Aim: Recrystallization of Benzanilide or Phenyl Benzoate or P-Bromoacetanilide
Reference: F. G. Mann and B. C. Saunders, Practical Organic Chemistry, 4th Ed, 1974, P. No. 13-15.
Requirement:
Apparatus: Conical flasks, Filter funnel, Buchner flask, Buchner funnel, etc.
Chemicals: Benzanilide or Phenyl Benzoate or P-Bromoacetanilide, Ethanol
Theory/Principle:
Organic compounds that are solids at room temperature are usually purified by recrystallization. The general technique involves dissolving the material to be recrystallized in a hot solvent (or solvent mixture) and cooling the solution slowly. The solid that crystallizes out from the solution is very pure material. During the recrystallization process, solid impurities (such as dust, filter paper, etc.) that do not dissolve in hot solution are normally eliminated through filtration. The dissolved impurities remain in the cold solution while the pure compound recrystallizes out of the solution. The process of recrystallization relies on the property that for most compounds, as the temperature of a solvent increases, the solubility of the compound in that solvent also increases
The general procedure for recrystallization is as shown in the flow chart below and the steps in the recrystallization of a compound are:
1. Find a suitable solvent for the recrystallization
2. Dissolve the impure solid in a minimum volume of hot solvent
3. Use decolorizing carbon (if necessary)
4. Remove any insoluble impurities by filtration
5. Slowly cool the hot solution to crystallize the desired compound from the solution
Fig.1 Method of Recrystallization
Procedure:
- Weigh about 1.0 g sample into a 100 mL conical flask. Add 10 ml of ethanol and anti-bumping granules (3-5 pieces).
- Heat the mixture on a hot plate until the solvent boils. Add successive small volumes of ethanol (2-3 mL) if required and continue boiling until compound has been dissolved (apart from insoluble impurities).
- If the solution is coloured, remove the solution from the hot plate.
- Cool the solution to room temperature and add decolourising charcoal (0.2-0.3 g) if required.
- Mix thoroughly and boil the mixture for several minutes.
- Filter the hot mixture of sample through a filter paper into the preheated conical flask/beaker.
- If crystallization occurs on the filter paper, add a minimum volume of hot ethanol to re-dissolve the crystals, and allow the solution to pass through the funnel.
- After filtration, boil the filtrate to produce a more concentrated solution.
- Cover the conical flask with a watch glass and allow the solution to cool to room temperature, then in an ice-bath after the crystallization has occurred.
- When all the crystals of compound have crystallized out, filter the crystals through a Buchner funnel at the suction pump.
- Wash the crystals with a little cold ethanol and dry.
- Weigh the pure compound recovered, calculate the percentage yield, report melting point and compare with melting point of the crude sample.
