Final answer:
To rank free radicals by stability, one must consider the degree of substitution of the carbon atom with the unpaired electron. The stability increases from primary radicals, which are the least stable, to tertiary radicals, which are the most stable. This increase in stability is due to inductive effects and resonance stabilization from alkyl groups.
Step-by-step explanation:
The question is asking to rank free radicals in order of decreasing stability. Free radicals are species with an odd number of electrons, which makes them highly reactive. The stability of free radicals generally increases in the following order: primary (RCH₂·) < secondary (R₂CH·) < tertiary (R₃C·). That is, the more substituted the carbon atom holding the unpaired electron, the more stable the free radical will be, due to factors such a inductive effects and resonance stabilization in the case of alkyl groups.
Based on these principles, the stability ranking for a series of hypothetical free radicals would be: tertiary free radical > secondary free radical > primary free radical. Each additional alkyl group donates electron density through an inductive effect and delocalizes the charge, contributing to the stability of the free radical.