Comparing the Rates of Hydrolysis

The rates of hydrolysis of different halogenoalkanes can be compared using silver nitrate: the faster the halide ion is released, the sooner the silver halide precipitate appears.Method:
1. Put equal amounts (e.g. 3 drops) of 1-chlorobutane, 1-bromobutane and 1-iodobutane in separate test tubes, each with 1 cm³ of ethanol.
2. Warm these, and a tube of aqueous silver nitrate, in a water bath at about 50°C.
3. Add equal volumes of the silver nitrate solution to each tube at the same time and start a timer.
4. Record the time for a precipitate to appear in each tube.Keep the temperature, volumes, concentrations and chain length the same so that only the halogen changes.Results: the yellow AgI precipitate appears first, then cream AgBr; the white AgCl precipitate forms very slowly. So the rate order is1-iodobutane > 1-bromobutane > 1-chlorobutaneThis matches the C-X bond enthalpies: the C-I bond is weakest and breaks most easily.Estimating rate: rate is proportional to 1/time.Worked example: if a precipitate takes 40 s to appear, the relative rate = 1/40 = 0.025 s⁻¹.Changing the structure: the same method can compare primary, secondary and tertiary isomers with the same halogen, such as 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane. The tertiary halogenoalkane is hydrolysed fastest and the primary slowest.