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Organic
Chemistry
Lesson 5
Alkenes
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https://quizlet.com/_1r6zhg

Write an equation for the reaction between methane
and chlorine to form chloromethane.

Explain this reaction in terms of a free-radical
mechanism.
We Are Here
Lesson 5: Alkenes
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Objectives:
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Describe the main addition reactions of the alkenes
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Extract an alkene from a citrus fruit
Organic vocabulary
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Nucleophile
Nucleus loving
Seeking +
Electron rich with lone pairs and/or negative charges
Examples Cl-, OH-, :NH3
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Electrophile
Electron loving
Seeking –
Electron deficient with positive or partial positive
charges
Examples NO2+, H+,
Nu :
E+
Nu - E
Reactivity of Alkenes
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Alkenes are considerably more reactive
than alkanes and are a major industrial
feedstock
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The reactivity is due to the double bond:
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The double bond contains 4 electrons
This is a significant amount of charge which:
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Makes it attractive to electrophiles
Enables it to polarise approaching molecules
Most reactions of alkenes are addition
reactions where two molecules come
together to make one new one
Trigonal planar, 120⁰
Alkenes and hydrogen
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Alkene + hydrogen  alkane
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Reaction conditions:
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Hot
Ni catalyst
This is an addition reaction, in which the hydrogen
adds across the double bond
Alkenes and hydrogen halides
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Alkene + hydrogen halide  halogenoalkane
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Reaction conditions:
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This reaction occurs very readily and needs no special
conditions
This is an addition reaction, in which the hydrogen
halide adds across the double bond
Markovnikov’s Rule
There are 2 possible products for hydrogen halide
additions. Which occurs? Discuss.
Markovnikov’s Rule states
"when an unsymmetrical alkene reacts with a hydrogen halide
to give an alkyl halide, the hydrogen adds to the carbon of the
alkene that has the greater number of hydrogen substituents,
and the halogen to the carbon of the alkene with the fewer
number of hydrogen substituents."
This is an empirical rule based on Markovnikov's experimental
observations in 1870 on the addition of hydrogen halides to alkenes.
Today we say that the reaction occurs via protonation to give the more
stable carbocation.
The halide will add to the more stable 2⁰carbocation rather than a less
stable 1⁰carbocation.
Alkenes and halogens
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Alkene + halogen  dihalogenoalkane
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Reaction conditions:
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This reaction occurs very readily and needs no special
conditions
If the halogen used is an aqueous solution of bromine
(bromine water), the orange-brown colour of bromine
solution is decolourised.
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This is the standard test for alkenes.
Alkenes and water
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Alkene + water  alcohol
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Reaction conditions:
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Water must be steam
Phosphoric or sulphuric acid catalyst
This is the process used to make industrial ethanol
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Fermentation from sugar would be far too expensive!
Extracting an alkene
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Alkenes occur widely in
nature, one such being the
compound limonene which is
found in many citrus fruits.
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In this experiment you will
extract limonene from orange
skin and test it to confirm its
alkene properties.
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Follow the instructions here
Polymerisation
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Under the right conditions, alkene molecules will add to each
other creating a polymer
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In this case, 1-bromo-2-fluoroethene polymerises to form
poly-1-bromo-2-fluroethene
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Conditions:
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Vary from alkene to alkene but often include high pressure,
temperature and a catalyst
The carbons in the C=C double bonds form the carbon chain,
everything else hangs off this chain
Drawing polymers
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Draw three-monomer lengths of the polymers
formed by:
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Propene
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Styrene
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Pent-2-ene
Homework
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Research the economic importance of alkenes
including:
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Manufacture of margarine
Manufacture of ethanol
Polymerisation
Key Points
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Alkenes undergo addition reactions with:
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Hydrogen
Hydrogen halides
Halogens
Water (steam)
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Alkenes undergo addition polymerisation
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Alkenes are very economically important due to the
range of products they can make
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We can deduce the product of asymmetric addition of
hydrogen halides using Markovnikov’s rule