Biuret
Biuret is an organic chemical compound with the molecular formula C₂H₅N₃O₂, systematically named as imidodicarbonic diamide or more commonly as carbamoylurea, formed by the condensation of two urea molecules with the loss of ammonia.[1] It appears as a white, crystalline solid that is slightly soluble in water (2 g/100 mL at 25 °C) and more soluble in hot water, and decomposes upon heating at 185–190°C without boiling.[1][2] Chemically stable but hygroscopic, biuret exhibits a pKa of about 10.17 and is incompatible with strong oxidizing agents or bases.[1] The compound is best known for its role in the biuret test, a colorimetric assay used to detect and quantify proteins and peptides in solutions by forming a violet-colored complex with copper(II) ions in alkaline conditions, due to its structural similarity to peptide bonds.[3][4] In this test, the intensity of the purple color, measured at 540 nm absorbance, correlates with peptide bond concentration, making it suitable for analyzing serum proteins (typically 1–10 g/dL) or solubilized cell fractions, though it requires at least 1 mg/mL for detection and can be interfered with by substances like hemoproteins.[3][4] Beyond analytical applications, biuret serves as a safer alternative to urea in ruminant feed additives, providing a slow-release nitrogen source to enhance protein synthesis without the toxicity risks of high urea levels (biuret content should be limited to under 2% in fertilizers to avoid plant damage).[1] It is also utilized as a pharmaceutical intermediate, growth hormone regulator, foaming agent in plastics, and component in paints and adhesives.[1]Chemical Characteristics
Molecular Structure and Formula
Biuret has the molecular formula C₂H₅N₃O₂ and can also be represented as HN(CONH₂)₂, which arises from the condensation of two urea molecules with the elimination of ammonia.[5][5] The molecule features a linear chain consisting of two ureido groups (-NHCONH₂) connected via a central carbonyl linkage, forming a symmetric structure with two amide functional groups and a urea-like -CO-NH-CO-NH- moiety.[5] In its trans configuration, the molecule is nearly planar, with the largest torsion angle measuring only 3°. Key bond distances include C=O lengths of 1.23–1.24 Å, imide C–N bonds at 1.38 Å, and amide C–N bonds at 1.32–1.33 Å; notable bond angles are N–C–N at 114–119°, C–N–C at 128°, amide N–C–O at 123–124°, and imide N–C–O at 118–122°.[6] The IUPAC name for biuret is carbamoylurea, and its molecular weight is 103.09 g/mol.[5][6] In the solid state, biuret crystallizes in the monoclinic system with space group C2/c (No. 15) and Z = 8 molecules per unit cell; the lattice parameters are a = 15.4135(8) Å, b = 6.6042(3) Å, c = 9.3055(4) Å, and β = 91.463(3)°.[6] X-ray diffraction studies reveal a hydrogen bonding network featuring one intramolecular O⋯H–N interaction at 1.92 Å and two intermolecular hydrogen bonds, which collectively form skew ribbon motifs stabilizing the crystal packing.[6]Physical Properties
Biuret is a white, hygroscopic crystalline solid, often appearing as an odorless powder, pellets, or large crystals; it forms elongated plates when crystallized from ethanol and needles from aqueous solutions.[5] The compound has a melting point of 190 °C, at which it begins to decompose.[5] Its density is 1.467 g/cm³, measured at -5 °C.[5] Biuret exhibits moderate solubility in water, increasing with temperature: 2.01 g/100 g at 25 °C, 7 g/100 g at 50 °C, 20 g/100 g at 75 °C, and up to 53.5 g/100 g at 105.5 °C.[5] It is freely soluble in hot alcohol but only slightly soluble in ether.[5] Under normal conditions, biuret is chemically stable but hygroscopic, readily absorbing moisture from the air.[5] At elevated temperatures above approximately 193 °C, it undergoes thermal decomposition, releasing ammonia and other gaseous products such as isocyanic acid.[7]Chemical Properties and Reactivity
Biuret exhibits notable reactivity in forming coordination complexes with metal ions, particularly producing a violet-colored complex with Cu²⁺ ions in alkaline conditions, which is the basis for its use in qualitative analysis.[5] This complexation arises from the deprotonated nitrogen atoms coordinating to the copper center. Additionally, biuret undergoes hydrolysis under acidic or basic conditions, breaking down to urea and ammonia, a process that can be accelerated by heating under pressure.[8] The compound demonstrates thermal instability, decomposing above 193 °C and yielding melamine upon pyrolysis, which limits its handling at high temperatures.[5] During urea production, biuret forms as an unwanted byproduct due to sensitivity to prolonged heating, often requiring process controls to minimize its concentration.[9] Biuret displays low acute toxicity, with an oral LD50 exceeding 14,000 mg/kg in rats, indicating minimal risk at typical exposure levels.[10] However, biuret can be phytotoxic at concentrations exceeding approximately 1.5-2% in fertilizers, potentially causing damage to foliage and impaired growth in sensitive plants and seedlings, especially when applied near germinating seeds.[11] It is safe for ruminant digestion owing to slow microbial hydrolysis in the rumen, reducing ammonia toxicity risks compared to urea, but can induce toxicity in non-ruminants if overdosed due to limited degradation capacity.[12] The pKa for deprotonation of its imino groups is approximately 10.2, reflecting weak acidity.[1] Spectroscopically, biuret is identified by infrared absorption peaks at around 1700 cm⁻¹ corresponding to the C=O stretch and 3300 cm⁻¹ for the N-H stretch, characteristic of its urea-like functional groups.[13] These features aid in confirming its presence in mixtures.Production
Laboratory Synthesis
The classic laboratory synthesis of biuret is achieved by heating urea to 150–160 °C for 30–60 minutes, resulting in the condensation reaction where two molecules of urea combine to form biuret and ammonia:$2 \ \ce{NH2CONH2} \rightarrow \ce{HN(CONH2)2} + \ce{NH3}
This process requires an inert atmosphere or reduced pressure (typically 50–75 mm Hg) to remove the evolved ammonia gas and limit the formation of side products such as cyanuric acid.[14] Following the reaction, the crude product is purified by dissolving it in hot water (or a dilute alkaline solution at 50–70 °C), filtering to remove insoluble impurities, and cooling the filtrate to induce crystallization of biuret as white needles or crystals, which are then filtered, washed with ice-cold water, and dried at around 110 °C.[15][14] An alternative laboratory route involves the reaction of ammonia with phosgene (COCl₂) or urea with cyanic acid (HNCO) to yield biuret, though the phosgene-based method is less commonly employed due to the toxicity of phosgene and is typically reserved for specialized syntheses.[16] Under optimized conditions, this synthesis provides conversion yields of around 40% biuret (with up to 85% recovery of pure product after purification), with the primary byproduct being ammonia.[15] Due to the release of ammonia gas during the heating step, the procedure must be conducted in a well-ventilated fume hood to ensure safety.[14]