Orgo Chen lecture 1-29

Reaction Coordinate Diagram

  • Purpose: Used to illustrate the energy changes during a chemical reaction.

Energy Axis

  • Bottom Axis: Represents low energy (starting materials).

  • Top Axis: Represents high energy (products and intermediates).

General Overview

  • Starting Materials: Typically have low energy, e.g., stable benzene.

  • Products: Slightly altered energy due to aromaticity reformation.

  • Intermediate: Often a carbocation, known to have high energy, thus requiring energy input.

Energy Barriers

  • Energy Barrier: The amount of energy required to reach the transition state.

  • Carbocation Intermediate: High in energy despite possible resonance stabilization.

  • Uphill vs. Downhill:

    • First energy barrier: High due to conversion from starting materials to carbocation (uphill).

    • Second energy barrier: Lower energy transition to products (downhill).

Determining Reaction Rate

  • The rate of the reaction is determined by the height of the energy barriers.

  • Higher Barrier#1: Determines the rate of the reaction due to greater energy requirement.

Electrophilic Aromatic Substitution (EAS)

  • Learning Objective: Understand the mechanism of the Friedel-Crafts alkylation.

Starting Materials and Mechanism

  • Starting Material: Benzene, alkyl halide as electrophile (weak).

  • Electrophile Activation: Requires generation of a strong electrophile using a strong Lewis acid (AlCl3).

Mechanism Overview

  1. Form Strong Electrophile: Alkyl halide reacts with AlCl3 to form a carbocation.

  2. Nucleophilic Attack: Benzene ring acts as nucleophile attacking carbocation.

  3. Deprotonation: Loss of H+ to regain aromatic stability.

Generating Strong Electrophiles

  • Common Methods:

    • From alkyl halides.

    • By using alkenes with strong acids (e.g., sulfuric acid).

    • From alcohols through protonation followed by loss of water.

Carbocation Stability

  • Primary Carbocation: Generally unstable, can lead to reaction no-go due to inability to rearrange.

  • Tertiary Carbocation: Most stable due to existing pathways for rearrangement.

Resonance Stabilization

  • Allylic and Benzylic Carbocations: Unique stabilization due to hybridization and resonance structures.

    • Allyl group allows primary carbocation formation.

    • Benzylic carbocations can also stabilize due to resonance.

Learning Objectives on Carbocation Rearrangement

  • Recognize and apply knowledge of carbocation rearrangements through hydrogen or methyl shifts to form more stable carbocations.

Reaction Conditions

  • Acylation Reactions: Use of acyl chloride

  • Formation of Acylium Ion: Strong electrophilic species formed through interaction with Lewis acids (e.g. AlCl3).

Summary of Reactions

  • Electrophiles can be generated from various starting materials (alkyl halides, alkenes, alcohols) through established mechanisms.

  • Stability and rearrangement of carbocations are critical in determining the outcome of EAS reactions, impacting the reaction pathways and products.