Chemistry Mechanisms and Reactions Study Notes

Review and Practice Sessions Overview

  • Review Days Scheduled for Practice

    • On March 16, focus on understanding reagents, mechanisms, and substrate applications to produce different products.

    • Key Goal: Determine the major product emerging from given reactants.

Mechanistic Considerations

  • Substrate Types and Mechanisms:

    • Mechanisms Based on Substrate Type:

      • Primary Substrates: Favor SN2 over SN1.

      • Secondary Substrates: Both SN2 and SN1 possible, but reactivity differs.

Reagents Discussion

  • Reagent Types and Effects on Mechanisms:

    • SH- (Strong Nucleophile):

      • Mechanism: SN2 preferred due to primary substrate.

      • Result: Replacement of bromine with SH group, e.g. R-SH.

    • Potassium tert-Butoxide (tBuOK):

      • Strong nucleophile and base; bulky structure.

      • Mechanism: SN2 minor, E2 major for primary substrates.

      • Explanation: Bulky base hinders nucleophilic attack = favors elimination (E2).

    • DBN (Strong Base Only):

      • Mechanism: E2 only due to primary substrate focus.

    • NaOMe (Sodium Methoxide):

      • Characteristics: Small base promotes SN2 as major, E2 as minor for primary substrates.

      • Product: R-O-Me with inversion at the carbon.

    • Reagents with Secondary Substrates:

      • Nucleophiles (e.g., Iodide):

        • Functions as only a nucleophile yielding both SN2 and potential SN1 mechanisms.

        • Polar aprotic solvent (DMSO) increases SN2 rate significantly.

      • Weak Nucleophiles (e.g., Ethanol):

        • Function: Weak nucleophile and base.

        • SN1 mechanism favored under normal conditions; E1 reaction more prominent under heating.

Elimination Reactions and Product Predictions

  • E2 Mechanism Preference:

    • Zaitsev vs. Hoffman Products:

      • Zaitsev is favored by smaller bases; leads to more stable alkenes.

      • Hoffman products result from bulky bases.

      • Followed by:

        • Alkene Varieties: Examples include pent-1-ene, trans-pent-2-ene, and cis-pent-2-ene.

  • Regio- and Stereochemistry Overview:

    • SN2 Reactions:

      • No rearrangements; backside attack leads to inversion of configuration; stereochemistry flips on reactants.

    • SN1 Reactions:

      • Attack occurs on carbocations (rearrangements possible).

      • Product mixture: retention and inversion possible due to carbocation stability.

    • E2 Mechanisms:

      • Stereochemistry: Selectivity influenced by beta-hydrogen availability; competing pathways influence stereochemical outcome.

      • Zaitsev elimination leads to the more stable alkene configurations with trans and cis isomers respecting sterics.

      • E1 pathways favor the more stable products without influence from the base size.

Final Notes and Practices

  • Review overall structures, mechanisms, and product distributions from substrate interactions and reagent properties.

  • Work on various substrate types and variations to deepen understanding of reaction outcomes.

  • Upcoming class will provide a key to practice problems to ensure understanding of materials, focusing on both mechanistic approaches and product yield assessments.