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
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)
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller bookishresearcher. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £2.99. You're not tied to anything after your purchase.
