Question

4. What is the most important criterion in choosing a recrystallization solvent?

5.  Using resonance, briefly (less than 20 words) explain why only the alcohol (phenol) is acetylated and not the carboxylic acid.

Answer 1

4. The following are the most important criterion in choosing a recrystallization solvent -

a) The compound should be very soluble at the boiling point of the solvent and only sparingly soluble in the solvent at room temperature. This difference in solubility at hot versus cold temperatures is essential for the recrystallization process. If the compound is insoluble in the chosen solvent at high temperatures, then it will not dissolve. If the compound is very soluble in the solvent at room temperature, then getting the compound to crystallize in pure form from solution is difficult. For example, water is an excellent solvent for the recrystallization of benzoic acid. At 10°C only 2.1 g of benzoic acid dissolves in 1 liter of water, while at 95 °C the solubility is 68 g/L.

b) The unwanted impurities should be either very soluble in the solvent at room temperature or insoluble in the hot solvent. This way, after the impure solid is dissolved in the hot solvent, any undissolved impurities can be removed by filtration. After the solution cools and the desired compound crystallizes out, any remaining soluble impurities will remain dissolved in the solvent.

c) The solvent should not react with the compound being purified. The desired compound may be lost during recrystallization if the solvent reacts with the compound.

d) The solvent should be volatile enough to be easily removed from the solvent after the compound has crystallized. This allows for easy and rapid drying of the solid compound after it has been isolated from the solution.

5. When anion is created by removing the hydrogen atom from OH- group in phenol, the negative ion will be strongly stabilised by resonating in the benzene ring while in case of carboxylic acid, anion will be stabilised within the carboxyl group.