AS/A-level Chemistry - Alkenes

• 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.

• 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.

Do you remember the mechanisms for electrophilic addition reactions in AS/A-level Chemistry?

• 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 1^st 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.

• 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).

• 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.

References:

1. https://byjus.com/chemistry/tests-for-unsaturation/

2. https://byjus.com/chemistry/physical-properties-of-alkenes/

That's the end of the topic!

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Drafted by Bonnie (Chemistry)