How to be a Successful Organic Chemist

20 Troubleshooting sublimation 1. The crude material does not sublime. The most common reasons are insufficient heat, and/or the pressure is not low enough. 2. The sample sublimes, but the material crystallizes on the sides of the filter flask, and not on the cold finger. The bottom of the flask might be hot enough, but the sides are not. The material is allowed to sublimate, but cannot reach the cold trap and is able to crystallize earlier. Insulation of the lower part of the filter flask can be appropriate. 3. I don’t know where the sample went! The crude material was added, but during the process of heating the flask, the material is gone, and there are no crystals.There can be several reasons, but the most likely contender is that the cold finger did not efficiently work as a trap. Lowering the position of the cold finger is a good solution. 4. The crude material is boiling! The system can be too hot, and less energy should be supplied to heat the system. Potentially the pressure should be reduced. 5. The product crystallizes on the cold finger, but looks pasty and wet and not crystalline. The ice cold water in the cold finger has been in the finger for too long. The cold surface has started con- densing water from the atmosphere, and once the product starts to reach the cold surface it is met by water. In general, the ice cold water should be added to the cold finger right before the sublimation starts to eliminate this problem. Figure 2.10. Key steps for sublimation (C) As the temperature rises, more crude material sublimates. Inside the filter flask, the vapor reaches the cold finger. The vapor then crystallizes immediately. (D) Once the sublimation is complete, the cold finger is care- fully removed. Notice the crystals that have grown on the cold finger. This can be scraped off, and quantified.