- Alkenes are unsaturated hydrocarbons because they can make more bonds with extra atoms in addition reactions.
- They contain at least 1 double C-C covalent bond meaning high electron density so they are very reactive.
Electrophilic Addition Reactions
- Double bond opens up, atoms added to C atoms.
- This happens because double bond has plenty of electrons and easily attacked by electrophiles.
- Electrophiles – electron pair acceptors.
Mechanisms for electrophilic addition reactions
General equation for this reaction using ethane and electrophile X-Y.
CH2CH2 + X-Y --> CH2XCH2Y
- C=C double bond repels X-Y e- which polarises X-Y bond.
- 2 electrons from C=C attack slightly +ve X atom which creates bond between 1st C and X.
- X-Y bond breaks, e- from bond taken by Y atom and –ve ion formed with lone pair e-.
- C 2 is left +ve so now there is a carbocation intermediate.
- Y- ion acts as nucleophile, attacks +ve carbocation and donates its e- pair and forms new bond with 2nd C.
- So X-Y molecule added to alkene across double bond to form saturated compound.
Bromine Test for Unsaturates
Shake alkene with orange bromine water solution quickly changes from orange to colourless.
Bromine added across double bonds to form colourless dibromoalkane (happens by electrophilic addition).
Major and Minor products
Adding hydrogen halides to unsymmetrical alkene gives us two possible products.
The amount of each one formed depends on how stable carbocation formed in middle of reaction.
3 possible carbocations:
- Carbocation’s with more alkyl groups are more stable because alkyl group feeds e- toward +ve charge.
- More stable ones more likely to form than less stable so more product formed via more stable carbocation than less stable.
That's the end of the topic!
Drafted by Bonnie (Chemistry)