solvolysis

Sometimes you don't need to add a separate reagent at all — you just dissolve the substrate in a reactive solvent and let the solvent do the substituting. Solvolysis is a substitution (or elimination) in which the solvent itself acts as the nucleophile. Dissolve an alkyl halide in water and the water attacks; dissolve it in methanol and the methanol attacks. When water is the solvent the reaction is specifically called hydrolysis; in an alcohol it is alcoholysis.

Because the solvent is present in huge excess (it is the whole sea the substrate swims in), its concentration is effectively constant and does not appear as a variable in the rate law. That makes solvolysis the textbook way to run and study SN1 reactions: the solvent is a weak nucleophile but a great ion-stabilizer, so it lets the substrate ionize to a carbocation at its own slow pace (the rate-determining step), then mops up the cation afterward. The observed kinetics are first order in substrate — exactly the SN1 signature.

Solvolysis is a classic experimental probe of mechanism. By measuring how solvolysis rates change with substrate structure (tertiary >> secondary > primary, reflecting carbocation stability) and with solvent polarity (faster in more ionizing, polar protic solvents), chemists confirmed the SN1 picture and the role of carbocations. A caution: solvolysis usually runs alongside its cousin, E1 elimination — the same carbocation that a solvent molecule attacks can instead lose a proton to form an alkene — so solvolysis products are often a substitution/elimination mixture.