Some chemical reactions of organic compounds
4 marks · NormalThe following reactions involve organic compounds.
Write the type of reaction (addition, substitution or combustion) for each equation below:
Explain how an addition reaction differs from a substitution reaction, using an example from the equations above.
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(a(i)) addition
(a(ii)) combustion
(b) An addition reaction adds atoms to a molecule without removing any atoms, as in the first equation where Br2 is added across the double bond of ethene. In a substitution reaction, one atom or group is replaced by another, but nothing is added across a double bond.
Common mistakes: Omitting differences in mechanism; confusing addition with substitution; not using provided examples.
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Marking points
B1 (a(i)) · 1 mark · Correctly identifies first reaction as addition.
B1 (a(ii)) · 1 mark · Correctly identifies the second reaction as combustion.
M1 (b) · 1 mark · States or describes that addition involves adding atoms/groups to a molecule.
A1 (b) · 1 mark · Gives a correct comparison/example showing difference from substitution, with reference to the reactions above.
Worked solution
a)(i) The first reaction adds Br2 to ethene—this is addition.
a)(ii) The second reaction is with oxygen to produce CO2 and H2O—combustion.
b) In an addition reaction, atoms are added to a molecule without taking any away (e.g., Br2 adds across the double bond in ethene). In a substitution reaction, one atom (or group) is swapped for another, not added across a double bond.