E1 Reaction Mechanism

Jun 27, 2024

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E1 Reaction Mechanism and Rate

Introduction

  • Presenter: Laya from Labor SICOM.
  • Topic: E1 reaction rate and mechanism.
  • E1 Breakdown: Elimination (E1) stands for elimination reaction and
  • Key Note: '1' in E1 indicates a unimolecular reaction.

Elimination and Unimolecular

  • Elimination: Removal of a beta hydrogen and a leaving group.
  • Beta Elimination: Correct term since we remove a beta hydrogen.
  • Unimolecular: Indicates a first-order reaction, not necessarily single-step, can involve multiple steps.

Beta Elimination Reaction

  • Step 1: Leaving group breaks away from the parent chain, creating a positive charge on the parent chain.
  • Alpha and Beta Carbons:
    • Alpha Carbon: Initially held the leaving group and now carries a positive charge.
    • Beta Carbons: Adjacent carbons holding beta hydrogens.

Mechanism of Beta Elimination

  • Final Step: A base grabs a beta hydrogen, forming a pi-bond between the alpha and beta carbon.
  • Product: Formation of a double bond between alpha and beta carbons.

Rate-Determining Step

  • Slow Step: Departure of the leaving group results in a positive charge, making it the rate-determining step.

Kinetics of E1 Reaction

  • Rate Law: Rate = k [substrate]
    • 'k' is a constant.
    • Rate depends only on the substrate concentration.
  • First-Order Reaction: Doubling substrate concentration doubles the reaction rate, base concentration has no effect.

Example: 2-Bromo-2-ethylpentane with H2SO4, Water, and Heat

  • **Analysis Based on Substitution and Elimination checklist: (Detailed videos on website)
    • **Alkyl Chain: **Tertiary alpha carbon can form a stable carbocation (SN1 or E1 likely).
      • Beta carbons have removable hydrogens.
    • **Leaving Group: **Bromine is a good leaving group (supports E1 and SN1).
    • **Attacking Molecule (Base): **Although H2SO4 is an acid, water acts as a weak base for solvolysis.
    • **Solvent: **Polar protic stabilizes charges, supporting E1 and SN1.

Mechanism and Outcome

  • Mechanism Under Heat: Heat favors elimination over substitution.
    • Step 1: Leaving group departs, forming a stable tertiary carbocation (alpha carbon)
  • Step 2: Water grabs a beta hydrogen, forming a double bond between alpha and beta carbon.
    • Resulting molecule: Double bond between alpha and beta carbon.
    • Byproducts: BR- and hydronium (H3O+) maintain charge balance.

Conclusion and Further Resources

  • Products: Analyzed based on Zaitsev's rule (future video).
  • Series and E-books: Information on substitution and elimination series, e-books, and online tutoring available on the website.
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  • E-books: '10 Secrets to Acing Organic Chemistry.'

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Note: Look out for competition between SN1 and E1 reactions. The mechanism described focuses on E1 for this session.

Summary:

E1 Reaction involves elimination, forming a double bond between alpha and beta carbons. The rate depends on substrate concentration and not on the base, being a first-order reaction. Mechanisms likely in presence of tertiary carbons and good leaving groups, with polar protic solvents stabilizing intermediate carbocations. Heat favors E1 over SN1.

Follow-up:

Part two will explore products of an E1 reaction and Zaitsev's rules for alkene stability.

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