A Level Chemistry OCR Practice Exam

Radicals play a crucial role as intermediates in substitution reactions, particularly in mechanisms such as free radical substitution. During such reactions, a molecule undergoes a process where one atom or a group of atoms is replaced by a radical. This typically occurs in several stages: initiation, propagation, and termination. In the initiation phase, radicals are generated, often through the initiation of a reaction by heat or light, leading to the formation of reactive radicals. These radicals then engage in the propagation steps where they react with stable molecules to create new radicals, continuing the chain reaction. Hence, their ability to stabilize and react further underscores their role as intermediates. The ambiguity surrounding the other options is important to note: while some radicals can have catalytic roles in certain specific reactions, they do not generally act as catalysts in substitution reactions. Rather, they directly influence the reaction by participating in bond-making and bond-breaking processes. Radicals do not simply serve as end products; they are transient species during the reaction. Understanding these dynamics is key to comprehending substitution reactions, especially in organic chemistry, where the nature of the intermediates determines the reaction pathway and its overall outcome.