Thirty-six students enrolled in Organic Chemistry II participated in individual, semistructured, think-aloud interviews about the factors that contribute to the stability and reactivity of organic species in the context of unimolecular and bimolecular nucleophilic substitution and elimination reactions. The students were provided with the mechanistic steps for these reactions. Most students correctly identified the leaving groups in these reactions and referred to them as "good leaving groups". However, less than half of the students could explain the electronic and structural factors that justify characterizing a species as a "good" leaving group. Nearly one-third of the students who were interviewed were unable to provide any explanation of what factors result in a chemical species being a "good" leaving group. These findings are discussed through the lenses of both Perry's scheme of intellectual development and Ausubel and Novak's theory of meaningful learning.