Nucleophilic Substitution, Elimination, and Electrophilic Addition Reactions
This page provides a comprehensive overview of key reaction mechanisms in A Level Chemistry, focusing on nucleophilic substitution, elimination, and electrophilic addition reactions. These mechanisms are crucial for understanding organic Chemistry reactions and are frequently tested in AQA A Level Chemistry exam questions.
Nucleophilic Substitution Reactions
Nucleophilic Substitution with -OH
Definition: Nucleophilic substitution with -OH involves the replacement of a halogen X with a hydroxyl group OH.
- Reagent: NaOH or KOH sourceofhydroxide
- Conditions: Heat under reflux with warm NaOH/KOH
- General Equation: R-X + NaOH → R-OH + NaX
Example: The mechanism shows the OH- nucleophile attacking the carbon bonded to the halogen, displacing the halogen and forming an alcohol.
Nucleophilic Substitution with NH3
- Reagent: NH3 dissolved in ethanol
- Conditions: Heated under pressure in a sealed container
- General Equation: R-X + 2NH3 → R-NH2 + NH4X
Highlight: This reaction produces primary amines and requires excess ammonia to neutralize the hydrogen halide byproduct.
Nucleophilic Substitution with -CN
- Reagent: Ethanolic KCN
- Conditions: Heating under reflux with warm KCN
- General Equation: R-X + KCN → R-CN + KX
Vocabulary: The cyanide ion CN− acts as the nucleophile, replacing the halogen to form a nitrile compound.
Elimination Reactions
- Reagent: Ethanolic KOH/NaOH
- Conditions: Hot ethanolic KOH/NaOH
- General Equation: R-CH2-CHX-R' + KOH → R-CH=CH-R' + KX + H2O
Highlight: The hydrogen being eliminated must be adjacent leftorright to the carbon with the C-X bond.
Electrophilic Addition Reactions
Electrophilic Addition with Br2
- General Equation: C=C + Br2 → BrC-CBr
Example: The mechanism involves the formation of a bromonium ion intermediate, followed by nucleophilic attack by the bromide ion.
Electrophilic Addition with HBr
- General Equation: C=C + HBr → HC-CBr
Highlight: This reaction follows Markovnikov's rule, with the bromine attaching to the more substituted carbon.
Electrophilic Addition with H2SO4
- General Equation: C=C + H2SO4 → HC-C-OSO3H
Vocabulary: The sulfuric acid acts as both the electrophile and the catalyst in this reaction.
Electrophilic Addition with H2O
- Conditions: Strong acid catalyst
- General Equation: C=C + H2O → HC-C-OH
Definition: This reaction, also known as hydration of alkenes, produces alcohols and requires a strong acid catalyst.
These reaction mechanisms are fundamental to A Level Chemistry and are essential for understanding more complex organic Chemistry reactions. Mastering these mechanisms will greatly aid in solving AQA A Level Chemistry mechanisms exam questions and similar problems in other exam boards like OCR.
