free radicals

Meaning: A radical has an unpaired electron, written with a dot (Cl•, CH₃•). It forms by homolytic cleavage, in which a bond breaks and each atom keeps one electron. Heat or light (hν) acting on a weak bond such as Cl–Cl, Br–Br or O–O in a peroxide is the usual trigger. Alkyl radicals are stabilized by hyperconjugation with neighboring C–H and C–C bonds, so stability runs 3° > 2° > 1° > methyl. Resonance makes allylic and benzylic radicals especially stable. In biology, reactive oxygen species such as superoxide are radicals that damage cell components.

On the MCAT: The classic setup is radical halogenation of an alkane with X₂ and hν. You label the steps. Initiation makes radicals from X₂. Propagation consumes one radical and generates another. Termination joins two radicals, so the radical count falls. You then predict the major product. Br₂ is more selective and gives mostly the most substituted halide. Cl₂ is less selective and gives mixtures. NBS brominates allylic and benzylic positions.

Common confusion: Radicals are not ions, and a radical need not carry a charge. Homolysis gives two radicals, shown with single-barbed (fishhook) arrows, while heterolysis gives ions, shown with full-headed arrows, because both bonding electrons go to one atom.