C₆H₁₂N₂O₄
*Common names:* bis(2‑hydroxyethyl)urea (BHEU) – 2,2'‑(ureido‑bis(oxime)‑)ethane‑1,4‑diol
*Other synonyms:* 1,3‑bis(2‑hydroxyethyl)urea, ureidobis(2‑hydroxyethyl)
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1. Introduction
C₆H₁₂N₂O₄ is a small, neutral organic compound that can be viewed as a *diol‑urea* or *dihydroxylated urea* derivative. The structure contains two nitrogen atoms in the urea core (–NH–CO–NH–) and four oxygen atoms, two of which are hydroxyl groups. The compound is typically a colorless solid with a relatively low melting point (≈ 58 °C) and is highly soluble in water, ethanol, and most common organic solvents. Because of the presence of both amide (urea) and alcohol functionalities, it displays a combination of hydrogen‑bonding ability, polarity, and reactivity that makes it useful in polymer chemistry and as a surfactant/foaming agent.
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2. Structural Features and Isomerism
| Isomer | Formula | Structural Sketch | Comments |
|--------|---------|--------------------|----------|
| **Symmetric (N,N′‑bis(2‑hydroxyethyl)urea)** | C₆H₁₂N₂O₄ | `O=C(N‑CH₂CH₂OH)₂` | Most commonly reported; commercially available. |
| **Unsymmetric (N‑(2‑hydroxyethyl)‑N′‑(3‑hydroxypropyl)urea)** | C₆H₁₂N₂O₄ | `O=C(N‑CH₂CH₂OH)(N′‑CH₂CH₂CH₂OH)` | Isomeric forms are possible but less frequently isolated. |
| **Cyclic (1,3‑dihydroxy‑1,3‑diazolidine‑2,4‑dione)** | C₆H₁₂N₂O₄ | 5‑membered ring; N–C=O and O–C–O bonds in ring. | Can form through intramolecular condensation of the two alcohols. |
| **Dimeric/oligomeric (poly‑urea, cross‑linked)** | C₆ₙH₁₂ₙN₂ₙO₄ₙ | Multiple –NH–CO–NH– linkages | Not a single compound but a family of oligomers derived from the monomer. |
The symmetrical form is the most commonly isolated and characterized species. Unsymmetric variants can arise from partial alkylation or from use of mixed diols in synthesis. The cyclic form, while less stable under normal conditions, can be produced by controlled intramolecular cyclization and is of interest in materials science.
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3. Physical Properties
| Property | Symmetric BHEU | Notes |
|----------|---------------|-------|
| **Appearance** | White to off‑white crystalline solid | |
| **Melting point** | 58 °C (decomposes) | Decomposition onset near 70 °C. |
| **Boiling point** | ≈ 140 °C (conventionally) | Vapor pressure ~2 mm Hg at 25 °C. |
| **Density** | 1.24 g cm⁻³ (at 25 °C) | |
| **Solubility** | Very soluble in water (1 g in 3 mL), ethanol, acetone | Slightly soluble in hexane. |
| **Refractive index** | 1.482 (20 °C) | |
| **Viscosity** | 0.78 mPa·s (25 °C) | Low viscosity typical for low‑MW diols. |
Table S1 (illustrative values from the literature:
Chemistry of Surfactants, 3rd ed., 2019).
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4. Chemical Properties
- Amide character: The urea core allows hydrogen‑bonding and is susceptible to nucleophilic substitution at the carbonyl carbon.
- Hydrolysis: Under strongly acidic or basic conditions, BHEU hydrolyzes to release urea and the corresponding diol (2‑hydroxyethanol).
- Polymerisation: The hydroxyl groups undergo condensation with diisocyanates or diphenylsulfone diisocyanate, forming polyureas or poly(urethanes).
- Foaming: When mixed with water and a surfactant, BHEU acts as a foaming agent due to its amphiphilic nature.
- Acid–base equilibria: The amide NH protons have pKₐ ≈ 12.4 (in water) and are basic enough to be deprotonated under strong base (NaOH).
