Question

A compound is slightly soluble in hexanes, soluble in water, soluble in DMSO, and the compound is insoluble in benzene, and is insoluble in toluene. Based on this, the following is true.

a) DMSO is a good recrystallization solvent

b) water is a good recrystallization solvent

c) hexanes is not a good recrystallization solvent

d) hexanes is a good recrystallization solvent

e) benzene is a good recrystallization solvent

Answer 1

The criteria used to choose an appropriate recrystallization solvent includes:
1) finding a solvent with a high-temperature coefficient. The solvent must not dissolve the compound at low temperatures (that includes room temperature) but must dissolve the compound at high temperatures. The solute must dissolve in order to rid its lattice of impurities, but must not remain dissolved at room temperature.

2) using a solvent that dissolves impurities readily or not at all. If the solvent dissolves the impurities readily (even at room temperature) then the impurities will not become trapped in the developing crystal lattice but will remain dissolved in the solvent. If the impurities do not dissolve (even at elevated temperatures) then they can be easily removed by gravity filtration.

              3) ensuring the solvent will not react with the solute. Recrystallization does not chemically alter a molecule. No chemical bonds must be broken in the solute molecule. The crystal lattice is dissolved at elevated temperatures, but this only involves overcoming the intermolecular attractive forces.

4) using a solvent that is non-flammable, inexpensive and volatile. Solvents with low boiling points (i.e., volatile) can be easily removed from the resultant crystals by simply allowing the solvent to evaporate.

Like dissolves like.

The compound is insoluble in benzene and toluene means Compound is polar compound because toluene and benzene both are non-polar molecule.

The compound is slightly soluble in hexane. Hexane is less polar that's why slightly soluble to the polar compound.

The compound is soluble in water and DMSO. Both water and DMSO are polar compounds. The compound is also polar.

Answer -

(a) DMSO is a good recrystallization solvent. (True) because it dissolves the compound.

(b) Water is a good recrystallization solvent. (True) because it dissolves the compound.

(c) Hexane is not a good recrystallization solvent. (True) because it doesn't dissolve the compound properly.

(d) Hexane is a good recrystallization solvent. (False)

Because hexane is not a good recrystallization solvent.

(e) Benzene is a good recrystallization solvent. (False)

Because Benzene is not a good recrystallization solvent.