Why is ethanol a good solvent for the recrystallization of biphenyl, but acetone and water are not good choices?
The Correct Answer and Explanation is:
Correct Answer: Ethanol is a good solvent for the recrystallization of biphenyl because biphenyl is moderately soluble in hot ethanol but poorly soluble in cold ethanol, allowing effective recrystallization. In contrast, acetone dissolves biphenyl too readily at all temperatures, while water does not dissolve biphenyl at all.
Explanation
Recrystallization is a purification technique that relies on the differential solubility of a compound in a solvent at different temperatures. The ideal solvent for recrystallization should dissolve a significant amount of the solute when hot but only a minimal amount when cold. This allows the compound to be dissolved at high temperatures and then form pure crystals upon slow cooling.
Ethanol fits these criteria well for biphenyl, a nonpolar aromatic hydrocarbon. Biphenyl has moderate solubility in hot ethanol and very low solubility in cold ethanol. Ethanol, being a polar protic solvent, can interact with biphenyl’s aromatic structure just enough to dissolve it when heated. As the solution cools, biphenyl precipitates out of solution because its solubility decreases significantly with temperature. This makes ethanol a suitable solvent for the purification process.
On the other hand, acetone is a poor choice because it is a polar aprotic solvent that dissolves biphenyl very well at both high and low temperatures. This high solubility at all temperatures prevents biphenyl from recrystallizing properly during cooling, leading to poor recovery or impure crystals.
Water is also an unsuitable solvent for recrystallizing biphenyl due to its high polarity and hydrogen bonding capacity, which makes it very poor at dissolving nonpolar compounds like biphenyl, even at elevated temperatures. Since biphenyl does not dissolve appreciably in hot water, it cannot be recrystallized from it.
In summary, ethanol provides the right balance of solubility and temperature dependence, making it ideal for the recrystallization of biphenyl, unlike acetone and water.
