4.2.1Organic Chemistry
Hydrolysis of Halogenoalkanes — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 37 of 42
Section 1 of 5The Hydrolysis Reaction
Hydrolysis replaces the halogen with an OH group through nucleophilic substitution.
Nucleophile
- The hydroxide ion from NaOH or KOH has a lone pair and a full negative charge, so it attacks the carbon bonded to the halogen.
Equation
- CH₃CH₂CH₂Br + OH⁻ → CH₃CH₂CH₂OH + Br⁻, giving an alcohol and a halide ion.
Water as nucleophile
- Water can also hydrolyse halogenoalkanes, but it is much slower since oxygen carries only a partial negative charge.
Warming
- The reaction is slow at room temperature, so the mixture is warmed in a water bath.
Section 2 of 5Rate Experiment
Acidified silver nitrate in ethanol detects halide ions as they form.
1
Place equal volumes of ethanol and acidified silver nitrate solution in three test tubes standing in a water bath at about 50 °C.
2
Add the same number of drops of a chloroalkane, a bromoalkane and an iodoalkane to separate tubes and start a stopwatch.
3
Silver ions react with halide ions as they are released: Ag⁺(aq) + X⁻(aq) → AgX(s).
4
Time how long each tube takes to show a precipitate, keeping volumes and temperature identical.
5
Compare the times: the faster the precipitate appears, the faster the hydrolysis.
Section 3 of 5Results and Explanation
Results and Explanation
Iodoalkane
- Gives a yellow precipitate of AgI first because the C–I bond is the weakest, about 228 kJ mol⁻¹.
Bromoalkane
- Gives a cream precipitate of AgBr at an intermediate rate, with a C–Br bond of about 290 kJ mol⁻¹.
Chloroalkane
- Gives a white precipitate of AgCl slowest because the C–Cl bond is stronger, about 346 kJ mol⁻¹.
Conclusion
- Rate of hydrolysis depends on how easily the carbon-halogen bond breaks, not on the polarity of the bond.
| Halogenoalkane | Precipitate | Relative rate |
|---|---|---|
| Chloroalkane | White AgCl | Slowest |
| Bromoalkane | Cream AgBr | Intermediate |
| Iodoalkane | Yellow AgI | Fastest |
Section 4 of 5Safety and Control
Variables
- Keep temperature, amount of halogenoalkane and concentration of silver nitrate the same in each tube.
Ethanol solvent
- Ethanol allows the halogenoalkane and aqueous silver nitrate to mix, since the organic liquid is insoluble in water.
Flammability
- Ethanol is flammable, so warm it with a water bath and never with a naked flame.
Acidified silver nitrate
- The nitric acid prevents other silver salts such as carbonates from precipitating and spoiling the result.
Section 5 of 5Exam Checklist & Tips
4.2.1 – Key Points for the Exam
- Hydrolysis A reaction in which a bond is broken by reaction with water or hydroxide ions.
- Nucleophile A species that donates a lone pair of electrons to form a new covalent bond.
- Bond enthalpy The energy needed to break one mole of a covalent bond in the gas phase.
- Nucleophilic substitution Replacement of an atom or group by a nucleophile.
Common Mistakes
- Saying iodoalkanes react fastest because C–I is most polar; it is the lower bond enthalpy that matters.
- Mixing up the colourschloride is white, bromide is cream and iodide is yellow.
- Adding silver nitrate without first acidifying it with nitric acid.
- Using different volumes or temperatures in each tube, which makes the comparison unfair.
Last updated:
\n