48 exam-style questions on Electron-pair sharing reactions (nucleophilic/electrophilic) — topic R3.4 of Reactivity 3: What are the mechanisms of chemical change? in the IB Chemistry syllabus — with answers and explanations. Samples below; the full set is free with an account.
A tertiary halogenoalkane undergoes nucleophilic substitution. Which mechanism does it favour and what is the rate-determining step?
Answer: SN1; formation of a tertiary carbocation
Tertiary halogenoalkanes favour SN1 because the tertiary carbocation formed is stabilised by three electron-donating alkyl groups. The rate-determining (slow) step is the heterolytic loss of the halide to form this carbocation, so the rate depends only on [halogenoalkane]. Primary halogenoalkanes favour SN2.
Ethene reacts with bromine water. What type of reaction is this and what is observed?
Answer: Electrophilic addition; orange bromine decolourises
The electron-rich C=C double bond in ethene attracts the electrophile Br2 and undergoes electrophilic addition across the double bond (with pure bromine the product is 1,2-dibromoethane). The orange/brown bromine is decolourised, which is the standard test for unsaturation (C=C double bonds).
By which mechanism does a tertiary halogenoalkane undergo nucleophilic substitution?
Answer: SN1
A tertiary halogenoalkane forms a relatively stable tertiary carbocation, favouring the two-step SN1 mechanism. Primary halogenoalkanes favour the one-step SN2 mechanism.
Define a nucleophile.
Answer: An electron-pair donor that is attracted to regions of positive charge
A nucleophile is an electron-rich species that donates a lone pair to an electron-deficient (positive or δ+) centre; contrast with electrophiles, which accept electron pairs.
A primary halogenoalkane reacts with aqueous sodium hydroxide. Determine the mechanism and the product type.
Answer: SN2, forming an alcohol
Primary halogenoalkanes favour SN2 (concerted, one-step, back-side attack); the hydroxide nucleophile replaces the halogen to give an alcohol.
Explain why the C–X bond of a halogenoalkane is susceptible to nucleophilic attack at the carbon atom.
Answer: The halogen is more electronegative than carbon, giving carbon a δ+ charge
Because the halogen is more electronegative, the bonding electrons are pulled towards it, leaving carbon δ+; nucleophiles are attracted to this electron-deficient carbon.
That was 6 of 48 Electron-pair sharing reactions (nucleophilic/electrophilic) questions.
Practise all 48 freeExplainAO2 · 1–4 marks
Give reasons or causes: say what happens AND why, linking each step with 'because' or 'so that'.
DrawAO1 · 1–2 marks
A labelled, accurate drawing with a ruler where lines are straight; axes and units on a graph.
DefineAO1 · 1–2 marks
Give the precise meaning — one sentence, the key terms in it, no example needed unless asked.
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48 Electron-pair sharing reactions (nucleophilic/electrophilic) questions — free with an account. Spaced repetition, streaks and full exam simulations included.
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