01:160:311 Lecture Notes - Lecture 3: Ethyl Acetate, Supersaturation, Paper Towel

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22 Sep 2019

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Concepts involved in recrystallization and how they are employed. Picking solvent: like dissolves like, balance of polarity that is needed for recrystallization/requirements for solvents with respect to solute and impurities. Like dissolves like means polar compound will dissolve well in polar solvent and opposite for nonpolar. Compound should be completely soluble at boiling point of solvent but insoluble in solvent at room temperature. Impurities should be insoluble in the hot solvent (or soluble in the solvent at room temperature) Solvent should be volatile enough to easily be removed after compound has crystallized (allows for easy and rapid drying of compound) Why solvent pairs are used and procedure of how to use solvent pairs/what happens in the process of using solvent pairs and precautions when using them. Desirable and undesirable qualities of water, ethanol and ethyl acetate and acetone (common crystallization solvents) Used because one solvent may not have all the desired properties to purify sample.

Related Questions

A process of recrystallization involves dissolving the solid inan appropriate solvent at an elevated temperature and allowing thecrystals to re-form on cooling. Residual amount of solvent isremoved by drying the crystals and therefore usually does notconsider to be an impurity, although if the crystals are not driedproperly the residual solvent will affect the melting pointmeasurements.
The choice of solvent is the most critical step in the process ofrecrystallization. Which of the following criteria should besatisfied while choosing a solvent for recrystallization?

Select one:

a. A compound to be purified should be completely soluble in areasonable amount of hot solvent.

b. A compound to be purified should be insoluble, nearlyinsoluble, or slightly soluble in cold solvent.

c. Impurities should be either insoluble in the solvent at alltemperatures so they can be filtered out or soluble in a coldsolvent staying in it while the desired compound willprecipitate.

d. The boiling point of the solvent should be low enough so theresidual amount of it will be readily removable from the crystalsof pure compound.

e. The boinling point of the solvent should generally be lowerthan the melting point of the desired compound so the compound willnot melt while heating the solvent giving the impression that ithas been dissolved.

f. The solvent should not react chemically with the compoundbeing purified.

g. All of these considerations are correct.

Suppose you were trying to find a solvent for a recrystallization of each compound below. You did one test with a solvent, get the results stated - from the table of solvents, what should your next likely solvent to try be? Be sure to explain your answer and be specific.

Table of Solvents:

Solvent	Dielectric constant
Benzene	2.28
Ether	4.34
Isopropanol	18.3
Water	78.5

a.) DDT is very soluble in ether at room temperature

b.) Glucosamine does not dissolve in even hot isopropanol

01:160:311 Lecture Notes - Lecture 3: Ethyl Acetate, Supersaturation, Paper Towel

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