1-Hydroxybenzotriazole hydrate (HOBt·H₂O, CAS 123333-53-9) is a white to off-white crystalline powder. It is the monohydrate form of 1-hydroxybenzotriazole, a heterocyclic compound containing a benzotriazole ring system with a hydroxyl substituent. It is a widely used and crucial reagent in organic synthesis, particularly in peptide chemistry. Its primary and most significant application is as an additive or co-reagent in peptide coupling reactions, especially when used with carbodiimide reagents like DCC (dicyclohexylcarbodiimide) or EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide). HOBt acts as a racemization-suppressing agent and significantly accelerates the amide bond formation by forming an active ester intermediate, which is more reactive than the O-acylisourea intermediate formed by the carbodiimide alone. This minimizes side reactions and improves coupling yields. Beyond peptide synthesis, HOBt·H₂O finds use in other condensation reactions for preparing esters, anhydrides, and other amides. It is generally considered safer to handle than its anhydrous counterpart, which can be shock-sensitive and potentially explosive when dry. The hydrate form is the standard and preferred commercial form for laboratory use due to its enhanced stability.