JEE Main · 2006 · 30 Apr 2006 · Offline · General Organic Chemistry

Free-radical stability grows with both hyperconjugation from neighbouring alkyl groups and resonance delocalisation into aromatic rings, with resonance the more powerful effect. The dimethyl (secondary) radical $(\mathrm{C}{\mathrm{H}}_3{)}_2\dot{\mathrm{C}}\mathrm{H}$ is the least stable; the tertiary radical $(\mathrm{C}{\mathrm{H}}_3{)}_3\dot{\mathrm{C}}$ is a little better through more hyperconjugation; the diphenyl radical $({\mathrm{C}}_6{\mathrm{H}}_5{)}_2\dot{\mathrm{C}}\mathrm{H}$ is stabilised by two rings; and the triphenyl radical $({\mathrm{C}}_6{\mathrm{H}}_5{)}_3\dot{\mathrm{C}}$ is the most stable, delocalised over three rings. Stability therefore increases as $(\mathrm{C}{\mathrm{H}}_3{)}_2\dot{\mathrm{C}}\mathrm{H}<(\mathrm{C}{\mathrm{H}}_3{)}_3\dot{\mathrm{C}}<({\mathrm{C}}_6{\mathrm{H}}_5{)}_2\dot{\mathrm{C}}\mathrm{H}<({\mathrm{C}}_6{\mathrm{H}}_5{)}_3\dot{\mathrm{C}}$.