Quick answer: Alkanes react with halogens in UV light by free radical substitution, a chain reaction of initiation, propagation and termination. This Chemistry 11-12 lab on halogenoalkanes gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How does the identity of the halogen and the structure of the carbon chain change how fast a halogenoalkane reacts? You will compare hydrolysis rates by timing the silver halide precipitate and then explain the pattern with bond enthalpies and the SN1 and SN2 mechanisms.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R3.4
Aim
To compare the rates of hydrolysis of halogenoalkanes with warm ethanolic silver nitrate, and to show how the halogen and the structure (primary, secondary, tertiary) control the rate.
Equipment
- 1-chlorobutane, 1-bromobutane, 1-iodobutane, 2-bromobutane, 2-bromo-2-methylpropane (about 5 drops each)
- Ethanolic silver nitrate (0.10 mol dm⁻³), ethanol
- Test tubes, rack, 250 cm³ beaker for a water bath at 50 °C, thermometer, stopwatch, droppers
Safety
Wear goggles and gloves. Halogenoalkanes and ethanol are flammable and harmful: use a hot-water bath, never a naked flame. Silver nitrate stains skin black. Dispose of waste in the halogenated waste container.
Method
- Place 1 cm³ of ethanolic silver nitrate in each of five test tubes and stand them in the 50 °C water bath for 3 minutes.
- Add 5 drops of one halogenoalkane to a tube and start the stopwatch at once.
- Stop the watch when the first cloudiness appears. Record the colour and time. Stop at 600 s if nothing forms.
- Repeat for each halogenoalkane. Keep the temperature, volumes and number of drops the same.
Results
| Halogenoalkane | Type | Precipitate colour | Time (s) |
|---|---|---|---|
| 1-chlorobutane | primary | none | > 600 |
| 1-bromobutane | primary | cream | 62 |
| 1-iodobutane | primary | yellow | 9 |
| 2-bromobutane | secondary | cream | 24 |
| 2-bromo-2-methylpropane | tertiary | cream | 3 |
Analysis questions
Question 1 Identify the precipitates
Name the precipitates formed from the chloro, bromo and iodo compounds, and write the ionic equation for the precipitate from 1-iodobutane.Question 2 Effect of the halogen
Use the three primary compounds to describe how the rate changes with the halogen, and explain using bond enthalpies.Question 3 Relative rates
Take 1-bromobutane as 1.0 and calculate the relative rate (1 ÷ time) of 1-iodobutane, 2-bromobutane and 2-bromo-2-methylpropane.Question 4 Effect of structure
Describe the trend among the three bromo compounds and give the mechanism that explains the tertiary result.Question 5 Controlled variables
State three variables that must be controlled and why the tubes are pre-warmed.Question 6 Source of error
Give one source of error in judging the first cloudiness and a way to reduce it.Question 7 Hydrolysis equation
Write the equation for the hydrolysis of 2-bromo-2-methylpropane in water.
