Diethyl ether and methylene chloride are generally not suitable for use in recry

Question

Diethyl ether and methylene chloride are generally not suitable for use in recrystallization, but are frequently used as extraction solvents. Explain. Acetone and isopropyl alcohol are generally not suitable for use in extraction, but are frequently used as recrystallization solvents. Explain. After separating layers in an extraction, it is necessary to "dry" the organic layer with a substance that absorbs water. Explain why this is necessary. Suggest two methods for determining which layer in an extraction is the aqueous layer and which layer is the organic layer. An organic compound has a distribution coefficient of 1.5 in methylene chloride and water, if 50 mg of this compound is dissolved in 15 mL of water, in which case will more of the compound be extracted into the methylene chloride: one extraction with 15 mL of methylene chloride or three extractions using 5 mL of methlene chloride each time? Refer to Appendix B if necessary.

Explanation / Answer

Question 1:

As the boiling points of diethyl ether and methlene chloride are very low, they evaporate off very quickly. The cooling that accomapanies quick evaporation makes the water vapor of the atmosphere to condense on the recrystallized crystals, resulting in wet crystals. Thus they are not recommended as solvents for recrystallization.

Nevertheless, they stand as good solvents for extraction. The lesser density compared to common solvents such as water and nonpolar nature of diethyl ether accounts for this use.

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