Key Concepts in Free Radical Chemistry

  • Radicals Formation:

    • Formed by homolysis, a reaction where atoms in a covalent bond each take an electron from the bond to have unpaired electrons.
    • Energy Requirement: Breaking bonds requires energy (heat or light) for homolysis.

  • Alkyl Hydrogen Atom Abstraction:

    • Important step in radical reactions.
    • Example: ( X + H-R \rightarrow X-H + R )
    • Each contributes an electron to form a new bond, generating a new radical.

  • Addition to Pi Bond:

    • A chain propagation step in free radical polymerization.
    • Example: ( R + \text{Ethene} \rightarrow \text{CH}2(R)\text{CH}2 )
    • The radical causes the double bond to split, forming a new radical.

  • Homolytic Bond Dissociation Energies:

    • Bond Formation: Exothermic (( \Delta H < 0)).
    • Bond Breaking: Endothermic (( \Delta H > 0)).

  • Alkyl Radical Stability:

    • Stability trend: Tertiary > Secondary > Primary > Methyl.
    • Radicals are reactive but generally unstable.

  • Free Radical Halogenation of Alkanes:

    • Reaction: ( R-H + X_2 \rightarrow R-X + H-X \quad ( ext{heat/light}) )
    • Diatomic halogen bond breaks, creating radicals and forming products.
    • Various products can form depending on molar equivalents used.

  • Non-Selectivity of Free Radical Chlorination:

    • Chlorine is less selective leading to a mix of products unless only one type of hydrogen atom is present.
    • Example: ( CH3CH2-H + Cl \rightarrow CH3CH2^{radical} + HCl )
    • The next step forms products with the remaining radicals continuing to react.

  • Selectivity of Free Radical Halogenation:

    • Bromine is less reactive and more selective than chlorine.
    • Selectivity order: ( Br2 > Cl2 > F_2 ).

  • Chirality Centers in Free Radical Halogenation:

    • Starting from an achiral molecule can yield enantiomers or achiral products.
    • Starting from a chiral molecule results in diastereomers.