Final answer:
In an E2 mechanism, X would be a leaving group like a halide. In radical reactions, X and X' could be any species that combine in a termination step as radicals.
Step-by-step explanation:
The question asks about the identity of radicals X and X' in a mechanism, presumably related to free radical or eliminations reactions in organic chemistry. In the context of an E2 mechanism, X would typically be a leaving group such as a halide (e.g., I-, Br-, Cl-). The E2 mechanism involves the concerted elimination of X and a proton from adjacent carbon atoms to form an alkene. In radical reactions, X and X' could be any species with an unpaired electron that combine in a termination step to give a non-radical product. For example, in homolytic cleavage, a Co3+-CH2R bond could produce a Co2+ species and a CH2R radical (23.6.10).