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Curly Arrow Mechanisms for A-Level Chemistry
May 10, 2025
Summary of Curly Arrow Mechanisms for AQA A-Level Chemistry
Introduction
Overview of curly arrow mechanisms in organic chemistry as per AQA A-level Chemistry specification.
Focus on drawing mechanisms for exam marks, not on reagents or catalysts.
Nucleophilic Substitution
Starting Point
: Haloalkane (e.g., bromine or chlorine).
Process
: Nucleophile attacks the delta positive carbon attached to the halogen, substituting halogen for the nucleophile.
Key Nucleophiles
:
Hydroxide ion (OH⁻)
Cyanide ion (CN⁻)
Ammonia (NH₃)
Mechanism Insights
:
Curly arrows represent electron pair movement.
Formation of dative covalent bonds with lone pair donation.
Leaving group: the halogen, forming a bromide or chloride ion.
Hydroxide Ion
Formation of an alcohol and a bromide ion.
Cyanide Ion
Similar mechanism to hydroxide, forming a nitrile.
Ammonia
Intermediate stage forming an ammonium ion and eventually primary amine.
Possibility of subsequent nucleophilic substitutions forming secondary and tertiary amines, and quaternary ammonium salts.
Elimination Reactions
Starting Point
: Haloalkanes with hydroxide ions.
Process
: Formation of an alkene by attacking a hydrogen, forming a double bond, and removing the halogen as a leaving group.
Product
: Alkene and water.
Electrophilic Addition (Alkenes)
Electrophiles
:
Diatomic halogens (e.g., Cl₂, Br₂)
Hydrogen halides (e.g., HBr)
Sulfuric acid (H₂SO₄)
Diatomic Halogens
Formation of dihaloalkanes.
Hydrogen Halides
Formation of haloalkanes.
Sulfuric Acid
Formation of alkyl hydrogen sulfate.
Acid-Catalyzed Addition (Hydration of Ethene)
Catalyst
: Phosphoric acid (H₃PO₄).
Process
: Formation of alcohol.
Alcohols to Alkenes (Elimination)
Mechanism
: Acid-catalyzed elimination forming alkenes.
Catalyst
: Sulfuric or phosphoric acid.
Nucleophilic Addition (Ketones and Aldehydes)
Nucleophiles
:
Hydride ion (H⁻)
Cyanide ion (CN⁻)
Product
: Alcohols or hydroxy nitriles.
Reagents
: Strong acids for cyanide reactions.
Nucleophilic Addition-Elimination (Acyl Chlorides)
Nucleophiles
:
Water, Alcohol, Ammonia, Primary Amine
Products
: Carboxylic acids, esters, primary amides, secondary amides.
Electrophilic Substitution (Benzene)
Reactions
:
Nitration (NO₂⁺ ion)
Friedel-Crafts Acylation (RCO⁺ ion)
Products
: Nitrobenzene, phenyl ketones.
Conclusion
Practice mechanisms through questions to understand better than mere memorization.
Focus on reagents, conditions, and equations, especially regarding catalysts.
Best of luck with revision and exams.
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