Why does the stability of free radicals decrease as we go from left to right across the periodic table?
1 Answer
The stability of free radicals decreases as we go from left to right across the Periodic Table because the central atoms become more electronegative.
It is difficult to measure the energy of a specific free radical. But we can get the trends by looking at bond dissociation energies (BDEs).
H₃C-H → H₃C· + ·H; BDE = 435 kJ/mol
H₂N-H → H₂N· + ·H; BDE = 431 kJ/mol
HO-H → HO· + ·H; BDE = ~498 kJ/mol
F-H → F· + ·H; BDE = ~565 kJ/mol
A low BDE reflects a more stable free radical. A high BDE reflects a less stable free radical.
So the order of stability is
H₃C· ≈ H₂N· > HO· > F·
The central atom in a free radical has only seven valence electrons.
So free radicals are electron-deficient species, as are carbocations.
The same factors that stabilize carbocations also stabilize free radicals.
The stability of cations decreases in the order
H₃C⁺ > H₂N⁺ > HO⁺ > F⁺
The F⁺ ion is least stable (highest energy) because the F atom is strongly electronegative. It is difficult to remove an electron from an F atom.
The pattern repeats itself with free radicals. The most electronegative element has the least stable free radical.
