Organic Chemistry MCAT Practice Exam

In distillation, when would you apply fractional distillation?

1. When boiling points differ by greater than 30 °C.

2. To separate diastereomers with small differences in boiling points.

3. When separating non-volatile impurities.

4. When requiring very high temperatures for purity.

The correct answer is: To separate diastereomers with small differences in boiling points.

Fractional distillation is particularly useful when separating components that have close boiling points, often within a few degrees of each other. This method employs a fractionating column, which increases the surface area for repeated vaporization and condensation, allowing for more precise separations based on boiling point differences. In the case of diastereomers, which may have similar but distinct boiling points, fractional distillation can effectively exploit these slight differences to achieve separation. This is because the interaction of the diastereomers with the packing in the column will allow for distinct vaporization behaviors, enabling the collection of different fractions based on their boiling points. The other scenarios such as large boiling point differences or the presence of non-volatile impurities typically do not require the advanced separation techniques that fractional distillation offers. Instead, simpler distillation methods may suffice in those cases. Furthermore, requiring very high temperatures for purity does not directly relate to the principles of fractional distillation itself, as the technique relies more on the effective separation based on boiling point rather than the temperature conditions used.