Key Concepts in Radical Reactions from Organic Chemistry

  • Chapter 10: Radical Reactions

  • Allylic Substitution and Radicals

    • Hydrogen atoms bonded to allylic carbons are allylic hydrogens.
    • Substitution reaction occurs with high temperature and low concentration of halogens (
      X2X_2).

  • Allylic Chlorination

    • At 400 °C, extCl2ext{Cl}_2 gas leads to formation of allyl chloride.
    • Chain Initiating and Propagating steps involve allylic radicals and chlorine species.

  • Allylic Bromination with NBS

    • NBS provides a low, constant concentration of bromine; reaction typically in nonpolar solvent.
    • Chain reactions involve HBr reacting with NBS to generate bromine.

  • Radical Stability

    • Allylic radicals are more stable than secondary (situation 3^{ ext{o}} > 2^{ ext{o}} > 1^{ ext{o}} > methyl > vinyl).
    • Benzylic radicals have even higher stability due to resonance.

  • Benzylic Free Radical Halogenations

    • Process includes reactions with halogens, yielding benzyl bromide and other derivatives.

  • Anti-Markovnikov Addition of HBr

    • Involves extH+ext{H}^+ leading to a 2exto2^{ ext{o}} carbocation intermediate via radical addition; requires peroxides.

  • Chain Polymerization and Process Overview

    • Common polymers include polypropylene, PVC, and Teflon, formed from respective monomers.
    • Ethene polymerization mechanism includes initiation, propagation, and termination, leading to high molecular weight polymers.

  • Head-to-Tail Polymerization of Styrene

    • Preference for this pathway due to stability of benzylic radical intermediate over 1exto1^{ ext{o}} radical.

  • Alternative Alkene Polymerization

    • Cationic and anionic processes shown, with examples including super glue formation and other reactions generating polymers.