NHCONH

NHCONH is the shorthand representation of the di-amide fragment NH-CO-NH, a ureylene or carbamoyl linkage. In structural terms, it describes a carbonyl carbon connected to two nitrogen atoms, giving the motif NC(=O)N in SMILES notation. This is the simplest example of a urea-like linkage and underpins a wide range of related compounds.

Structure and properties: The NHCONH fragment is planar due to resonance within the amide bond, giving partial double-bond character to the C–N bonds. Both nitrogens bear hydrogen atoms in many contexts, enabling hydrogen-bonding networks. The motif is highly polar, contributing to solubility in water and many polar organic solvents. Its chemistry is dominated by amide-like behavior, with relatively stable carbonyl chemistry and characteristic hydrogen-bonding capacity.

Occurrence and uses: The NHCONH linkage is central to ureas and biurets, and it forms the repeating unit in polyurea polymers. It also appears in numerous pharmaceutical and agrochemical compounds as a structural motif. In synthetic chemistry, it is commonly formed by the reaction of an amine with an isocyanate, yielding a urea-type product via a carbamoyl transfer: RNH2 + R'–NCO → RNH–CO–NH–R'. The motif serves as a versatile binding and linkage element due to its hydrogen-bonding potential and stable carbonyl.

Reactivity and stability: As a urea-type amide, the NHCONH fragment is relatively resistant to hydrolysis under mild conditions but can be cleaved under strong acidic or basic conditions or by enzymatic pathways in certain contexts. Its stability and hydrogen-bonding properties make it favorable in supramolecular design and polymer chemistry.

See also: Urea, Biuret, Polyurea, Amide, Carbamoyl.