Urea (CO(NH2)2) is a white crystalline compound with a wide range of applications, ranging from agriculture as a nitrogen-rich fertiliser to pharmaceuticals and chemical industries. Although urea is known for its extremely high solubility in water, its solubility in various organic solvents varies greatly.
Urea solubility in organic solvents can affect its use in synthesis reactions, product formulations, and even its removal or recovery processes.
In this article, we will explore the solubility of urea in different organic solvents, the factors that affect this solubility, and its practical significance.
Urea molecules have a unique structure, consisting of a scoring group (C=O) and two corrosive groups (- NH2). These functional groups endow urea with a significant upper portion, enabling it to form a large number of hydrogen bonds. It is precisely this powerful hydrogen bonding ability that enables urea to exhibit excellent solubility in water as a solvent. At present (about 25 ° C), urea can dissolve in about 108 grams per 100 millilitres of water, and the solubility further increases with increasing temperature.
However, when the solvent changes from water to an organic solvent, the solubility of urea undergoes a significant change. This difference mainly depends on factors such as the oxygen, hydrogen bond donor/absorption capacity, and temperature of the organic solvent.
Experiments have shown that urea can gradually dissolve in ethanol, but it requires heating treatment. This is because heating can increase the movement speed of solvent molecules, thereby increasing the chance of collision between urea molecules and ethanol molecules, and promoting their dissolution. Of course, this is also related to factors such as the ratio of urea and ethanol and temperature.
According to experimental measurements, the solubility of urea in methanol is 2.655 mol/L (20 ℃), which is equivalent to a mass concentration of approximately 159.3 g/L (with a molecular weight of 60 g/mol).
Methanol, like ethanol, is a polar organic solvent. The polar OH groups in methanol can form hydrogen bonds with the carbonyl and amino groups of urea, promoting dissolution. This solubility characteristic has been utilised in the application of methanol-based solutions for treating or processing urea. For example, in some industrial cleaning formulations targeting urea residues, methanol can be used as a solvent to effectively dissolve and remove urea.
Glycerol is a polyol with three hydroxyl groups and has a relatively high solubility in urea. 1 gram of urea is soluble in about 1 millilitre of glycerol. Multiple hydroxyl groups in glycerol can form a vast network of hydrogen bonds with urea molecules. This high solubility in glycerol is beneficial in pharmaceutical applications. For example, in some formulations of topical creams or ointments, urea is used for moisturising or dissolving keratin. In contrast, glycerol can be used as a solvent to keep urea in solution and ensure its uniform distribution in the formulation.
The solubility of urea in acetonitrile is relatively low, about 4 g/L. Acetonitrile is a polar non-protonic solvent. Compared with proton solvents such as ethanol or methanol, acetonitrile has lower solubility, which may be due to the lack of strong hydrogen bond donors in acetonitrile. Although the carbonyl group of urea can interact with the polar CN group of acetonitrile, the lack of hydrogen bond donors like – OH in proton solvents limits the solubility of urea.
However, this low solubility may be advantageous in separation techniques. For example, if urea needs to be separated from a mixture of substances, acetonitrile can be used as a solvent to selectively dissolve other components while leaving urea or only dissolving a small amount of urea for further purification.
Urea is almost insoluble in ether and completely insoluble in chloroform. The polarity of ether is relatively low, and the oxygen atoms in ether are spatially hindered, making it difficult for them to form strong interactions with urea molecules. On the other hand, chloroform is a nonpolar solvent. The lack of significant polar groups in chloroform means that it cannot effectively interact with polar urea molecules through hydrogen bonding or other strong intermolecular forces.
This insolubility in ether and chloroform can be utilised during the separation process. For example, suppose urea is mixed with other substances soluble in ether or chloroform. In that case, these solvents can be used to extract different components, leaving urea as a solid residue for easy separation.
This is because these solvent molecules themselves also have solvent groups that can form hydrogen bonds with nitro oxygen atoms and potassium atoms in urea molecules. For example, alcohol solvents such as methanol (CH3 OH) and ethanol (C2H ₅ OH), as well as propylene glycol, dimethyl sulfoxide (DMSO), and others, all form hydrogen bonds with urea to promote its dissolution. However, these hydrogen bonds are usually not as strong and solid as the hydrogen bonds between urea and water.
This is because non-methanol solvents lack methanol groups that can form hydrogen bonds with urea, making it difficult to effectively overcome the strong hydrogen bonding forces between urea molecules and disperse and dissolve them.
Similar to most solids, the solubility of urea in organic solvents typically increases with increasing temperature. Raising the temperature can increase the mobility of molecules, which is beneficial for breaking the solute-solute and solute-solvent interactions and promoting the formation of solute-solute interactions.
The presence of other solutes in the solution also affects the solubility of urea. For example, an increase in ionic strength may reduce the solubility of urea. On the contrary, in some drug formulations, urea can even serve as a “co-solvent” to increase the solubility of poorly soluble drugs.
Although urea is a neutral compound, the pH value of organic solvent-based solutions can affect its solubility, especially in solvents that can act as weak acids or weak bases. For example, in solvents containing acidic or alkaline impurities, the amino group of urea may react with protons or hydroxide ions, respectively. Similarly, in alkaline solutions, urea molecules may undergo reactions that affect their dissolution behaviour.
In organic synthesis, urea may serve as a reactant, catalyst, or part of the reaction medium. For example, in the synthesis of specific polymers or drugs, urea can be used as a reactant. Its solubility in solvents such as ethanol or benzene ensures uniform distribution in the reaction mixture, ensuring effective reaction with other reagents. For example, in the production of urea formaldehyde resin, the solubility of urea in methanol or water is crucial for adequate mixing and reaction.
Urea is used as a humectant and lotion solubiliser in dermatology. Its solubility in specific solvents (such as water/propylene glycol) is the key to preparing adequate external preparations. In addition, urea can also break non-covalent bonds of proteins at high concentrations and is used in the laboratory to increase the solubility of specific proteins.
The solubility of urea in organic solvents is also helpful in separation and purification processes. For example, organic solvents can be used when recovering urea from industrial waste streams or purifying products containing urea. If urea is mixed with other substances, solvents with different solubility characteristics of urea can be used to selectively dissolve or precipitate urea. The insolubility of urea in ether and chloroform can be used to separate it from substances soluble in these solvents. By carefully selecting solvents and controlling conditions such as temperature and pH, effective separation and purification of urea can be achieved.
The solubility of urea in organic solvents is a complex but fascinating research field. It is influenced by factors such as solvent polarity, temperature, the presence of other substances, and pH value.
The solubility range of different organic solvents for urea is broad, ranging from high solubility in polar solvents such as ethanol and glycerol to almost no solubility in nonpolar solvents such as chloroform. This dissolution behaviour has wide applications in various industries, including organic synthesis, product formulation, and separation processes.
By understanding the subtle differences in solubility of urea in organic solvents, chemists and engineers can develop more efficient processes and products that fully utilise this multifunctional compound.
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