Nucleophilic substitution — AQA A-Level Chemistry
Test yourself on Nucleophilic substitution with AQA A-Level practice questions.
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Nucleophilic substitution explained
Halogens are more electronegative than carbon, so the shared pair in a carbon–halogen bond is pulled towards the halogen.
Read the full explanation
The bond is therefore polar: the carbon carries a small positive charge, δ⁺, and the halogen a small negative charge, δ⁻. That electron-deficient carbon is the site every reaction of a halogenoalkane starts from, because a nucleophile is attracted to positive charge. Polarity falls from C–F through C–Cl and C–Br to C–I as electronegativity falls down the group, so C–I is the least polar bond of the four. Do not turn that into a prediction about rate: iodoalkanes still react fastest because the C–I bond is the weakest, and bond enthalpy, not polarity, controls how quickly the bond breaks.
Halogenoalkanes undergo substitution reactions with the – – nucleophiles OH , CN and NH Students should be able to: outline the nucleophilic substitution mechanisms of these reactions explain why the carbon–halogen bond enthalpy influences the rate of reaction.
A nucleophile is an electron-pair donor attracted to the δ⁺ carbon. The mechanism is the same for all three: a curly arrow runs from the lone pair on the nucleophile to the δ⁺ carbon, and a second arrow runs from the C–X bond to the halogen, which leaves as X⁻. Warm aqueous sodium hydroxide gives an alcohol; potassium or sodium cyanide in aqueous ethanol, heated under reflux, gives a nitrile and lengthens the chain by one carbon; excess ammonia in ethanol in a sealed tube gives an amine, with a second ammonia molecule removing the H⁺. Rate is set by the carbon–halogen bond enthalpy: C–I is the weakest bond so iodoalkanes hydrolyse fastest, and C–F the strongest so fluoroalkanes are slowest.
Your focus
- Label the partial charges on a carbon–halogen bond and justify the direction from electronegativity values.
- Deduce which atom in a given halogenoalkane a nucleophile will attack, and say why.
- Distinguish the polarity argument from the bond enthalpy argument when explaining relative rates of hydrolysis.
Show all 6 objectives
- Draw the full mechanism, arrows included, for the reaction of 1-bromopropane with hydroxide, cyanide and ammonia.
- Identify the reagent and conditions needed to convert a named halogenoalkane into a nitrile, an alcohol or an amine.
- Rank the hydrolysis rates of a chloro-, bromo- and iodoalkane using bond enthalpy data and justify the order.
Nucleophilic substitution exam tips
Marking Points
- one mark for stating that the halogen is more electronegative than carbon
- one mark for the δ⁺ label on the carbon and the δ⁻ label on the halogen, shown on the bond
- one mark for identifying the δ⁺ carbon as the atom attacked by a nucleophile
- one mark for the correct order of bond polarity, C–F most polar through to C–I least polar
- one mark for the curly arrow from the lone pair on the nucleophile to the δ⁺ carbon
- one mark for the curly arrow from the C–X bond to the halogen, with X⁻ shown as a product
- one mark for the correct organic product, such as the nitrile from cyanide
- one mark for naming the reagent as a bottle chemical with its conditions, for example KCN in aqueous ethanol, heated under reflux
- one mark for explaining the rate order by carbon–halogen bond enthalpy rather than by polarity
Examiner Tips
- 💡Always draw the δ⁺ and δ⁻ on the bond itself, not floating next to the molecule.
- 💡Say "more electronegative than carbon" explicitly; naming the halogen as electronegative on its own rarely scores.
- 💡Keep polarity and bond enthalpy as two separate arguments so you do not use one where the other is wanted.
- 💡Name reagent and conditions together; cyanide needs aqueous ethanol and heating under reflux, ammonia needs excess in a sealed tube.
- 💡Check every arrow starts on a lone pair or a bond and ends where the new bond forms.
- 💡Make the comparison explicitly: the C–I bond has the lowest bond enthalpy so breaks most easily and the iodoalkane hydrolyses fastest; use bond enthalpy values only where the question supplies them.
Common Mistakes
- labelling the carbon δ⁻ and the halogen δ⁺, which reverses the whole mechanism
- writing full charges C⁺ and Cl⁻ rather than the partial charges δ⁺ and δ⁻
- claiming C–I is the most polar bond because iodine is the largest atom
- predicting that the most polar bond reacts fastest, when the rate follows bond enthalpy instead
- giving the reagent as CN⁻ or OH⁻ when AQA requires a reagent that could be taken from a bottle, such as KCN or NaOH(aq)
- starting the curly arrow at the negative charge rather than at a lone pair, which gains no credit
- explaining the rate trend by the polarity of the carbon–halogen bond instead of its enthalpy
- leaving the lone pair off NH₃, or forgetting the second ammonia molecule that removes H⁺ from the salt