Answers:
LEVEL: AS
CHEMISTRY – Organic chemistry
12. Alkenes: electrophilic addition mechanism, formation by cracking and
geometrical isomers (15 minutes)
(a) The mechanism for the addition of gaseous hydrogen iodide to propene can be
represented as:
(i) Explain what the curved arrows represent.
(1 mark)
The movement or shift of a pair of electrons.
(ii) State the name of the type of ion formed in the first stage of this mechanism.
(1 mark)
Carbonium ion (or carbon cation)
(iii) Write the systematic name of the product.
(1 mark)
2-iodopropane
(iv) Explain why this mechanism is described as electrophilic addition.
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Answers:
LEVEL: AS
CHEMISTRY – Organic chemistry
(2 marks)
Electrophilic because propene is a nucleophile being attacked by the H- atom at the positive end of the
polar HI molecule behaving as an electrophile.
Addition reaction because the two reactants combine or add together to form one product.
(b) In the petrochemical industry, hydrocarbons are cracked to form products that include
ethene and propene. Write a balanced equation to show the cracking of a molecule of pentane,
C5H12, to produce propene and one other product.
(1 mark)
C5H12 → C3H6 + C2H6
(c) (i) Describe the shape of a but-2-ene molecule and explain why the hydrocarbon can exist as
geometrical isomers.
(2 marks)
Planar or flat molecule (two H atoms and four carbon atoms in the same plane). Geometrical isomers
exist because the molecule cannot rotate freely about the C=C bond.
(ii) Draw the full structural formulae of the two geometric isomers of but-2-ene and give the
name of each.
Isomer 1:
(2 marks)
Structural formula:
Name:
cis-but-2-ene
Isomer 2:
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2
Answers:
LEVEL: AS
CHEMISTRY – Organic chemistry
(2 marks)
Structural formula:
Name:
trans-but-2-ene
(d) Limonene is a colourless liquid that can be extracted from orange peel. Describe a test tube
experiment you could do to confirm the presence of carbon–carbon double bonds in limonene.
In your answer you should state the reagents you would use and the observations you would
expect to make.
(3 marks)
Add one drop of limonene to bromine water, shake the mixture and observe the red-brown/orange
colour of the bromine water quickly turn colourless.
(You could also use bromine in a non-aqueous solvent or as a vapour but remember it is
dangerous. If you use alkaline potassium manganate(VII), you must warm the mixture to
observe the purple colour disappear and the mixture may turn colourless or brown.)
(Total marks 15)
ANSWERS
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