Question

P u Purpose lule TOTTOwing in this exptum a. using hot filtration b. not cooling (chilling) the hot filtered solution immediately c. washing the crystals in the final step with ice cold water d. using gravity filtration (not vacuum filtration) in the hot filtratio step e. using a stemless funnel in the hot filtration step. 2. What are the requirements for a good recrystallizing solvent. 3. Using the data of Table 6.1, compare the following pairs of liquids as recrystallization solvents (Note: Review the conditions for a good recrystallizing solvent): a. water vs. acetone b. methanol vs. toluene c. benzene vs. cyclohexane d. chloroform vs. ethyl acetate.

number 3

Answer 1

Criteria for a good recrystallization solvents are -
1. Recrystallization solvent should NOT dissolve the substance to be purified at
room temperature, but it should dissolve it well at the solvent’s boiling point
and the solvent should dissolve soluble impurities well at room temperature.
2. The solvent should not dissolve insoluble impurities even at the solvent’s boiling
point.
3.These insoluble impurities can then be removed by gravity filtration.
4. The solvent must not react with the substance to be purified.
5. The solvent should be volatile.
6. The solvent should be inexpensive.

When these criterias are not met by a single solvent , there comes a need to use a pair of them . As recrystallisation is a method used to purify a desired compound the solvent pair that we choose will be such that , one is the soluble solvent in which our compound should dissolve and the other is the insoluble solvent in which our compound should not dissolve, and the 2 solvents should be miscible with eachother.

So water and acetone are a pair of such recrystallisation solvents.

They are both soluble in eachother but acetone is organic solvent and water is the aqueous or inorganic solvent so hence dissolves the compound to be purified accordingly. And hence it is a good recrystallisation solvent pair.

Benzene is more polar and cyclohexane is less polar. So good recrystallisation solvent pair .

Ethylacetate and chcl3 has comparable polarity, both are polar but chcl3 is highly polar than ethylacetate.. so hence its also an ok pair.

Methanol and toluene both are organic solvente but methanol is a very good recrystallisation solvent but toluene is not .

So they can be a good pair.