How to be a Successful Organic Chemist

12 Please also note that some compounds simpl crystallize more easily than others. More rigid molecules are, as a rule, easier to crystalize. 8 Rigid, in this context, mean compounds that con- tain fewer bond capable of undergoing internal rotation, so that there are fewer possible conformers possible. Let us go through a recrystallization process, focusing on technical aspects and trouble shooting. How to perform a recrystallization: 1. The crude impure solid is dissolved in hot solvent. If some solid remains undissolved after adding solvent, it is likely to be an impurity and should be removed by filtrering the (hot!) solution. Typical problems: Adding too much solvent so that the product does not crystallize later. Filtering the hot solution too slowly so that the solvent cools and the solid starts crystallizing in the funnel and/or on the sides of glassware. 2. The solution is allowed to stand without being disturbed. The temperature is allowed to gradually drop, leading to growth of large crystals.The flask should not be placed on a surface (it will shock-cool the solution), but either placed in an insulated jar, or clamped. Typical problems: Crystals do not form at all (too much solvent), pre- cipitate forms instead of crystals (temperature has dropped too quickly, or an oil forms). 3. The solution is allowed to stand until crystallization is complete. Typical problems: Crystallization can be a slow process, and impa- tience can lead to low recovery. 4. The solution is placed in an ice-water bath to lower the tem- perature even further, and allow more crystals to form. At this point, most crystals should already have formed. 5. The crystals are filtered and air-dried. How can we tell if a recrystallization has been a success? Sim- ple visual inspection is a good start: The crystals should have shiny surfaces and catch the light. They should appear uniform, and you should have crystals of similar structure and size. A melting point analysis should also show a narrower and elevated melting point range compared to the crude material. Like any purification technique, recrystallization has some limitations. First of all the compound you crystallize should be a solid at standard conditions. Greases, waxes and oils cannot be crystallized at standard conditions. Secondly, the crude ma- terial should be mostly pure. There is not any minimum purity 8  I have prepared new compounds that I have tried on and off to recrystallize for the better part of a whole year, without success. (A) The crude material is transferred to a suitable crystallization vessel. (B) The crude material is dissolved in a solvent, and gently heated. (C) The solution is allowed to gently and slowly cool down. Notice the crystals growing in the solu- tion. (D) The solution is cooled to room temperature, leading to the formation of large crystals. Figure 2.1. Key steps in a recrystallization process