Question

5. Discuss the principal of crystallization on a molecular level (lattice) that explains why crystallization provides a pure compound. Describe the technique known as 'crystal seeding' and state how "seeding could be used to accelerate the crystal growing process 7. State the solubility of each of the components in neutral water . State if any unexpected compound was observed, via TLC, either in the crystals or after the extraction of the filtrate. If so identify the unexpected compound and state reasons why the unexpected compound was extracted out of the organic solution at that point during the experiment.

Answer 1

Ans5.) The principle behind the crystallisation is that the amount of solute that can be dissolved by a solvent increases with temperature.In crystallisation, the impure substance is dissolved in a suitable solvent to reach its nearly saturated solution at a temperature higher than the room temperature.

To crystallize an impure, solid compound, add just enough hot solvent to it to completely dissolve it. The flask then contains a hot solution, in which solute molecules - both the desired compound and impurities - move freely among the hot solvent molecules. As the solution cools, the solvent can no longer hold all of the solute molecules, and they begin to leave the solution and form solid crystals. During this cooling, each solute molecule in turn approaches a growing crystal and rests on the crystal surface. If the geometry of the molecule fits that of the crystal, it will be more likely to remain on the crystal than it is to go back into the solution. Therefore, each growing crystal consists of only one type of molecule, the solute. After the solution has come to room temperature, it is carefully set in an ice bath to complete the crystallization process. The chilled solution is then filtered to isolate the pure crystals and the crystals are rinsed with chilled solvent.

Ans6.) Crystal seeding is the process of adding homogeneous or heterogeneous crystals to a crystallising solution to nucleate and/or grow more crystals.Crystal seeding is said homogeneous if the seeds are made of the same molecule to crystallize, as occurs in macroseeding, microseeding, and streak seeding, and heterogeneous if it foresees the use of foreign material, such as porous silicon and mica , as in the case of cross seeding and epitaxial growth. Whatever the kind of used seeds, their presence generally makes less energetically expensive the entire crystallization process, because it allows the passing directly to the crystal growth stage without dealing with the intrinsic uncertainties of the nucleation.

Seeding allow to efficiently seperate nucleation and growth of crystals during large scale crystallization process. This avoid the burden of controlling nucleation in large scale operations and set optimum condition for crystal growth, thereby controlling the crystal size and crystal polymorphism. Some of the critical parameters include seed loading, temperature profile of crystallization, seed size and shape seed surface area and porosity and seed chemistry