Carbonyldiimidazole a organic compound used primary as a reagent in organic synthesis and for coupling of amino acids for peptide synthesis.  CDI is a white crystalline solid. Mainly used to convert amines into amides, carbamates, ureas. Conversion of alcohols into ester can also be done.

The molecular formula for Carbonyldiimidazole is  (C₃H₃N₂)₂CO. It can be prepared with the reaction of phosgene and four equivalents of imidazole under anhydrous conditions. The results are a ~90% yield after removing solvent and  imidazolium cholride.

Conversion using 4 C₃H₄N₂ + C(O)Cl₂ → (C₃H₃N₂)₂CO + 2 [C₃H₃N₂H₂]Cl the imidazole becomes both the base and the nucleophile. Using another precursor  1-(trimethylsilyl)imidazole needs more effort in preparative getting the advantage that coproduct trimethylsilyl chloride is volatile.

Carbonyldiimidazole hydrolyzes giving back imidazole. You can determine the purity by the amount of CO_{2. (C3H3N2)2CO + H2O → 2 C3H4N2 + CO2}

CDI promotes the amide formation even though it's reactivity is less than acid chlorides but its easier to handle so  to avoid using thionylchloride in the formation of acid chloride which would be able to cause side reactions. One of the early applications of this reaction was seen in the peptide formation bonds using the driving force of formation of CO_{2 .} 

The formation of amide is promoted by CDI. Although the reactivity of CDI is less than acid chlorides, it is more easily handled and avoids the use of thionyl chloride in acid chloride formation, which can cause side reactions.^[3] An early application of this type of reaction was noted in the formation of peptide bonds (with CO₂ formation as a driving force).