How to be a Successful Organic Chemist

19 Like any of the techniques we have discussed in this chapter, TLC-analyses require a fair amount of care to get right. The most likely problems can be traced to the application and preparation of the TLC-plate. Let us have a closer look at some TLC plates that are of inadequate quality and their causes. 2.5 Sublimation Sublimation is a purification technique for solids and in the context of this book, for organic compounds with lower melting points. Sublimation describes the process of a solid becoming a gas, without passing through the liquid state.The gas phase is then typically crystallized on a cold surface. However, many subli- mations require reduced pressure, and we will primarily focus on that variation of sublimations. The reason for many sublimations happening under reduced pressure, is because the sublimation point is decreased with decreasing pressure. To allow organic compounds to sublimate, and not simply melt, a reduced pressure is often necessary. A crude, but efficient sublimation apparatus can be made from of a filter flask, where the side neck is con- nected to an aspirator or vacuum trap, and the neck is equipped with a cold finger (a tube with ice water, or other cooling material). The impure solids are placed in the bottom of the filter flask, and the cold finger is inserted.While an aspirator or pump is reducing the pressure inside the filter flask, the material is carefully heated on a hot plate (take care to avoid melting or boiling the material). The organic compound will start to sublime forming a gas. Once this gas reaches the cold finger, it will immediately crystallize on the cold finger, where it can be collected. ! Figure 2.9. Various TLC scenarios. (A) The plate is good. The two spots are small and clearly de- fined. (B) The plate is not good. The spots have “drift- ed” which most likely was due to the plate not being perpendicu- lar to the solvent when it was inserted into the chamber (C) The plate is not good. The two spots are large, because the sample was too con- centrated. (D) The plate is not good The spots have not wandered because the solvent system used, was not polar enough. (E) The plate is not good The spots have wan- dered to the solvent front, because the sol- vent was too polar (A) A filter flask is clamped. The side arm is attached to a tube connected to a water aspirator, leading to reduced pressure inside the flask. The cold finger is fitted through the neck. There is ice-cold water in the cold finger. (B) The filter flask is heated. The crude material starts to sub- limate. Notice the rising purple vapor.