Define nucleophilic substitution and elimination reactions in the context of halogenoalkanes. What is the key difference between these two types of reactions?

Nucleophilic substitution is a reaction where a nucleophile replaces a leaving group in a halogenoalkane, while elimination involves the removal of a hydrogen atom and a leaving group, resulting in the formation of a double bond. The key difference is that substitution results in a new compound with the same number of atoms, whereas elimination produces an alkene with fewer atoms. Nucleophilic substitution typically occurs under conditions favoring polar solvents, while elimination is favored by higher temperatures and less polar solvents.

This question tests the student's understanding of two fundamental organic reaction mechanisms involving halogenoalkanes. By defining both concepts and identifying their key difference, students demonstrate their grasp of reaction conditions and outcomes, which are crucial for predicting reaction behavior in organic chemistry.