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

Methyl alcohol (\(CH_3OH\)) is soluble in water in all proportions due to its polar structure and ability to form hydrogen bonds with polar water molecules. On the other hand, stearyl alcohol \([CH_3(CH_2)_{16}OH]\) is insoluble in water because its long nonpolar hydrocarbon chain dominates the molecule's properties, preventing it from forming strong enough interactions with water. This results in stearyl alcohol being a waxy solid that does not dissolve in water.

Step-by-step solution

Introduction to molecular solubility

Molecules dissolve in a solvent when the forces between the solute molecules and the solvent molecules are strong enough to overcome the forces holding the solute molecules together. The rule of thumb for solubility is "like dissolves like." This means that polar molecules generally dissolve in polar solvents, and nonpolar molecules dissolve in nonpolar solvents.

Analyzing the polarity of water

Water (\(H_2O\)) is a polar molecule due to its bent molecular geometry and the difference in electronegativity between oxygen and hydrogen atoms. The oxygen atom is more electronegative, causing the electrons to be more attracted to the oxygen atom rather than the hydrogen atoms, resulting in a polar molecule with a partially positive (δ+) and a partially negative (δ-) charge.

Analyzing the polarity of methyl alcohol

Methyl alcohol (methanol, \(CH_3OH\)), is a polar molecule. The oxygen atom in methanol is more electronegative than carbon and hydrogen atoms, creating a polar bond between them. The polar bond and the bent geometry of the molecule make it overall polar.

Solubility of methyl alcohol in water

Methyl alcohol, being a polar molecule, can form hydrogen bonds with the polar water molecules. These hydrogen bonds are strong enough to overcome the forces holding the methanol molecules together, which allows the methanol molecules to dissolve in the water. This makes methyl alcohol soluble in water in all proportions.

Analyzing the polarity of stearyl alcohol

Stearyl alcohol \([CH_3(CH_2)_{16}OH]\) is a long-chain alcohol. The long-chain part (CH3(CH2)16) of the molecule is nonpolar due to the symmetric distribution of the carbon and hydrogen atoms. The OH group at the end of the molecule forms a polar bond between the oxygen and hydrogen atoms.

Solubility of stearyl alcohol in water

Stearyl alcohol has a small polar OH group and a very long nonpolar chain. The nonpolar hydrocarbon chain dominates the overall molecular properties. Therefore, the very limited interaction between the polar OH group and the water molecules is not sufficient to overcome the forces holding the stearyl alcohol molecules together. This makes stearyl alcohol insoluble in water and forms a waxy solid. In conclusion, the difference in solubility between methyl alcohol and stearyl alcohol in water can be attributed to their molecular structures and polarities. Methyl alcohol, with its smaller and polar structure, can easily form hydrogen bonds with water, making it soluble. On the other hand, the nonpolar and long hydrocarbon chain structure of stearyl alcohol prevents it from forming strong enough interactions with water, making it insoluble and waxy.