How to be a Successful Organic Chemist

15 4. Nothing distills but the amount of liquid in the still pot mysteriously goes down. Typical problems: There is a leak in the system, or several leaks. The vapor is escaping though openings between joints. Make sure that all joints are properly sealed. 2.3 Liquid-liquid extraction Liquid-liquid extraction involves the exchange of certain com- pounds between two solvents that are immiscible or only par- tially miscible. Liquid-liquid extraction is also very commonly used for washing an organic phase, for example to remove inor- ganic compounds, or to protonate or deprotonate bases or acids, respectively, so they become soluble in the aqueous phase. A very typical extraction flow diagram is shown below, where a reac- tion mixture is quenched with water, extracted (several times), washed with brine, dried, filtered and finally evaporated to yield a crude product or a pure product. Figure 2.4. A typical flow-chart for an extraction Liquid-liquid extraction (we will refer to it simply as extraction from now on) is typically conducted with one aqueous phase (either pure water, or an aqueous solution) and one organic phase. It is important to note that the desired compound (usually an organic molecule) can in theory be in either phase. It depends on both the nature of the compound, but also the nature of the aqueous phase. We will first examine some typical scenarios involving extraction, then move on to discuss technical details to keep in mind. 1. Extraction of neutral compounds. If the desired organic com- pound is neutral (i.e. is neither acidic nor basic), the extraction sequence usually involves simply extracting with an organic solvent several times. 2. Extraction of acidic compounds. If the desired compound is acidic, we can selectively deprotonate that compound by using an aqueous base. This will pull the deprotonated compound into the aqueous phase. Because very few organic compounds are soluble in water, we can discard the organic phase which now contains any byprod- ucts and/or unreacted starting materials. Acidification of the aque- ous phase can precipitate the desired product. 3. Extraction of basic compounds. If the product is basic, we can perform a sequence very similar to the acidic compound above. We can protonate the basic compound by using an aqueous acid, pulling the protonated compound into the aqueous phase and discarding the organic phase. Neutralizing the acidic phase will depro- tonate the basic compound, which may precipitate, or may require a second extraction with an organic solvent. The typical apparatus used in an extraction, is the separatory funnel. These come in many different sizes, but the typical size is 100 mL, which you will encounter in several labs at PSU.The funnel is equipped with a top and a nozzle, and the nozzle can be closed or opened with a stopcock. The funnel is narrow at the nozzle, which is a structural feature that allows us to accurately separate the two phases. START Post-reaction mixture is quenched by water mixture added to seperatory funnel Is reaction flask covered in residue? Rinse with organic solvent and/or water Yes Extract, and allow phases to seperate Seperate the organic layer from the aqeous page 2 No Figure 2.4. A typical flow-chart for an extraction. Figure 2.5. Adding HCl to the basic, aqueous phase leads to precipitation of benzoic acid, here seen as a white precipitate.