Question

You want to recrystallize the product from a nucleophilic substitution reaction. After adding a minimal amount of hot 95% ethanol to dissolve all the crystals and letting the flask cool to room temperature crystals do not form. To induce crystallization and maximize the % recovery, one should:_______.
A) Boil off some solvent
B) Scratch the flask
C) Add a seed crystal
D) Cool the flask in an ice bath
E) Any of the above

Answer 1

Answer:

B) Scratch the flask

C) Add a seed crystal

Explanation:

Two important methods to induce crystallization in this case are;

1) Scratch the flask

2) Add a seed crystal

However, the rate of crystallization becomes slower at lower temperature hence cooling in an ice bath should only be applied before the crystals form. When the crystals start forming, the system should be maintained at room temperature.

The question clearly stated that a minimal amount of hot 95% ethanol was added to dissolve all the crystals hence you do not need to boil off some solvent. Boiling off solvent is applied when too much solvent is used.