Organic Chemistry CH7 Lecture (10/29/25

Coordination with Email and Meetings

  • Suggestion to stop by tomorrow or reach out via email for coordination.

Overview of Alkane Stability and Elimination Reactions

  • Context of Discussion: Continued from previous discussion about alkane stability, moving into elimination reactions.

  • Main Concept: Conducting elimination reactions typically requires a strong base.

Alkoxides: Preparation and Examples

  • Definition of Alkoxide: Conjugate base derived from an alcohol, typically characterized as RO⁻ where R is an alkyl group (the negative charge is on oxygen).

  • Common Alkoxides Discussed:

    • Ethoxide: C₂H₅O⁻ (Sodium ethoxide, Na⁺C₂H₅O⁻)

    • Methoxide: CH₃O⁻ (Sodium methoxide, Na⁺CH₃O⁻)

    • Potassium hydroxide: KOH

  • Making an Alkoxide:

    • Start with a typical alcohol.

    • Use a strong base to deprotonate the alcohol.

    • Notable strong bases include Sodium Hydride (NaH) which deprotonates alcohols by providing H⁻ ions.

  • Mechanism of Alkoxide Formation:

    • Strong base (e.g., NaH) extracts a proton (H⁺) from the alcohol, creating an alkoxide (RO⁻).

    • Reaction produces an alkoxide and a correspondingly charged counter ion (e.g., Na⁺).

Reaction Mechanism Involvement

  • Role of Sodium in Alkoxides:

    • Sodium acts as a counter ion balancing the charge of the alkoxide and aids the formation of the alkoxide.

  • Example of Alkoxide Naming:

    • For a methyl groups (C₁H₃) alkoxide:

    • Example: Sodium methoxide is Na⁺OCH₃ (alkoxide is OCH₃⁻).

    • For ethyl groups (C₁H₂):

    • Example: Sodium ethoxide is Na⁺OCH₂CH₃ (alkoxide is OCH₂CH₃⁻).

Strong Bases and Elimination Reaction Types

  • Reactions mediated by sterically hindered bases (e.g., potassium tert-butoxide) lead to different product outcomes. Sterically hindered bases prevent actions on more substituted internal carbons.

Types of Elimination Reactions

  • E2 (Bimolecular Elimination):

    • Identified by the presence of two reactants during the rate-determining step, specifically the base and the substrate.

  • E1 (Unimolecular Elimination):

    • The rate determined solely by substrate concentration, similar to SN1 reactions.

General Mechanism of E2 Reactions

  • Generic Reaction Setup:

    • Base attacks a beta hydrogen and donates electron density, resulting in the cleavage of C-H and C-X (leaving group).

    • Forming a double bond between carbons while expelling the leaving group.

  • Transition State Dynamics:

    • Discussed electron density shifts, bond formations, and how orientation affects product configurations.

Importance of Anticoplanar Configuration

  • Definition: Hydrogen and leaving group must be anticoplanar to allow for successful elimination.

  • Geometry Importance: Maintaining this orientation ensures proper hybridization transitions necessary for the formation of a double bond (sp³ to sp² hybridization).

Example of E2 Reactions with Specific Substrates

  • Examining 2-bromo-2-methylbutane:

    • Identify alpha (attached to leaving group) and beta (adjacent carbons) hydrogens.

    • Emphasis on pathway analysis for hydrogen abstraction, with multiple products leading to stability comparisons (di-substituted vs. tri-substituted outcomes).

  • Zaitsev's Rule: Small bases favor formation of more substituted alkenes (more stable).

  • Hoffman Product: Larger bases yield less substituted alkenes due to steric hindrance.

Bases and Their Effects on Elimination Reactions

  • Effect of Base on Product Formation:

    • Small, less sterically hindered bases tend to product more stable products.

    • Conversely, more sterically hindered bases produce less stable products due to spatial limitations in accessing beta protons.

Case Study: Substrate Analysis with Steric Considerations

  • Menthol Analogue Comparison:

    • Differentiation of neomenthyl chloride vs menthyl chloride as diastereomers with the same elimination reaction; analysis focuses on product identification based on anticoplanar positioning of substituents.

Conclusion and Future Learning

  • Identification of products from elimination reactions leads to deeper understanding through discussing transitions, base effects, and spatial orientation compliance.

  • Homework: Review structures of alkoxides and practice product predictions for elimination reactions.