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Alkenes

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What are alkenes?
The alkenes are a homologous series of hydrocarbons
with the general formula CnH2n and names ending –ene.
Alkenes contain a carbon – carbon double bond and so are
unsaturated.
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No. of
carbon atoms
Molecular
formula
2
C2H4
ethene
3
C3H6
propene
4
C4H8
butene
5
C5H10
pentene
6
C6H12
hexene
Name
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Naming alkenes
Alkenes with four or more carbon atoms display positional
isomerism because the carbon–carbon double bond may
appear between different carbon atoms.
If there are two or more possible positions for the double
bond, a number is used before the –ene to indicate the first
carbon involved.
but-1-ene
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but-2-ene
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Structure of alkenes
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Bonding in alkenes
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Isomerism in alkenes
Rotation around the double bond in alkenes is restricted
by the presence of the  bond. This leads to E–Z
stereoisomerism in some alkenes.
E-pent-2-ene
Z-pent-2-ene
The two E-Z stereoisomers have the same structure; the
only difference between them is the arrangement of the
atoms in space.
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Structure of alkenes summary
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Double bonds and electrophiles
The double bond of an alkene is an area of high electron
density, and therefore an area of high negative charge.
The negative charge of the double bond may be
attacked by electron-deficient species, which will
accept a lone pair of electrons.
These species have either a full positive charge or
slight positive charge on one or more of their atoms.
They are called electrophiles, meaning ‘electron loving’.
Alkenes undergo addition reactions when attacked by
electrophiles. This is called electrophilic addition.
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Electrophilic addition mechanism: 1
In the first stage of electrophilic addition,
the positive charge on the electrophile is
attracted to the electron density in the
double bond.
δ+

As the electrophile approaches the
double bond, electrons in the A–B
bond are repelled towards B.
δ-
The pi bond breaks, and A bonds to the
carbon, forming a carbocation – an
ion with a positively-charge carbon
atom. The two electrons in the A–B
bond move to B forming a B- ion.
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Electrophilic addition mechanism: 2
In the second stage of electrophilic addition,
the B- ion acts as a nucleophile and attacks
the carbocation.

The lone pair of electrons on the B- ion
are attracted towards the positivelycharged carbon in the carbocation,
causing B to bond to it.
Because both electrons in the bond that joins B- to the
carbocation ion come from B-, the bond is a co-ordinate bond.
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What is hydrogenation?
Hydrogen can be added to the carbon–carbon double bond
using a nickel catalyst in a process called hydrogenation.
C2H4 + H2  C2H6
Vegetable oils are unsaturated
and may be hydrogenated to
make margarine, which has a
higher melting point.
As well as a nickel catalyst,
this requires a temperature
of 200 °C and a pressure
of 1000 kPa.
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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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More on the bromine water test
A simple equation for the bromine water test with ethene is:
CH2=CH2 + Br2 + H2O  CH2BrCH2Br + H2O
However, because water is present in such a large amount,
a water molecule (which has a lone pair) adds to one of the
carbon atoms, followed by the loss of a H+ ion.
CH2=CH2 + Br2 + H2O  CH2BrCH2OH + HBr
The major product of the test is not 1,2-dibromoethane
(CH2BrCH2Br) but 2-bromoethan-1-ol (CH2BrCH2OH).
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Electrophilic addition reactions
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Addition to unsymmetrical alkenes
When an electrophile
(e.g. HBr) attacks an
alkene with three or
more carbon atoms
(e.g. propene), a mix
of products is formed.
This is because
these alkenes are
unsymmetrical.
minor product:
1-bromopropane
HBr
major product:
2-bromopropane
Unequal amounts of each product are formed due to the
relative stabilities of the carbocation intermediates.
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Structure of carbocations
A chain of carbon atoms can be represented by R when
drawing organic structures. This is an alkyl group (general
formula CnH2n+1).

Primary (1°) carbocations
have one alkyl group attached to
the positively-charged carbon.

Secondary (2°) carbocations
have two alkyl groups attached
to the positively-charged carbon.

Tertiary (3°) carbocations
have three alkyl groups attached
to the positively-charged carbon.
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Stability of carbocations
The stability of carbocations increases as the number of alkyl
groups on the positively-charged carbon atom increases.
primary
secondary
tertiary
increasing stability
The stability increases because alkyl groups contain a
greater electron density than hydrogen atoms. This
density is attracted towards, and reduces, the positive
charge on the carbon atom, which has a stabilizing
effect.
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Structure of carbocations
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Type of addition reaction
• Hydrogenation
• Halogenation
• Hydration
• Addition of hydrogen halide
• Oxidation
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Electrophiles: true or false?
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Polyalkenes
Alkenes can undergo addition reactions with themselves
to form a long chain polymer molecule. This reaction is
addition polymerization.
The polymer can be represented by showing the repeating
unit with square brackets around it. The n stands for a
unspecified number of monomer units.
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Polymerization of ethene
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LDPE and HDPE
The reaction conditions under which ethene polymerizes
affect the structure and properties of the poly(e)thene.

Low-density polythene (LDPE) is formed under a high
pressure (1400 atm) and a temperature of about 170 °C.
These conditions cause a high level of branching,
meaning that the polymer chains cannot pack
tightly together.

High-density polythene (HDPE) is formed with a catalyst,
a pressure of 2 atm and a temperature of about 70 °C.
Little branching occurs under these conditions,
resulting in chains that can pack tightly together to
create a denser material.
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Other polyalkenes
Propene undergoes addition polymerization to form
polyproprene:
Chloroethene undergoes addition polymerization to form
polychloroethene:
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Which alkene?
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More about LDPE
LDPE is a soft, flexible and stretchy plastic, with a melting
point of about 120 °C. It is used to make:

plastic bags

squeezable bottles, and
general purpose
containers and trays

other items that need to
be soft and flexible, such
as tubing.
LDPE has the recycling symbol ‘4’.
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More about HDPE
HDPE is a tough and flexible plastic, with a melting point of
about 130 °C. It is used to make:

containers such as milk and
detergent bottles

rigid items such as folding
tables, chairs and pipes.
HDPE has the recycling symbol ‘2’.
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More about polypropene
Polypropene is a tough and flexible plastic, with a melting
point of about 160 °C. It is used to make:

ropes, carpets, rugs and other
textiles

medical, laboratory and
kitchen items that need to
withstand temperatures in
autoclaves and dishwashers

certain bottles, buckets, containers
and other items such as bottle
tops and moulded fittings.
Polypropene has the recycling symbol ‘5’.
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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