Alkenes
6 marks · NormalHex-2-ene, C6H12, is a liquid alkene. A sample is tested and burned under different conditions.
State what is meant by an 'addition reaction' in the context of alkenes.
Write a balanced chemical equation for the complete combustion of hex-2-ene, C6H12.
Draw the structural formula of the compound formed when hex-2-ene reacts with bromine.
Describe how you would test a sample of hex-2-ene using bromine water, and state the expected observation.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) An addition reaction is when atoms are added across the carbon–carbon double bond so that the double bond becomes a single bond.
(b) C6H12(l) + 9O2(g) → 6CO2(g) + 6H2O(l)
(c) CH3CH2CHBrCHBrCH2CH3 or structural formula showing dibromo addition at positions 3 and 4, i.e. 3,4-dibromohexane · must show Correct dibromo structure: Br atoms on adjacent carbons, double bond removed.
(d) Add bromine water to the sample of hex-2-ene and shake. The orange colour of the bromine water will disappear (decolourise), showing the presence of a double bond. · must show Test: addition of bromine water and shaking, Observation: bromine water decolourises (NOT clear)
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Marking points
M1 (a) · 1 mark · Defines addition reaction in correct context for alkenes (atoms added across double bond).
M2 (b) · 1 mark · Correct reactants and products identified (hex-2-ene, O2, CO2, H2O).
A1 (b) · 1 mark · Fully balanced equation with correct coefficients.
A2 (c) · 1 mark · Correct structural formula for 3,4-dibromohexane formed by addition of Br2 to hex-2-ene (Br on former double bond).
M3 (d) · 1 mark · Describes adding bromine water to the sample and shaking.
A3 (d) · 1 mark · States orange colour disappears/decolourises (do not accept 'clear').
Worked solution
a) In alkenes, an addition reaction is when new atoms are added to the molecule across the carbon–carbon double bond, converting it into a single bond.
b) The balanced equation for the complete combustion of hex-2-ene is:
c) The product of the reaction between hex-2-ene and bromine is 3,4-dibromohexane. Its structural formula is:
CH3–CH2–CHBr–CHBr–CH2–CH3 (or a drawn structural formula showing Br atoms on carbons 3 and 4, no double bond).
d) Add bromine water to the sample of hex-2-ene in a test tube and shake the mixture. The orange colour of the bromine water will be decolourised if a double bond is present, so the final mixture becomes colourless (not clear), indicating that hex-2-ene is unsaturated.