Electrophilic Addition in Organic Chemistry

Introduction

• Electrophilic Addition: A reaction mechanism in chemistry characterized by the addition of an electrophile to an organic compound.
• Key Terms:
  • Electrophile: An electron pair acceptor, attracted to negative electron-rich areas, opposite of a nucleophile.
  • Addition Reaction: Two reactants create one product without any by-products.

Key Concepts

Electrophile

• Attracted to areas of negativity.
• Defined as an electron pair acceptor.

Addition Reaction

• Combines two reactants into one product.
• Example format: A + B → C (one product).

Organic Component: Alkene

• Electrophilic addition involves alkenes (e.g., ethene), which contain a carbon-carbon double bond.
• The double bond is an area of high electron density, attracting electrophiles.

Important Electrophiles

1. Bromine Molecule (Br₂)
   • Reacts through induced dipole formation.
2. Hydrogen Bromide (HBr)
   • Reacts due to natural polarity between hydrogen and bromine.
3. Sulfuric Acid (H₂SO₄)
   • Similar reaction mechanism as HBr.

Example Mechanisms

Bromine (Br₂) with Ethene

• Initial Steps:
  • Induced dipole formation in Br₂ due to proximity to ethene.
• Curly Arrows:
  • From double bond to Br₂, forming a bond and an intermediate.
• Intermediate:
  • Formation of a carbocation and a negative bromine ion.
• Final Product:
  • Dibromoethane (1,2-dibromoethane), with no byproducts.

Hydrogen Bromide (HBr) with Ethene

• Natural Polarity:
  • Between H and Br allows for initial reaction.
• Intermediate and Final Steps:
  • Similar to Br₂, carbocation formation and nucleophilic attack by Br⁻.
• Outcome:
  • HBr adds across double bond, forming bromoethane.

Sulfuric Acid (H₂SO₄) with Ethene

• Molecule Structure:
  • Shows natural polarity aiding the reaction.
• Steps:
  • Similar mechanism to HBr, forming an ethyl hydrogen sulfate.
• Further Reaction Possibility:
  • Can add water to form ethanol, using H₂SO₄ as a catalyst (not required for exam mechanism).

Stability of Carbocations

• Carbocation stability is essential for understanding product formation.
• Primary Carbocation: One alkyl group, less stable.
• Secondary Carbocation: Two alkyl groups, more stable.
• Tertiary Carbocation: Three alkyl groups, most stable.
• Inductive Effect: Alkyl groups donate electrons, stabilizing the carbocation.

Exam Tips

• Recognize and apply the correct mechanism for given reactants.
• Pay attention to carbocation stability and its influence on product formation.
• Clearly represent mechanisms with proper arrow notation.
• For complex reactions like sulfuric acid, focus on the main electrophilic addition without delving into further hydration reactions unless specified.