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AQA A Level Chemistry

3.3.3
Halogenoalkanes

Section 3.3.3 covers the polar carbon–halogen bond, nucleophilic substitution with hydroxide, cyanide and ammonia with their mechanisms, the effect of bond enthalpy on the rate of hydrolysis, elimination and its mechanism with the reagent acting as both nucleophile and base, and the role of chlorine radicals from CFCs in the depletion of the ozone layer.

Exam Paper
Paper 2
7405/2
Specification Points
3.3.3.1 – 3.3.3.3
3 sections covered
Topic Parts
6 pages
Revision notes available
Exam Board
AQA
7405 (2015 onwards)

Revision Notes

Work through 3.3.3 Halogenoalkanes in a structured sequence.

Seven types
Reaction Types
and Mechanisms
1
Available

Reaction Types, Mechanisms and Nucleophiles

Organic reaction types and mechanisms: addition, elimination, substitution, oxidation, reduction, hydrolysis, polymerisation, curly arrows, heterolytic fission, electrophiles, nucleophiles and bond polarity.

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C–X bond
Halogenoalkanes
and Bonding
2
Available

Halogenoalkanes: Naming, Classification and the C–X Bond

Halogenoalkanes: nomenclature and formulae, primary, secondary and tertiary, the polar carbon–halogen bond, bond enthalpies and uses.

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Lone pair attacks δ+
Nucleophilic
Substitution
3
Available

Nucleophilic Substitution Reactions

Nucleophilic substitution of halogenoalkanes: hydroxide, water, ammonia and cyanide as nucleophiles, conditions, products and the curly-arrow mechanism.

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OH⁻ as a base
Elimination
Reactions
4
Available

Elimination Reactions of Halogenoalkanes

Elimination of halogenoalkanes: ethanolic KOH, hydroxide as a base, alkene products and isomers, substitution versus elimination.

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AgNO₃ in ethanol
Rates of
Hydrolysis
5
Available

Rates of Hydrolysis and Bond Enthalpy

Rates of hydrolysis of halogenoalkanes: the silver nitrate experiment, precipitate colours, bond enthalpy trend for C–Cl, C–Br and C–I, and primary, secondary and tertiary comparison.

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Cl• catalyst
CFCs and
Ozone
6
Available

CFCs and the Ozone Layer

CFCs and the ozone layer: chlorine radicals, the equations for ozone decomposition, the catalytic cycle, legislation and HFC alternatives.

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Specification Coverage

3.3.3 Halogenoalkanes – AQA A Level Chemistry

The following AQA specification points outline the knowledge and skills students are expected to demonstrate. Wording follows the AQA A Level Chemistry (7405) specification.

3.3.3.1 Nucleophilic substitution

3.3.3.1i
know that halogenoalkanes contain polar bonds
3.3.3.1ii-a
know that halogenoalkanes undergo substitution reactions with OH⁻
3.3.3.1ii-b
know that halogenoalkanes undergo substitution reactions with CN⁻
3.3.3.1ii-c
know that halogenoalkanes undergo substitution reactions with NH₃
3.3.3.1iii-a
outline the nucleophilic substitution mechanism of halogenoalkanes with OH⁻
3.3.3.1iii-b
outline the nucleophilic substitution mechanism of halogenoalkanes with CN⁻
3.3.3.1iii-c
outline the nucleophilic substitution mechanism of halogenoalkanes with NH₃
3.3.3.1iv
explain why the carbon-halogen bond enthalpy influences the rate of reaction

3.3.3.2 Elimination

3.3.3.2i
know that a halogenoalkane can undergo concurrent substitution and elimination reactions (eg 2-bromopropane with potassium hydroxide)
3.3.3.2ii
explain the role of the reagent as both nucleophile and base in elimination reactions
3.3.3.2iii-a
outline substitution mechanisms of halogenoalkanes
3.3.3.2iii-b
outline elimination mechanisms of halogenoalkanes

3.3.3.3 Ozone depletion

3.3.3.3i
know that ozone in the upper atmosphere is beneficial because it absorbs ultraviolet radiation
3.3.3.3ii
know that chlorine atoms are formed in the upper atmosphere when ultraviolet radiation causes C-Cl bonds in CFCs to break
3.3.3.3iii
know that chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer
3.3.3.3iv-a
use the equation Cl + O₃ -> ClO + O₂ to explain chlorine-catalysed decomposition of ozone
3.3.3.3iv-b
use the equation ClO + O₃ -> 2O₂ + Cl to explain chlorine-catalysed decomposition of ozone
3.3.3.3v
appreciate that scientific evidence provided by different research groups led to legislation to ban the use of CFCs and that chemists have developed alternative chlorine-free compounds