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 (source of hydroxide)
• 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 (left or right) 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.