3.3.1Organic Chemistry
Nucleophilic Substitution
5 min read 5 parts Lesson 12 of 55
Section 1 of 5The Mechanism
1
The carbon–halogen bond is polar because the halogen is more electronegative, so the carbon has a partial positive charge.
2
A nucleophile uses its lone pair to attack the δ+ carbon, shown by a curly arrow from the lone pair to carbon.
3
At the same time the C–X bond breaks heterolytically, shown by a curly arrow from the bond to the halogen.
4
The halide leaves as X⁻, and the nucleophile now forms a new covalent bond to carbon.
Section 2 of 5Reagents and Products
Hydroxide
- Warm with aqueous NaOHCH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻, forming an alcohol.
Cyanide
- Heat under reflux with KCN in ethanol: CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻, forming a nitrile and adding a carbon.
Ammonia
- Excess ammonia in ethanol, heated in a sealed tube: CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br.
Water
- Water is a weak nucleophile, so hydrolysis with water alone is very slow compared with hydroxide.
Solvent
- Mixed ethanol and water dissolves both halogenoalkane and aqueous reagent, but too much ethanol favours elimination.
| Nucleophile | Reagent | Product |
|---|---|---|
| OH⁻ | NaOH(aq), warm | alcohol |
| CN⁻ | KCN in ethanol, reflux | nitrile |
| NH₃ | excess NH₃ in ethanol, sealed tube | primary amine |
Section 3 of 5Rate of Hydrolysis
Rate of Hydrolysis
Bond enthalpies
- C–F 467, C–Cl 346, C–Br 290 and C–I 228 kJ mol⁻¹, so the C–I bond is the weakest and breaks most easily.
Order of reactivity
- Iodoalkanes react fastest, then bromoalkanes, then chloroalkanes; fluoroalkanes hardly react.
Polarity is not the factor
- C–F is the most polar, but bond strength matters more than polarity for the rate.
Silver nitrate test
- Add AgNO₃(aq) in ethanol and warm: white AgCl, cream AgBr and yellow AgI appear at rates that increase from Cl to I.
Section 4 of 5Worked Example
Question Name the organic product and conditions when 1-bromopropane reacts with potassium cyanide, and state the reaction type.
- The nucleophile is CN⁻, so Br is replaced by –CN.
- The equation is CH₃CH₂CH₂Br + CN⁻ → CH₃CH₂CH₂CN + Br⁻.
- The conditions are KCN dissolved in ethanol and heated under reflux.
Answer
- Butanenitrile;
- nucleophilic substitution, reflux with ethanolic KCN.
Section 5 of 5Exam Checklist & Tips
3.3.1 – Key Points for the Exam
- Nucleophile An electron-pair donor that attacks a δ+ carbon.
- Nitrile An organic compound containing the –C≡N group.
- Hydrolysis Breaking a bond by reaction with water or hydroxide.
- Bond enthalpy The energy needed to break one mole of a bond in gaseous molecules.
Common Mistakes
- Saying C–F reacts fastest because it is the most polar; the weak C–I bond reacts fastest.
- Forgetting that the cyanide route adds a carbon to the chain.
- Writing H₂O as the reagent when aqueous NaOH is needed for a good rate.
- Starting the curly arrow from the carbon instead of the nucleophile lone pair.