Question

Explain why methyl alcohol is soluble in water in all proportions, while stearyl alcohol \(\left[\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{OH}\right]\) is a waxy solid that is not soluble in water.

Short answer

Methanol's high solubility in water is attributed to its polar hydroxyl group that forms strong hydrogen bonds with water molecules. On the other hand, stearyl alcohol's insolubility in water is due to its long nonpolar hydrocarbon chain, which causes strong London dispersion forces that impede its solubility. The hydroxyl group in stearyl alcohol is not enough to overcome the nonpolar chain's influence, resulting in its insolubility in water.

Step-by-step solution

Understanding the chemical structures of the two alcohols

First, let's look at the chemical structures of the two alcohols. Methanol has the chemical formula CH3OH and is the simplest alcohol. Its structure consists of a single carbon atom bonded to three hydrogen atoms and one hydroxyl group (-OH). In contrast, stearyl alcohol has the chemical formula CH3(CH2)16OH. It contains a chain of 18 carbon atoms, with a hydroxyl group at one end and the rest of the carbons connected by single bonds with hydrogen atoms filling the other valence positions around each carbon.

Understanding solubility and intermolecular forces

Solubility is the ability of a substance to dissolve in a solvent. In this case, we want to understand why methanol is soluble in water while stearyl alcohol is not. Factors that impact solubility include the intermolecular forces between solute and solvent, as well as the temperature and pressure of the system. When examining the intermolecular forces, we should consider factors like hydrogen bonding, dipole-dipole interactions, and London dispersion forces (also known as van der Waals forces).

Considering the role of hydrogen bonding and dipole-dipole interactions in solubility

Hydrogen bonding is a strong form of dipole-dipole interaction that occurs between molecules with polar covalent bonds involving hydrogen. Since water is a highly polar molecule with hydrogen bonds, alcohol molecules that are also polar and can form hydrogen bonds with water will generally be more soluble. In the case of methanol, the hydroxyl group (-OH) is highly polar and can form hydrogen bonds with water molecules. These hydrogen bonds outweigh the relatively minimal London dispersion forces from the single carbon in methanol, allowing the molecules to mix easily and completely in water.

Examining the role of the hydrocarbon chain in stearyl alcohol

For stearyl alcohol, the hydroxyl group at one end of the molecule can form hydrogen bonds with water, making it polar like methanol. However, the long (nonpolar) hydrocarbon chain of 17 carbon atoms, CH3(CH2)16-, plays a significant role in the molecule's overall behavior. As the hydrocarbon chain becomes larger, the London dispersion forces between stearyl alcohol molecules become stronger, making the molecule less soluble in water. These nonpolar interactions make it difficult for the polar water molecules to become evenly dispersed and dissolve the stearyl alcohol.

Concluding the explanation

In conclusion, the difference in solubility between methanol and stearyl alcohol in water can be attributed to their respective chemical structures and the resulting intermolecular forces. Methanol, with its single polar hydroxyl group, can form strong hydrogen bonds with water, leading to high solubility in all proportions. In contrast, the long hydrocarbon chain in stearyl alcohol results in strong London dispersion forces that impede its solubility in water. The hydroxyl group alone is not enough to make stearyl alcohol soluble in water, due to the overwhelming influence of the nonpolar hydrocarbon chain.