3.9 Radicals and Biradicals
Some species have an odd number of valence electrons, and so at least one of their
atoms cannot have an octet. Species having electrons with unpaired spins are called
radicals. They are generally highly reactive. One example is the methyl radical,
CH3, which is so reactive that it cannot be stored. It occurs in the flames of burning
hydrocarbon fuels. The single unpaired electron is indicated by the dot on the
C atom in CH3.
Radicals are of crucial importance for the chemical reactions that take place in
the upper atmosphere, where they contribute to the formation and decomposition of
ozone. Radicals also play a role in our daily lives, sometimes a destructive one. They
are responsible for the rancidity of foods and the degradation of plastics in sunlight.
Damage from radicals can be delayed by an additive called an antioxidant, which
reacts rapidly with radicals before the radicals have a chance to do their damage.
Human aging is believed to be partly due to the action of radicals, and antioxidants
such as vitamins C and E may delay the process (see BOX 3.1). The nitric oxide molecule,
NO, has an unpaired electron and plays an important role as a neurotransmitter
and vasodilator. Because it is a radical, NO is very reactive and can be eliminated
within a few seconds. Because it is small, the NO molecule can move easily throughout
the body. These properties allow NO to play several roles, including blood pressure
regulation and helping to fight infection during immune response.
A biradical is a molecule with two unpaired electrons. The unpaired electrons
are usually on different atoms, as depicted in (20). In that biradical, one unpaired
electron is on one carbon atom of the chain and the second is on another carbon
atom several bonds away. In some cases, though, both electrons are on the same
atom. One of the most important examples is the oxygen atom itself. Its electron
configuration is [He]2s22px
22py
12pz
1 and its Lewis symbol is O
$$
# #. The O atom has
two unpaired electrons, and so it can be regarded as a special type of biradical.
However, there is a difference: whereas in a normal biradical the relative orientations