The term free radicals in organic chemistry may simply be defined as the chemical species that carries odd or unpaired electrons on the carbon with only seven valence electrons. Free radicals are electrically neutral chemical species with unpaired electrons, making them highly reactive and often very short-lived. Since the carbon in free radicals has only seven electrons, it is electron deficient and therefore, acts as an electrophile in chemical reactions.
Generation of Free Radicals
Free radicals species are formed by the homolytic cleavage of the covalent bond at high temperature in the gas phase, in non-polar solvents, by ultraviolet light or by the addition of other radicals. Some reactions involving the production of free radicals are given below.
Thermal Cleavage
Heat can break a either a strong and weak covalent bond.
Photochemical Cleavage
Light can break the bond if the wavelength of the light is correspond both to an energy greater than that of the bond to be cleaved, and to an electronic excitation of the molecule concerned.
Redox Cleavage
A covalent bonds may be broken by electron transfer process either by accepting an electron from a donor or donating an electron to an acceptor
Structure of Free Radicals
It has been experimentally found that the free radicals are trigonal planar around the carbon bearing odd electron. The carbon with three sigma bond and an odd electron is in sp2 hybridization with three hybrid orbitals oriented at 120° in a plane perpendicular to pz orbital occupied by the odd electron.
Factors That Stabilize Free Radicals
Free radical is an electron deficient species so it is stabilized by electron donar group (+I effect).
It is also stabilizede by resonance, hyperconjugation, adjacent atoms with lone pairs, electronegativity of the atom decreases, moving down the periodic table (larger size), hybridization from sp3 to sp2 to sp.
Stability of alky free radicals on the basis of Inductive Efeect: As free radicals are electron-deficient species, it is stabilized by Electron Donating Groups. Thus, greater the number of alkyl groups attached, more is the stability.
3° > 2° > 1° > *CH3
Stability of alky free radicals on the basis of hyperconjugation: More the number of α - hydrogens, more will be the stability of the free radical via hyperconjugation
Stability of ally and benzyl free radicals: Charge delocalization also stabilizes electron deficient species. Larger the number of resonating structure, more stable the radical species.
Stabilized by Adjacent Atoms with Lone Pairs: Oxygen, nitrogen, fluorine have lone pairs of electrons that can donate electron density to the half-empty p orbital, which is a stabilizing interaction.
*CH2-O-CH3 > *CH2-CH3
The stability of the α-heteroatom radical (radical on the C adjacent to N, O, or F) is dominated by resonance stabilization from the adjacent lone pair, and this resonance is most effective with the least electronegative heteroatom (N).
*CH2-NH2 > *CH2-OH > *CH2-F > *CH3
Free radical stability decreases with increasing electronegativity: The most electronegative element has the least stable free radical and this is reflected in the higher bond strength.
*CH2-OH > *CH2-F >
Stability of the free radical depends on Hybridization: Free radicals decrease in stability as the % of s-character in the orbital increases (i.e. as the half-empty orbital becomes closer to the nucleus). For that reason, free radical stability decreases as the atom goes from sp3 → sp2 → sp.
*CH2-CH3 > *CH=CH2 > *C≡CH
Reactivity of the Free Radicals
Free radicals are highly reactive because their unpaired electron seeks to pair up—leading to rapid reactions with other molecules.
- They commonly participate in chain reactions (initiation, propagation, termination).
- They can abstract atoms (often hydrogen) from other molecules.
- They readily add to multiple bonds or combine with other radicals to form stable molecules.
Their reactivity tends to be inversely related to their stability: Less stable free radicals are more reactive.