Introduction to radicals

•CH₂CH₃

Radicals are species that have unpaired electrons.
The radical at the right has an unpaired electron on carbon.

Radicals have no charge at the atom bearing the unpaired electron. The carbon bearing the unpaired electron in the radical example above owns the unpaired electron and three bonding electrons. Thus it has the same number (4) as a nonbonded carbon atom.

Radicals are formed when bonds are cleaved homolytically, that is, one atom in each of the products has one electron of the bond cleaved.

This type of reaction in a radical chain mechanism is referred to as the initiation step (radicals ONLY in products).

Note that these are transfers of one electron giving rise to intermediates with unpaired electrons.
Thus a fish hook is used instead of the double barb arrow (

) used to show movement of electron pairs.
Each fish hook shows one electron moving to the atom at the head of the arrow, leaving one electron at the tail of the arrow.

Radicals may also be the product of reaction of a radical with a molecule. These reactions involve both bond-breaking and bond-making.

In the example below, an H-Cl bond is made and an H-C bond is broken.
The hydrogen atom becomes bonded to the chlorine atom, leaving one electron behind on the carbon.

This type of reaction in a radical chain mechanism is referred to as a propagation (radical reactant AND radical product).
Steps in which two radicals react to give a molecule are referred to as termination (radical reactant ONLY).