Section 3 c: alkenes
3.6 recall that alkenes have the general formula CnH2n
3.7 draw displayed formulae for alkenes with up to four carbon atoms in a molecule, and name the
straight-chain isomers
3.8 describe the addition reaction of alkenes with bromine, including the decolourising of bromine water
as a test for alkenes
What do all alkenes have in common?
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same functional group; C=C : at least 1 double covalent bond between two carbon atoms in the
molecule
a general formula of Cn H2n
each alkene has at least 1 double carbon = carbon bond; in such a carbon=carbon bond each
carbon is only bonded onto 3 other different atoms. It uses up 2 of its 4 valence electrons to bond
with the same atom. We say the carbon compound with 1 or more double bonds is unsaturated.
We can add more atoms to the molecule by using the double bond which can pop open.
because each alkene is unsaturated they are reactive; most reactions will involve the double bonds.
unsaturated = has a double bond
Examples of alkenes and isomers.
Write down the names, molecular formulas and graphical/displayed formula for the first 3 straightchained members of the alkenes. Why does methene not exist?
Isomerism: for one of the alkenes you can draw
another isomer. Which alkene?
Draw the isomer below.
Chemical reactions of alkenes
Combustion: to produce water and carbon dioxide when combustion is complete; carbon monoxide and
water when combustion is incomplete.
Write equations for the following combustions:
1. Combustion of ethene:
Section 3c alkenes
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2. Combustion of propene:
Addition reaction
An addition reaction is a reaction during which a small molecule is added to an unsaturated organic
compound changing it into a saturated compound. This happens because the double bond pops open
and each carbon electron of the double bond can now be used to make a new covalent bond with
another atom or group of atoms.
The small molecule which reacts with the alkene breaks up into 2 parts and one part goes to one carbon
of the double bond and the other part goes to the second carbon atom of the double bond.
Example:
ethene + bromine

dibromoethane (=colourless)
Draw a structural equation to show what happens during this addition reaction.
Write a balanced equation and word equation for the above reaction.
Test of alkenes – IMPORTANT!!
During the reaction between an alkene and bromine (bromine water) the bromine loses its red-brown
colour rapidly and a colourless solution is produced. This is the case because bromomethane is a
colourless compound.
This reaction is used to test for unsaturated compounds. If bromine water changes rapidly from redbrown to colourless the other reactant must be an alkene or an unsaturated compound.
The more bromine water that is decolourized the greater the unsaturation of the compound.
Section 3c alkenes
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