Rates of hydrolysis of halogenoalkanes (Core Practical 5)

The method

Put equal amounts of each halogenoalkane in separate tubes with ethanol (a common solvent for the halogenoalkane and water) and warm them in a water bath at the same temperature. Add aqueous silver nitrate, start timing, and record the time for a precipitate to appear.

R-X + H2O -> R-OH + H+ + X-, then Ag+ + X- -> AgX(s).

Changing the halogen

Rate: iodoalkane > bromoalkane > chloroalkane. The C-I bond is the weakest (bond enthalpies about C-Cl 346, C-Br 290, C-I 228 kJ/mol), so it breaks most easily. Bond enthalpy matters more than bond polarity here: C-Cl is the most polar but reacts slowest.

Precipitates: AgI yellow (first), AgBr cream, AgCl white (last).

Changing the structure

For isomers with the same halogen, the rate is tertiary > secondary > primary. Tertiary halogenoalkanes react through a stable tertiary carbocation, which forms easily.

Fair testing

Control the temperature (water bath), the volumes and concentrations, and the number of moles of halogenoalkane. Rate is proportional to 1 / time for the precipitate to appear.

Worked example

Predict the order in which precipitates appear for 1-chlorobutane, 1-bromobutane and 1-iodobutane, and explain.

1. The C-I bond is weakest and C-Cl strongest.
2. The weakest bond breaks fastest.

Answer: 1-iodobutane (yellow) first, then 1-bromobutane (cream), then 1-chlorobutane (white).

Check yourself

1. Which halogenoalkane is hydrolysed fastest?
   • 1-bromopropane
   • 1-chloropropane
   • 1-iodopropane
   • 1-fluoropropane
   Show answer

   1-iodopropane. C-I has the lowest bond enthalpy.

2. Why is 1-chlorobutane hydrolysed more slowly than 1-iodobutane, even though C-Cl is more polar?
   • The C-Cl bond enthalpy is higher, so it is harder to break
   • Chlorine is less electronegative
   • AgCl is soluble
   • The chloro compound has more carbon atoms
   Show answer

   The C-Cl bond enthalpy is higher, so it is harder to break. Bond strength is the deciding factor, not polarity.

3. What colour precipitate forms from 2-bromo-2-methylpropane in this experiment?
   • White
   • Yellow
   • Cream
   • Brown
   Show answer

   Cream. Bromide ions give cream AgBr.

4. Why is ethanol used in Core Practical 5?
   • Halogenoalkanes do not mix with water, but both dissolve in ethanol
   • It is the nucleophile
   • It catalyses the reaction
   • It precipitates the halide
   Show answer

   Halogenoalkanes do not mix with water, but both dissolve in ethanol. Ethanol is a common solvent, so the reactants mix.

5. Which isomer of C4H9Br is hydrolysed fastest?
   • 1-bromobutane
   • 2-bromobutane
   • 1-bromo-2-methylpropane
   • 2-bromo-2-methylpropane
   Show answer

   2-bromo-2-methylpropane. Tertiary halogenoalkanes react fastest.

6. A precipitate appears after 125 s. Calculate 1/time.
   Show answer

   0.0080 /s. 1/125 = 0.0080 /s.

7. Which variable must be controlled in this experiment?
   • The colour of the precipitate
   • The time taken
   • The halogen in the compound
   • The temperature of the water bath
   Show answer

   The temperature of the water bath. Temperature strongly affects rate.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.