Question

6. when choosing a solvent for recrystallization of an organic solid, what rule of thumb is generally used? What characrsitics should an ideal recrystallizing solvent have?

7. During recrystallization, why is it important to let the solution cool slowly to room temperature before cooling in ice? what would happen if the hot solution was immediatly cooled in ice?

Answer 1

6. The rule of thumb -

The rule of thumb for choosing a solvent is like dissolves like. This means that compound with similar functional group are usually mutually soluble in each other. Sucrose that is polyhydroxy compound soluble in water. Alcohols and carboxylic acids, which also contain the - OH are soluble in water. Benzene is not soluble in water but will dissolve other hydrocarbon like hexane.

characrsitics should an ideal recrystallizing solvent have -

i) Compound should very soluble at boiling point of solvent and sparingly soluble at room tempreture.

ii)The unwanted impurities either very soluble at room tempreture or insoluble at boiling point of solvent.

iii)The solvent should not react compound being purified.

iv)Solvent should enough volatile after recrystalization it should remove easilly.

7.)

Crystallization is an equilibrium pricess and produces very pure material. A small seed crystal is formed initially, and it then grows layer by layer in a reversible manner. In a sense, the crystal " select " the correct molecule from the solution . When cooled rapidly, the crystal lattice is formed without too much selection and impurities trapped into lattice.Therefore is it important to let the solution cool slowly to room temperature before cooling in ice.