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Reaction types, mechanisms, nucleophiles and electrophilesEdexcel International A Level Chemistry: Flashcards

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List the seven reaction types for organic reactions in this topic.

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List the seven reaction types for organic reactions in this topic.
Addition, elimination, substitution, oxidation, reduction, hydrolysis and polymerisation.
What is a substitution reaction?
One atom or group is replaced by another.
What is an elimination reaction?
A small molecule is removed from a larger one, forming a C=C bond.
What is hydrolysis?
A reaction in which a bond is broken by water or hydroxide ions.
What do curly arrows show?
The movement of a pair of electrons.
What is a reaction mechanism?
A step-by-step account of which bonds break and form.
What is heterolytic bond breaking?
A covalent bond breaks and both bonding electrons go to one atom, forming ions.
Define a nucleophile.
A species that donates a pair of electrons to form a covalent bond.
Define an electrophile.
A species that accepts a pair of electrons to form a covalent bond.
Why is the carbon in a C–Br bond δ+?
Bromine is more electronegative than carbon, so it draws the bonding electrons towards itself.
Why does ethene react with electrophiles?
The C=C bond is electron-rich.
Name the mechanism of bromoethane with aqueous hydroxide.
Nucleophilic substitution.
Name the mechanism of ethene with bromine.
Electrophilic addition.

Exam questions on Reaction types, mechanisms, nucleophiles and electrophiles

  1. A student summarises five reactions of ethene and related compounds. Reaction 1: ethene reacts with hydrogen bromide to form bromoethane. Reaction 2: bromoethane is warmed with aqueous sodium hydroxide to form ethanol and sodium bromide. Reaction 3: bromoethane is warmed with ethanolic sodium hydroxide to form ethene, sodium bromide and water. Reaction 4: ethanol is warmed with acidified potassium dichromate(VI) to form ethanal. Reaction 5: many ethene molecules join under high pressure to form poly(ethene).
    Name the type of reaction in Reaction 3 and in Reaction 5, and state what happens to the organic molecules in each.2 marks
  2. Bromoethane reacts with aqueous hydroxide ions to form ethanol: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻. The reaction can be explained by a mechanism in which curly arrows show the movement of pairs of electrons.
    Describe, in terms of the movement of electron pairs, how bromoethane forms ethanol in this reaction.2 marks
  3. Ethene reacts with bromine at room temperature to form 1,2-dibromoethane: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br. The reaction occurs readily even though ethene and bromine are both non-polar molecules.
    Explain why ethene is attacked by electrophiles, and why bromine behaves as an electrophile in this reaction.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).