Question

What is the primary free radical?

Answer 1

Hint: The free radicals contain a single occupied orbital which is highly reactive and unstable. A free radical is formed when odd unpaired electrons form on the atom during a bond break.

Complete Step by Step Solution:
1. Free radicals: The homolytic cleavage of a bond into a single electron instead of an electron pair results in a neutral species (atom or group) containing an unpaired electron, and these species are called free radicals.
2. A homolytic cleavage in the presence of heat or light is shown below:
\[R - X \to \mathop R\limits^ \bullet + \mathop X\limits^ \bullet \]
3. There are basically three types of free radicals: primary, secondary, and tertiary free radicals.
4. Primary free radical: The carbon is attached to a single alkyl group and there is a formation of only one sigma bond. The formation of primary free radical is presented below:
\[C{H_3} - \mathop {C{H_2}}\limits^ \bullet \]
5. Primary free radicals are less stable than other free radicals because primary radicals have the least number of alkyl groups that contain the unpaired electrons, which results in a smaller amount of delocalization of unpaired electrons.
6. The radical stability is also least as compared to the other two radicals due to the donation of electron density by neighbours being less.
7. The requirement of bond dissociation energy is more for the primary free radicals which means high energy is required to break the bond between a Hydrogen atom and Methyl group.

Additional information:
The free radical mechanism proceeds through initiation, propagation, and termination steps. For example, the depletion of the ozone layer:
1. Initiation step: Formation of free radicals from Freons by ultraviolet radiation: \[C{F_2}C{l_2} \to \mathop {Cl}\limits^ \bullet + \mathop {C{F_2}Cl}\limits^ \bullet \]
2. Propagation step: The Chlorine-free radical attach to the ozone and forms another free radical: \[\mathop {Cl}\limits^ \bullet + {O_3} \to \mathop {OCl}\limits^ \bullet + {O_2}\]
3. Termination: The two free radicals form a molecule: \[\mathop {Cl}\limits^ \bullet + \mathop {Cl}\limits^ \bullet \to C{l_2}\].

Note: Free radicals are less stable as their s-character increases in the orbital, for that reason, the stability decreases from alkyl to alkenyl to alkynyl atom.