Download More reactions of alkenes Objective

Reaction of alkenes
More reactions of alkenes
Objective: To know some more electrophilic reactions of
alkenes
Success criteria:
• understand the addition reactions of alkenes with:
hydrogen, in the presence of a nickel catalyst, to form an
alkane
halogens to produce dihalogenoalkanes
steam, in the presence of an acid catalyst, to produce alcohols
potassium manganate(VII), in acid conditions, to oxidise the
double bond and produce a diol
• understand that heterolytic bond fission of a covalent bond
results in the formation of ions
• understand the mechanism of the electrophilic addition
reactions for the reaction of alkenes with halogens including
using curly arrow notation and other given binary compounds
Testing for alkenes
The presence of
unsaturation (a carbon–
carbon double bond) can
be detected using
bromine water, a
red/orange coloured
solution of bromine.
A few drops of bromine water are added to the test liquid
and shaken. If a carbon–carbon double bond is present, the
bromine adds across it and the solution turns colourless.
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Electrophilic Addition of Br2 to
Ethene
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The same mechanism with one small
difference…
Br2 has an induced dipole.
Draw out the mechanism
– Connect the stages with  just like an
equation.
Halgoens reacting with alkenes
Halogens react with alkenes to form
dihalogenalkanes
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Heterolytic Fission
In heterolytic fission the bond breaks unevenly
One of the bonded atoms receives both
electrons from the bonded pair
Two different substances can be formed
E.g. A positively charged cation (X+) and
negatively charged anion (Y-)
X – Y  X+ + YThe curly arrow shows the movement of an
electron pair
Halgoens reacting with alkenes
Mechanism
1) The double bond repels the electrons in
Br2 polarising Br - Br
Halgoens reacting with alkenes
Mechanism
2) Heterolytic (unequal) fission of Br2. The
closer Br gives up the bonding electrons to
the other Br and bonds to the C atom
Halgoens reacting with alkenes
Mechanism
3) You get a positiviely charged carbocation
intermediate. The Br- now zooms over……
Halgoens reacting with alkenes
Mechanism
4) ….. And bonds to the other C atom,
forming 1,2-dibromoethane
Halgoens reacting with alkenes
But-1-ene is an alkene. Alkenes contain at least one C=C
double bond.
1.Describe how bromine water can be used to test for C=C
double bonds.
2.Name and draw the mechanism for involved in the above
test.
More reactions of alkenes
Objective: To know some more electrophilic reactions of
alkenes
Success criteria:
• understand the addition reactions of alkenes with:
hydrogen, in the presence of a nickel catalyst, to form an
alkane
halogens to produce dihalogenoalkanes
steam, in the presence of an acid catalyst, to produce
alcohols
potassium manganate(VII), in acid conditions, to oxidise the
double bond and produce a diol
• understand that heterolytic bond fission of a covalent bond
results in the formation of ions
• understand the mechanism of the electrophilic addition
reactions for the reaction of alkenes with halogens including
using curly arrow notation and other given binary compounds
•
•
•
•
Alkenes reacting with hydrogen
Ethene will react with hydrogen gas in an
addition reaction to produce ethane
It needs a nickel catalyst and a
temperature of 150oC
Margarine is made by “hydrogenating”
unsaturated vegetable oils
By removing some double bonds you raise
the melting point of the oil so that it
becomes solid at room temperature
Alkenes reacting with hydrogen
• Ethene will react with hydrogen gas in an
addition reaction to produce ethene
• It needs a nickel catalyst and a
temperature of 150oC
Ni
H2C = CH2 + H2  CH3CH3
150oC
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•
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Alkenes reacting with steam
Alkenes can be hydrated by steam at 300oC
and a pressure of 60 – 70 atm
The reaction needs a solid phosphoric (V) acid
catalyst
The reaction is used to manufacture ethanol
from ethene
It is an electrophilic addition reaction and an
example of a hydration reaction
H3PO4
H2C = CH2 (g) + H2O (g)   CH3CH2OH (g)
300oC, 60 atm
Alkenes reacting with steam
Mechanism
Can you suggest a mechanism?
Alkenes reacting with acidified potassium
manganate (VII)
H2C = CH2 + [O] + H2O  CH2OH-CH2OH
ethane-1,2-diol
• If you shake an alkene with acidified potassium manganate
(VII), the purple solution is decolourised (no change is
observed with alkanes).
• You’ve oxidised the alkene and made a diol (an alcohol
with two –OH groups)
• The oxidising agent is potassium manganate (VII), KMnO4,
acidified with dilute H2SO4.
• This is represented [O].
Questions
Write the equation, and name the organic product, for the
reaction between but-2-ene and:
1.Steam
2.Acidified potassium manganate (VII)
More reactions of alkenes
Objective: To know some more electrophilic reactions of
alkenes
Success criteria:
• understand the addition reactions of alkenes with:
hydrogen, in the presence of a nickel catalyst, to form an
alkane
halogens to produce dihalogenoalkanes
steam, in the presence of an acid catalyst, to produce alcohols
potassium manganate(VII), in acid conditions, to oxidise the
double bond and produce a diol
• understand that heterolytic bond fission of a covalent bond
results in the formation of ions
• understand the mechanism of the electrophilic addition
reactions for the reaction of alkenes with halogens including
using curly arrow notation and other given binary compounds
Reactions of alkenes plenary
What can you remember about the reactions of
alkenes?
Anything from the last two lessons
More reactions of alkenes
Objective: To know some more electrophilic reactions of
alkenes
Success criteria:
• understand the addition reactions of alkenes with:
hydrogen, in the presence of a nickel catalyst, to form an
alkane
halogens to produce dihalogenoalkanes
steam, in the presence of an acid catalyst, to produce alcohols
potassium manganate(VII), in acid conditions, to oxidise the
double bond and produce a diol
• understand that heterolytic bond fission of a covalent bond
results in the formation of ions
• understand the mechanism of the electrophilic addition
reactions for the reaction of alkenes with halogens including
using curly arrow notation and other given binary compounds