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A haloalkane hydrolyses when a nucleophile replaces its halogen. In a suitable aqueous silver nitrate and ethanol mixture, hydrolysis releases halide ions which then form a silver-halide precipitate. Chloride gives white AgCl, bromide cream AgBr and iodide yellow AgI. The covalently bound halogen must be released first, so this is a rate investigation rather than simply a test of free halide already in solution.
Video by MaChemGuy. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Design a controlled haloalkane rate comparison
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What must happen before a covalently bound halogen gives an AgX precipitate?
Halide must be released by reaction. Hydrolysis releases X⁻, then Ag⁺ precipitates it.
After watching 2: Why is an iodoalkane often faster than a comparable chloroalkane in this test?
Its C–I bond is weaker. Ease of C–X cleavage is central to this comparison.
What you'll be able to do
A haloalkane hydrolyses when a nucleophile replaces its halogen. In a suitable aqueous silver nitrate and ethanol mixture, hydrolysis releases halide ions which then form a silver-halide precipitate. Chloride gives white AgCl, bromide cream AgBr and iodide yellow AgI. The covalently bound halogen must be released first, so this is a rate investigation rather than simply a test of free halide already in solution.
A convenient comparison uses haloalkanes with the same carbon skeleton and different halogens. For comparable primary chloro-, bromo- and iodoalkanes, C–I is weaker than C–Br and C–Cl, so iodoalkane hydrolysis is normally faster under the chosen comparable conditions. C–X bond polarity alone does not explain that observed trend.
Use the approved fixed amounts and concentrations, the same ethanol–water composition, silver nitrate concentration, temperature and mixing. Ethanol helps bring organic and aqueous components together; changing its proportion also changes the reaction environment. Equilibrate samples in the water bath before reaction and time to a consistently defined visible turbidity.
Comparing primary, secondary and tertiary structures introduces a different variable from changing the halogen on one skeleton. Their mechanism and relative rates depend on structure and conditions; do not treat both comparisons as the same test of C–X bond strength. Use a clear experimental question and alter one factor at a time.
Under a consistent detection threshold, 1/t can be a useful relative-rate proxy. It is not automatically an absolute hydrolysis rate because different precipitates have different appearance and detection properties. Repeat runs and use a consistent observer or an appropriate objective turbidity measurement. Very fast reactions are difficult to time manually; a negative early observation does not prove no hydrolysis.
Use the approved water-bath or electrical heating arrangements for flammable ethanol-containing mixtures. Haloalkanes and silver-containing waste need specified handling and disposal. A larger spread between repeated times at one condition can weaken a claimed ordering. Report actual measurements and uncertainty rather than forcing results into an expected trend.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A student changes both the halogen and the ethanol proportion between runs. Why is a bond-strength conclusion weakened?
Keep solvent composition constant and compare matched substrates before attributing the effect to C–X cleavage.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.