Crystallization Study Questions - Answers

Answer to question 1.

The main purpose of a crystallization in an organic chemistry procedure is to purify the desired compound. Crystallization also may be used to isolate an solid organic compound from a mixture of compounds in a step in the work-up of a reaction mixture. After this initial isolation (usually by a quick crystallization), the compound is crystallized (or, recrystallized) carefully to purify it.

Answer to question 2.

You want to purify 10 g of benzoic acid that has been contaminated with 0.2 g of salicylic acid. Solubilities in water of the two compounds are given in the following table:

compound solubility at 20 C (g/10 mL) solubility at 100C (g/10 mL)
benzoic acid 0.029 0.680
salicylic acid 0.22 6.67

Answer to question 3.

You want to purify 10 g of benzoic acid that has been contaminated with 10 g of salicylic acid.

compound solubility at 20 C (g/10 mL) solubility at 100C (g/10 mL)
benzoic acid 0.029 0.680
salicylic acid 0.22 6.67

Answer to question 4.

A compound A has the following solubility properties:

solubility at 25C solubility at 100C
0.01 g/mL 0.1 g/mL

You have a sample of 0.1 g of compound A which is contaminated with a compound B. To crystallize A, you would dissolve the 0.1 g in 1 mL of hot solvent and allow it to cool. When cool, 0.09 g of A would crystallize and 0.01 g would remain in solution.

Answer to question 5.

Answer not yet available.

Answer to question 6.

Since you do not know the exact solubility data for benzhydrol in hexanes, you will add small portions of hot hexanes to the benzhydrol, swirling after each addition, until just enough hot solvent is added to completely dissolve the benzhydrol.

Answer to question 7.

If you boil off too much solvent, it is likely that there is no longer enough hot solvent to completely dissolve the compound you are crystallizing, and it will form a precipitate in the hot solvent. If this occurs, add fresh hot solvent to the solution in small portions until all of the compound is dissolved.

Answer to question 8.

Suppose you have prepared a compound which is reported in the literature to have a pale blue color. When dissolving the substance in hot solvent prior to recrystallization, the resulting solution is blue. Should you use decolorizing charcoal before allowing the hot solution to cool? Explain your answer.

Answer to question 9.

a) Compound A: benzene, acetone, or chloroform

b) Compound B: carbon tetrachloride, methylene chloride, ethyl acetate

c) Compound C: methanol, ethanol, or water

d) Compound D: ethanol, acetone, or diethyl ether

Answer to question 10.

naphthalene: a hydrocarbon, try a hydrocarbon solvent like hexanes or petroleum ether or toluene

2-indanol: an alcohol, try ethanol

phenylacetic acid: a carboxylic acid, try ethanol

4-phenylcyclohexanone: a carbonyl compound, try ethyl acetate or acetone

dimethylfumarate: a carboxylic acid, try ethanol

Answer to question 11.

During a recrystallization, you should always wait until crystals appear at room temperature before placing on ice. If the cooling is too rapid, crystals of product form so rapidly that they trap impurities in the growing crystal lattice.

Answer to question 12.

Benzene freezes at 5C. Therefore, when the flask was placed on ice, the benzene froze - this is the solid material that appeared in the flask. During the vacuum filtration (done at room temperature), the solution warmed up enough to melt the benzene, and only crystals of the compound remained on the filter paper.