Alkenes Revision Notes

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  • Created on: 08-04-16 09:21
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Alkenes
Unsaturated hydrocarbons because can make more bonds with extra atoms in addition
reactions.
c
nH2n
Contain at least 1 double CC covalent bond meaning high electron density so they are
very reactive.
Addition Reactions
Electrophilic addition reactions
Double bond opens up, atoms added to C atoms.
Happens because double bond has plenty of e and easily attacked by electrophiles.
Electrophile ­ electron pair acceptors.
Mechanisms for electrophilic addition reactions
General equation for this reaction using ethane and electrophile XY.
CH2CH2 + XY CH2XCH2Y
1) C=C double bond repels XY e which polarises XY bond.
2) 2 e from C=C attack slightly +ve X atom which creates bond between 1st C and X. XY 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's a carbocation intermediate.
3) Y ion acts as nucleophile, attacks +ve
carbocation and donates its e pair, forms
new bond with 2nd C.
4) So XY molecule added to alkene across
double bond to form saturated compound.

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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.
Known as carbocation intermediate.…read more

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Carbocation's with more alkyl groups are more stable because alkyl group feeds e toward +ve
charge. Can show alkyl group is donating e by drawing arrow on bond pointing to where e
donated.
More stable ones more likely to form than less stable so more product formed via more stable
carbocation than less stable.…read more

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Addition Polymers
Formed from alkenes and substituted alkenes (where 1 H atom swapped for another atom
or group, e.g. swapping 1 H in ethane for Cl atom you get chloroethene when polymerised
you get poly(chloroethene)). Long chain formed by several monomers joined.
Can be natural or synthetic.
Double bonds in alkenes can open up and join together, make long chain called polymer.
This is called addition polymerisation.…read more

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IUPAC Nomenclature
Once found monomer used to form addition polymer you know they form poly(X) where X is the
name of the monomer.
Polymer Properties ­
Alkene monomers unsaturated, once polymers they are saturated.
Main C chain of polyalkenes is usually nonpolar which means polyalkenes are very
unreactive.
The monomers in polymer chain have strong covalent bonds but weak intermolecular
forces. Long chains, fewer branches have strong intermolecular forces so the polymers are
strong and rigid.…read more

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