ch 10: free radicals

radical

reactive intermediate with a single unpaired electron

• classified at primary, secondary, tertiary (like others)

  • tertiary most stable & take least energy to break, primary least stable & takes most energy

• radicals are sp² hybridized & trigonal planar

features of radical reactions

• need added energy (light or heat)

• radical initiator (peroxides) contain extra weak bonds that source radicals

  • heating it causes homolysis of the weak OO bond, forming 2 RO radicals

three main steps of a radical reaction

1. initiation: 2 radicals formed by homolysis of a sigma bond

2. propagation: radical reacts with another reactant to form a new sigma bond and another radical

3. termination: two radicals combine to form a stable bond

monohalogenation

when a single H atom on a carbon has been replaced by a halogen

radical chlorination vs bromination

• chlorination forms a 1:1 mixture of products (unselective)

• bromination often yields one major (markovnikov) product

• chlorination is faster

• abstraction of a primary or secondary hydrogen by Cl is exothermic, by Br is endothermic

reactivity of C-H bonds

• primary bonds are strongest and least reactive, take most energy to form

• tertiary bonds are weakest and most reactive, take least energy to form

radical vs ionic addition

• radical involves an initial attack by the radical

• ionic involved an initial attack by a proton (H⁺)

polymers

large molecules made up of repeating units of monomers

polymerization

process of joining together monomers to make polymers

• more substituted radical adds to less substituted end of monomer (head-to-tail polymerization)