Free Radical Addition and Related Reactions

Free Radical Addition to HTR

  • Introduction to Free Radical Reactions

    • Focus primarily on the alkene when dealing with complex molecules; do not react to the arrow group.
    • The reactivity centers around the alkene, despite other functional groups being present.

  • Hydrogenation

    • Introduction of H2 to alkenes occurs without heat.
    • Understanding hydrogenation lays the groundwork for further reactions.

  • Oxymercuration and Hydroboration

    • Distinction between these reactions is important:
    • Oxymercuration follows no rearrangement.
    • Hydroboration leads to anti-Markovnikov addition.
    • Knowledge of regio- (the area where atoms or groups attach) and stereochemistry (the spatial arrangement of atoms) is crucial.

  • Alkene Reagents

    • Exam preparation involves identifying starting materials with alkene groups to determine correct reagents.
    • Stereochemistry clues are vital during identification:
    • Identify whether groups add on the same or opposite sides.
    • Determine the product configurations based on the alkene's substitution pattern.

  • Mechanics of Reactions

    • Movement and stabilization of electron pairs:
    • Understand that carbocation formation dictates reactivity pathways.
    • Stability favors formation of more substituted carbocations.

  • Reactions Overview

    • Oxymercuration displays:
    • OH adding to more substituted side (no rearrangement).
    • Hydroboration leads to:
    • OH adding to less substituted side due to anti-Markovnikov addition.
    • Summary of product formation emphasizing stereochemistry inclusivity without the need for summary of all stereoisomers.

  • Free Radical Mechanism

    • Initiation: Formation of free radicals from weak bonds under light or heat.
    • Propagation: Free radicals destabilizing other molecules, creating more radicals; a chain reaction.
    • Termination: When two free radicals combine to form a stable compound.

  • Identifying Products

    • The presence of hydrogen and bromine in products dictates the path of addition:
    • Markovnikov addition versus anti-Markovnikov addition can shift depending on conditions (heat and light).
    • Use hydrogen peroxide, which influences pathways significantly based on whether it promotes radical or addition mechanisms.

  • Conclusion

    • Review and practice mechanisms are essential for mastering reaction pathways.
    • Emphasize the importance of understanding the nuances between different radical reactions, and how they compare with standard ionic addition pathways.
    • Draw out complete mechanisms for a better grasp of all steps involved.