Compare halogenoalkane reactivity using C–X bond strength
Relate C–X bond strength to the ease of substitution and use silver nitrate tests to compare reaction rates.
Section 15 · Halogen compounds
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the named reagents, solvents, temperatures and pressure conditions; begin curly arrows at a bond or lone pair and distinguish typical mechanisms from absolute rules. Written practice is self-checked
Before you begin
- Use covalent formulas, nucleophiles, electrophiles, homolytic and heterolytic fission, and the alkene and alcohol pathways in Topics 13–14.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which bond is easier to break in the comparable C–X series?
What you will learn
- Compare C–Cl, C–Br and C–I bond strengths
- Relate bond breaking to substitution rate
- Use a controlled silver nitrate comparison
The carbon–halogen bond affects reactivity
For comparable halogenoalkanes, the C–Cl bond is stronger than C–Br, which is stronger than C–I. The C–X bond becomes easier to break down this sequence.
This helps explain the observed substitution reactivity trend: iodoalkanes tend to react faster than bromoalkanes, which tend to react faster than chloroalkanes under comparable test conditions.
Silver nitrate gives a comparative reaction test
Warm each halogenoalkane with the same aqueous silver nitrate/ethanol mixture under the same conditions. As the C–X bond breaks and X⁻ is released, AgX precipitates.
Compare time to first persistent precipitate or another defined endpoint. Faster precipitate formation indicates faster halide release in that controlled comparison; keep concentration, temperature, volumes and mixing comparable.
Pause and explain
What does a shorter time to precipitate mean in a controlled silver nitrate test?
Worked example
Order 1-chloropropane, 1-bromopropane and 1-iodopropane by expected substitution rate under comparable conditions.
Show the worked solution
- Compare the C–X bonds for the same alkyl group.
- C–Cl is strongest, then C–Br, then C–I.
- The weaker C–X bond is more readily broken in the substitution test.
Answer Expected rate: 1-iodopropane > 1-bromopropane > 1-chloropropane under comparable conditions.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A matched test gives persistent precipitate times: 1-chlorobutane 95 s, 1-bromobutane 28 s, 1-iodobutane 7 s. State the observed order and relate it to bond strengths.
Give me a hint
A smaller time means faster precipitate formation.
Compare my reasoning
- Iodo compound forms precipitate first and fastest.
- Bromo compound is intermediate; chloro compound is slowest.
- C–I is weakest and C–Cl strongest among these bonds.
Rate: 1-iodobutane > 1-bromobutane > 1-chlorobutane, consistent with C–I < C–Br < C–Cl bond strength.
Look for these in your work
- Time order interpreted
- Rate ranking correct
- Bond strength linked
Why should concentration, volume and temperature be kept the same when comparing halogenoalkanes?
Give me a hint
Identify alternative factors that also change reaction rate.
Compare my reasoning
- Different concentrations change collision frequency.
- Different temperature changes kinetic energy and rate.
- Control them so the comparison mainly tests the halogenoalkane and C–X bond.
Keeping conditions constant prevents concentration or temperature from confounding the reactivity comparison.
Look for these in your work
- At least two confounders named
- Control purpose explained
- Comparison target stated
A student sees a cream precipitate quickly and concludes that the halogenoalkane must have a weak C–Br bond. What can be concluded, and what else is needed?
Give me a hint
Separate halide identity from reaction rate and control the comparison.
Compare my reasoning
- Cream AgBr supports release of Br⁻.
- A single time does not compare bond strength with other halogenoalkanes.
- Use matched compounds and conditions to compare reaction times.
The cream precipitate supports bromide release; a controlled comparison with other halogenoalkanes is needed to infer relative rate and relate it to bond strength.
Look for these in your work
- Halide inference limited
- Rate comparison requirement stated
- Control specified
Common mistakes
- Comparing different structures and attributing every rate change only to C–X strength
- Claiming precipitate color alone measures reaction rate
- Changing temperature or concentration between trials
What can you explain now?
Which bond is easier to break in the comparable C–X series?
Compare with the explanation
C–I rather than C–Cl
C–I is weaker than C–Cl and is more readily broken.
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Make it stick
How does relative C–X bond strength explain the matched silver nitrate reaction-rate trend?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.