recombination

Meaning: Recombination (also called radical coupling) is the combination of two radicals, each donating its one unpaired electron to form a new covalent bond. It is a termination step in a radical chain reaction, such as the light-driven chlorination of methane with Cl₂. Examples: Cl• + Cl• → Cl₂, CH₃• + Cl• → CH₃Cl, and CH₃• + CH₃• → CH₃CH₃. Two radicals are used up and none are made, so the chain stops. Bond formation releases energy, and the step needs little or no activation energy. In biology, "recombination" instead means exchange of DNA between homologous chromosomes (crossing over).

On the MCAT: A passage gives a radical mechanism and asks you to label each step as initiation, propagation or termination. Recombination is termination. Related questions ask why a small amount of a coupled product such as ethane appears when methane is chlorinated, or why radical halogenation gives a mixture of products. A trace of such a coupled product is evidence for a radical mechanism.

Common confusion: Students call any step that involves a radical a termination or a propagation step. Count the radicals instead. Propagation has one radical on each side, so the chain continues. Recombination has two radicals as reactants and none as products. Termination is also rare compared with propagation, because radical concentration is very low and a radical is far more likely to meet an abundant neutral molecule than another radical.