The S_N1 reaction, or unimolecular nucleophilic substitution, is a type of chemical reaction where a substrate undergoes substitution by a nucleophile. This process occurs in two main steps: first, the leaving group departs, forming a carbocation intermediate, and then the nucleophile attacks this positively charged species. The rate of the reaction depends only on the concentration of the substrate, hence the term "unimolecular." S_N1 reactions typically occur with tertiary or some secondary alkyl halides, as these substrates can stabilize the carbocation formed during the reaction. Factors such as solvent polarity and the nature of the nucleophile can influence the reaction rate and outcome. Common solvents for S_N1 reactions include polar protic solvents like water or alcohols.