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In ester substitution, a nucleophile attacks the carbonyl carbon, forming a tetrahedral intermediate; its collapse expels an alkoxide leaving group. Esters are less reactive than acid chlorides and anhydrides, with reactivity approximately comparable to carboxylic acids. Their reactions include acid- or base-mediated ester exchange and hydrolysis, reduction to alcohols or, under suitable conditions, aldehydes, and addition of organometallic reagents. Esters with α-hydrogens can also participate in Claisen condensation to form β-keto esters. The lesson connects these transformations to reaction conditions, equilibrium or irreversibility, and key structural and spectroscopic features, including lactone formation and characteristic carbonyl signals.