a slightly +ve
(δ+) atom. Attracted to regions of -ve charge or high electron density.
Electrophilic Addition Reaction Examples
General equation using ethene + an electrophile X-Y:
1) The electron rich C=C double bond repels the electrons in X-Y, which polarises the X-Y
bond (or the bond could already be polar, as in H-Br).
2) Pair of electrons from π bond attack the δ+ X atom creating a new
covalent bond between C1 + X. The X-Y bond breaks heterolytically,
so the Y atom takes both electrons + forms a -ve ion w/ a lone pair.
C2 = left w/ a +ve charge so you now have a carbocation
intermediate.
3) The Y- ion then acts as a nucleophile, attacking the carbocation,
donating its lone pair of electrons + forming a new dative covalent
bond w/ C2. A saturated molecule is formed.
With Bromine (Br2)
Use of Bromine to Test for Unsaturation:
When you shake an alkene w/ bromine water the solution quickly decolourises from
orange to colourless. This is due to electrophilic addition, where bromine is added
across the double bond to form a colourless dibromoalkane.
With Hydrogen Bromide (HBr)
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