What type of substitution occurs when an electron-rich group of atoms replaces a positively charged leaving group?

Nucleophilic substitution occurs when a nucleophile, which is an electron-rich species, attacks a substrate containing a positively charged leaving group, resulting in the replacement of that leaving group with the nucleophile. This mechanism is characteristic of reactions where there is a strong affinity of the nucleophile for the electrophilic center in the substrate, typically a carbon atom bonded to the leaving group.

The process involves the nucleophile donating a pair of electrons to form a bond with the carbon atom, displacing the positively charged leaving group in the process. This type of reaction is fundamental in organic chemistry, especially in the synthesis and transformation of various compounds.

In contrast, electrophilic substitution involves electrophiles—which are electron-poor species—attacking nucleophilic centers in a substrate. Radical substitution refers to processes involving free radicals, and hydrolysis typically refers to reactions where water acts as a reactant in the breakdown of compounds, usually rather than substitution. Thus, nucleophilic substitution is the correct classification for the scenario presented.