3.3.1Physical Chemistry
Reactivity of Halogenoalkanes — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 73 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4The Polar C–X Bond
The halogens are more electronegative than carbon, so the C–X bond is polar.
Partial charges
- Partial charges The carbon atom carries a partial positive charge (δ+) and the halogen a partial negative charge (δ−).
Nucleophiles
- Nucleophiles A nucleophile is an electron-pair donor, which is attracted to the δ+ carbon atom of the halogenoalkane.
Substitution
- Substitution The nucleophile replaces the halogen, which leaves as a halide ion, so these are nucleophilic substitution reactions.
Examples of nucleophiles
- Examples of nucleophiles Common nucleophiles are OH⁻, CN⁻, NH₃ and water, which all have a lone pair of electrons.
Part 2 of 4Bond Enthalpy Controls the Rate
Bond Enthalpy Controls the Rate
Bond strength matters most
- Bond strength matters most Although the C–F bond is the most polar, it is the strongest, so fluoroalkanes are very unreactive.
Trend
- Trend Reactivity increases from chloro- to bromo- to iodoalkanes because the C–X bond gets weaker.
Why weaker
- Why weaker Larger halogen atoms overlap less well with carbon orbitals, so the bond is longer and easier to break.
| Bond | Mean bond enthalpy (kJ mol⁻¹) | Reactivity |
|---|---|---|
| C–F | 467 | Least reactive |
| C–Cl | 346 | Slow |
| C–Br | 290 | Faster |
| C–I | 228 | Fastest |
Part 3 of 4Measuring Reactivity With Silver Nitrate
Hydrolysis releases halide ions, which are detected by a silver halide precipitate.
1
Warm each halogenoalkane in a water bath with aqueous silver nitrate in ethanol, which acts as a common solvent.
2
Water acts as the nucleophile: RX + H₂O → ROH + X⁻ + H⁺.
3
Released halide ions react immediately: Ag⁺(aq) + X⁻(aq) → AgX(s).
4
Iodoalkanes give a yellow precipitate fastest, bromoalkanes a cream precipitate more slowly, and chloroalkanes a white precipitate slowest.
5
The order of the precipitation rates shows that C–I breaks most easily and C–Cl least easily.
Part 4 of 4Exam Checklist & Tips
3.3.1 – Key Points for the Exam
- Nucleophile An electron-pair donor that attacks a region of positive charge.
- Nucleophilic substitution A reaction in which a nucleophile replaces an atom or group in a molecule.
- Hydrolysis A reaction with water (or hydroxide) that breaks a bond and adds an OH group.
- Bond enthalpy The energy required to break one mole of a given bond in gaseous molecules.
Common Mistakes
- Saying that C–F is the most reactive because it is the most polar; bond strength is more important.
- Forgetting that the nucleophile attacks the δ+ carbon, not the halogen.
- Mixing up the precipitate colours of silver chloride, bromide and iodide.
- Writing the nucleophile curly arrow from the wrong place, which should start at the lone pair.
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