Why is glucose or sucrose soluble in water, but cyclohexane soluble in water?

In daily life and chemical experiments, we often encounter differences in the solubility of different substances in water. For example, glucose or sucrose is readily soluble in water, whereas cyclohexane is insoluble in water. Why is glucose or sucrose soluble in water, but cyclohexane soluble in water? The principle involves molecular structure, polarity and solubility. We will analyze this problem in detail from several angles.

1. Glucose and sucrose polar structure

Glucose and sucrose are common carbohydrates, and their molecular structures have obvious polarity. Glucose and sucrose molecules contain multiple hydroxyl groups (-OH), which can form hydrogen bonds, which are a strong intermolecular interaction force. Water molecules also have a strong polarity, and their molecules contain polar bonds between oxygen atoms and hydrogen atoms. When glucose or sucrose is dissolved in water, hydrogen bonding forces between water molecules and sugar molecules enable the sugar molecules to be dispersed and dissolved in water. This is why glucose or sucrose is easily soluble in water.

2. Cyclohexane non-polar structure

Unlike glucose and sucrose, cyclohexane is a non-polar compound. The cyclohexane molecule consists of six carbon atoms and twelve hydrogen atoms, forming a closed ring structure. Since there is no significant difference in the electronegativity of the carbon-hydrogen bond in the cyclohexane molecule, the whole molecule has no obvious polarity. This makes cyclohexane unable to attract water molecules to each other through hydrogen bonding. Therefore, cyclohexane is not easily dissolved in water.

3. Solubility principle: similar solubility

There is a classic rule in chemistry: "similar and soluble". That is, a polar substance is easily dissolved in a polar solvent, whereas a non-polar substance is easily dissolved in a non-polar solvent. Water is a polar solvent, which is suitable for dissolving polar substances, while cyclohexane is a non-polar substance and cannot form effective molecular interactions with water, resulting in its difficulty in dissolving in water. Cyclohexane is usually dissolved with a non-polar solvent such as hexane or benzene and the like, rather than water.

4. Sugar and water interaction

The dissolution of sugars (such as glucose and sucrose) in water is not only due to the formation of hydrogen bonds, but also closely related to their molecular size and solubility. The sugar molecules are small, and the water molecules can form a stable solvation layer around the sugar molecules, allowing the sugar molecules to be dispersed in the water. The cyclohexane molecule is large and non-polar, and it is difficult for water molecules to form an effective solvation layer around it, which affects its solubility.

5. Temperature effect on solubility

Solubility is often affected by temperature. In water, when the temperature rises, the movement of water molecules is intensified, and sometimes the solubility of polar substances can be improved. The non-polar nature of cyclohexane dictates that its solubility remains very low even at elevated temperatures. In contrast, the solubility of glucose and sucrose generally increases with increasing temperature.

Summary

Glucose or sucrose is soluble in water, but the reason why cyclohexane is soluble in water is mainly related to the polar difference of its molecular structure. The polar hydroxyl groups in glucose and sucrose molecules allow them to form hydrogen bonds with water molecules and thus dissolve easily in water. Cyclohexane, on the other hand, is a non-polar molecule that cannot interact with water molecules through hydrogen bonds and is therefore difficult to dissolve in water. Understanding these dissolution principles will help us to better understand the differences in solubility of different substances in water and their applications.