Crude triphenylmethanol was recrystallized using a 2:1 mixture of hexane:ethanol. Select the most likely reason why this recrystallization solution was chosen over pure ethanol.
a. Formation of triphenylmethanol crystals in the presence of pure ethanol is rapid, resulting in lower purity.
b. Triphenylmethanol is very soluble in pure ethanol and its recovery during recrystallization would be low.
c. Hexane stabilizes the phenyl groups of triphenylmethanol, protecting it from decomposition in the presence of heat.
d. Triphenylmethanol is relatively insoluble in pure ethanol, and not enough of it would dissolve for a recrystallization.
The Correct Answer and Explanation is :
The correct answer is:
b. Triphenylmethanol is very soluble in pure ethanol and its recovery during recrystallization would be low.
Explanation:
Recrystallization is a method used to purify compounds, where a solid compound is dissolved in a solvent at high temperature, and then allowed to crystallize out as the solution cools. The choice of solvent is crucial to the success of recrystallization. Ideally, the solvent should dissolve a large amount of the compound at high temperature but only a small amount at low temperature, allowing for the purification of the desired compound by recrystallization.
In this case, triphenylmethanol is relatively soluble in ethanol at both high and low temperatures. If pure ethanol were used as the solvent for recrystallization, much of the triphenylmethanol would remain dissolved in the solvent even as the solution cools, leading to poor recovery of the compound. This happens because the solubility of the compound in ethanol is high enough that upon cooling, the compound would not crystallize efficiently. Therefore, pure ethanol would not be an ideal solvent for recrystallization of triphenylmethanol.
To address this issue, a mixture of hexane and ethanol is used. Hexane is a less polar solvent, and its presence reduces the overall solubility of triphenylmethanol in the solution, especially at lower temperatures. The 2:1 mixture of hexane:ethanol provides a balance in solubility such that triphenylmethanol dissolves well at high temperatures but crystallizes out effectively as the solution cools. This improves the overall yield and purity of the recrystallized triphenylmethanol.
Option a is incorrect because recrystallization, in general, results in slower formation of crystals, allowing for purer crystals to form. Option c is incorrect because hexane does not stabilize the phenyl groups, and there is no evidence to suggest decomposition at heat that requires such stabilization. Option d is incorrect because triphenylmethanol is not relatively insoluble in ethanol, which would make recrystallization difficult with pure ethanol alone.