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3.3Organic Chemistry

Halogenoalkanes — AQA A Level Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 3 of 8

📚 AQA A Level Chemistry Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Lesson summary

Halogenoalkanes: Reactivity and Ozone Depletion — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Compare the rates of substitution of halogenoalkanes using bond enthalpies and explain how CFCs destroy ozone.The halogenoalkanes have different rates of substitution reactions.

  • Chlorofluorocarbons (CFCs)
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Nucleophile
  • Hydrolysis

Part 1 of 4Reactivity of halogenoalkanes

The halogenoalkanes have different rates of substitution reactions.

1
The rate depends on the strength of the carbon-halogen bond, not on its polarity.
2
The bond enthalpies are: C-F 467, C-Cl 346, C-Br 290, C-I 228 kJ mol-1.
3
Weaker bonds break more easily, so reactivity increases in the order C-F < C-Cl < C-Br < C-I.
4
Iodoalkanes react the fastest, although C-F is the most polar.

Rates can be compared by hydrolysis with aqueous silver nitrate in ethanol (ethanol as a common solvent): R-X + H2O → ROH + H+ + X-, and Ag+ + X- → AgX(s).

Reactivity of halogenoalkanes
The precipitate appears fastest with iodo (yellow), then bromo (cream), then chloro (white).
The tertiary and primary differ too, with tertiary halogenoalkanes being hydrolysed faster by an SN1 route.

Part 2 of 4Reactions of halogenoalkanes

Because carbon is bonded to an electronegative halogen, the Cδ+ is attacked by nucleophiles. Substitution reactions include: with aqueous NaOH (warm) to give alcohols; with KCN in ethanol to give nitriles (adds a carbon); with ammonia in ethanol, excess, to give primary amines. With ethanolic NaOH, elimination gives alkenes. (The mechanisms are covered in the lesson on organic mechanisms.)

Part 3 of 4Ozone depletion

Ozone (O3) in the stratosphere absorbs harmful UV radiation.

Ozone depletion
Chlorofluorocarbons (CFCs)
It forms and breaks down naturally: O2 + UV → 2O·; O· + O2 → O3. Chlorofluorocarbons (CFCs) were used as refrigerants, aerosol propellants and solvents because they are unreactive and non-toxic.
In the stratosphere, UV light breaks the C-Cl bond to form chlorine radicals: CF2Cl2 → ·CF2Cl + Cl·.
The chlorine radicals act as catalysts for ozone destruction:
Ozone depletion
Cl· + O3 → ClO· + O2
ClO· + O· → Cl· + O2

Overall

  • O3 + O· → 2O2.
  • The Cl· is regenerated and so one radical can destroy many thousands of ozone molecules.
  • Nitrogen oxides (NO·) from aircraft and lightning have a similar effect.
  • CFCs have been banned in the Montreal Protocol, and replaced by HFCs and hydrocarbons.

Part 4 of 4Worked examples

  1. Explain why iodoalkanes are hydrolysed faster than chloroalkanes. [2]2 marks

  2. Write equations to show how chlorine radicals catalyse the breakdown of ozone. [2]2 marks

  3. Explain why Cl· is described as a catalyst in ozone depletion. [1]1 mark

Key terms

TermMeaning
HalogenoalkaneAn alkane in which one or more hydrogens are replaced by halogens.
NucleophileAn electron-pair donor.
CFCChlorofluorocarbon, a compound containing C, Cl and F that depletes ozone.
OzoneO3, a gas in the stratosphere absorbing UV radiation.
HydrolysisBreaking down of a compound by reaction with water.

Avoid these mistakes

1
Saying C-F reacts fastest because it is most polar - bond strength decides the rate.
2
Forgetting the dot on radicals in the ozone equations.
3
Stating that CFCs break down in the troposphere - UV is needed, which occurs in the stratosphere.
4
Saying ozone absorbs infrared rather than UV.
5
Not mentioning that radicals are regenerated.

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