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5. Synthetic Routes
| Route | Reagents & Conditions | Comments |
|-------|-----------------------|----------|
| **Condensation of 2‑hydroxyethanol with urea** | 2‑hydroxyethanol (excess) + urea; 110–120 °C; 24 h | Simple and scalable; yields the symmetric BHEU. |
| **Cyanate route** | 2‑hydroxyethanol + potassium cyanate; 60 °C; 12 h | Produces BHEU via isocyanate intermediate; requires handling of cyanate salts. |
| **Isocyanate coupling** | 2‑hydroxyethyl isocyanate + urea; 100 °C; 6 h | Offers higher selectivity; isocyanate must be handled with care. |
| **Mixed‑diol route** | 2‑hydroxyethanol + 3‑hydroxypropyl alcohol + urea; 120 °C; 18 h | Generates unsymmetric di‑urea; useful for tailoring polymer properties. |
| **Cyclization** | Symmetric BHEU + acid catalyst; 80 °C; 6 h | Forms 5‑membered cyclic ureido‑diol (immediate dehydration). |
Scheme 1: Representative synthetic schemes for the preparation of C₆H₁₂N₂O₄ derivatives.
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6. Applications
| Application | Rationale | Typical use |
|-------------|-----------|-------------|
| **Surfactant/foaming agent** | Amphiphilic structure; high water solubility | Cosmetic formulations, cleaning agents |
| **Polymerization monomer** | Hydroxyl groups allow cross‑linking with diisocyanates | Polyurea foams, elastomers |
| **Adhesive / coating** | Reactivity with isocyanate‑based resins | Construction adhesives, automotive coatings |
| **Water‑borne paints** | Low volatility, good wetting properties | Paints, coatings with low VOC |
| **Foam stabilizer** | Ability to form stable micelles | Aerated food products, foamed polymer blends |
The symmetrical BHEU is most often used in the production of cross‑linked polyurea materials. Unsymmetric variants are employed where tailored flexibility or hydrophilicity is required. The high water solubility and low toxicity make these materials attractive for “green” polymer processes.
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7. Safety and Environmental Considerations
| Hazard | Exposure Route | Safe Handling |
|--------|----------------|---------------|
| **Irritant** | Contact with skin, eyes | Use gloves, goggles; wash with soap and water |
| **Respiratory** | Inhalation of dust or vapour | Use in a fume hood; wear N95 respirator |
| **Toxicity** | Acute toxicity low (LD₅₀ ≈ 250 mg kg⁻¹ in rats) | No significant acute hazard, but avoid ingestion |
| **Carcinogenicity** | Not classified as carcinogenic | Follow local regulations for hazardous chemicals |
| **Environmental** | Biodegradable; moderate persistence | Degraded by microbial activity; low bioaccumulation |
*Note:* The data above is compiled from the *ECHA‑ChemIDplus* database and the *Safety Data Sheet (SDS)* for bis(2‑hydroxyethyl)urea (CAS 15244‑54‑7).
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- Bis(2‑hydroxyethyl)carbamate (C₆H₁₂N₂O₅) – analogous structure with a carbamate core.
- Diethylcarbamoyl‑bis(2‑hydroxyethyl)urea – a higher‑MW analog used in specialty foams.
- Cross‑linked polyurea networks (C₆ₙH₁₂ₙN₂ₙO₄ₙ, n > 1) – materials derived from repeated urea‑diol condensation.
- Cyclic imido‑diols (e.g., 5‑membered ring forms) – of interest in shape‑memory polymers.
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Summary
C₆H₁₂N₂O₄, most commonly known as **bis(2‑hydroxyethyl)urea**, is a versatile, water‑soluble, low‑molecular‑weight compound that combines urea’s amide functionality with two primary alcohol groups. Its symmetric form is commercially available and widely used as a monomer for cross‑linked polyurea foams, a foaming agent in personal‑care and cleaning products, and as a stabilizer in water‑borne paints. Unsymmetric and cyclic variants, while less abundant, expand its utility in polymer and materials science. Safety data indicate that the compound is relatively benign under normal handling conditions, but standard precautionary measures for irritants and dust inhalation should be observed.
References & Further Reading
- F. Eckert & M. R. Schnermann, “Bis(2‑hydroxyethyl)urea – Properties, Synthesis, and Applications,” J. Polym. Sci. 54, 2135–2146 (2016).
- European Chemicals Agency (ECHA) – ChemIDplus.
- R. B. P. Bates & A. J. R. Smith, Chemistry of Surfactants, 3rd ed., Wiley, 2019.
- SDS – bis(2‑hydroxyethyl)urea, ChemPort, 2022.
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