Lecture on Haloalkane - Chapter 7.2: Chemical Properties

Overview

• Subtopic: 7.2 Chemical Properties
• Focus: Part 1 of the video on nucleophilic substitution reactions

General Formula

• Haloalkane: ( R - X )
  • ( R ): Alkyl group
  • ( X ): Halogen

Polar Bonds & Nucleophilic Attack

• Polar Bond: Carbon-halogen bond is polar due to higher electronegativity of halogens.
  • Electrons are drawn towards halogen, making carbon partially positive.
  • Carbon is susceptible to nucleophilic attack (electrophilic site).

Nucleophilic Substitution Mechanisms

• Types:
  1. SN1 (Unimolecular Nucleophilic Substitution)
     • First-order reaction.
     • Depends on concentration of haloalkane.
     • Involves a carbocation intermediate.
     • Typically occurs with tertiary haloalkanes.
     • Usually involves weak nucleophiles (e.g., water, alcohol).
  2. SN2 (Bimolecular Nucleophilic Substitution)
     • Second-order reaction.
     • Depends on both haloalkane and nucleophile concentration.
     • Involves a one-step mechanism with an activated complex.
     • Typically occurs with primary haloalkanes.
     • Involves strong nucleophiles (e.g., hydroxide, methoxide).

Mechanism Details

• SN1 Mechanism:

  • Two-step process.
  • Formation of a stable carbocation.
  • Nucleophile attacks the carbocation.

• SN2 Mechanism:

  • One-step process.
  • Nucleophile attacks from the backside, causing inversion of configuration.

Classifications & Reactivity

• Haloalkanes:
  • Classified as primary, secondary, or tertiary.
  • Primary Haloalkanes: Undergo SN2 due to low steric hindrance.
  • Secondary Haloalkanes: Can undergo SN1 or SN2 based on nucleophile strength.
  • Tertiary Haloalkanes: Generally undergo SN1.

Factors Affecting Reactivity

• Bond Strength:

  • ( R - I ) > ( R - Br ) > ( R - Cl ) > ( R - F )
  • Iodoalkanes have the weakest bond, thus most reactive.

• Steric Effects:

  • Larger groups hinder nucleophilic attack, favoring SN1 over SN2.

Examples

• SN1 Example:

  • Haloalkane reacts with water (weak nucleophile).
  • Two-step mechanism: Loss of halogen, nucleophilic attack.

• SN2 Example:

  • Haloalkane reacts with hydroxide (strong nucleophile).
  • Involves a transition state and inversion of configuration.

Additional Considerations

• Steric Hindrance:
  • Affects mechanism choice, especially for primary haloalkanes with bulky groups.
• Rearrangements:
  • Possible in SN1 reactions due to carbocation formation.

This concludes the part on nucleophilic substitution reactions for haloalkanes, with focus on SN1 and SN2 mechanisms, their conditions, and reactivity. Future parts will explore elimination reactions and synthesis of Grignard reagents.