Guide to Tackling Multistep Synthesis Problems - Organic Chemistry …
Most solutions are relatively diluted (~5 %) and their density is not much different from that of water (i.e., 5 % HCl: 1.02 g/cm3, 5 % NaOH: 1.055 g/cm3). Thus, the density of a solid i.e., sodium hydroxide (2.1 g/cm3 in the solid) does not provide the information sought. The density is determined by the major component of a layer which is usually the solvent. About 5 % of a solute does not change the density of the solution much. However, this can change if very concentrated solutions are used (see table in the back of the reader)! Thus, diethyl ether and ethyl acetate, which are both less dense than the dilute solutions that are usually used for extraction, form the top layer, while dichloromethane and chloroform form the bottom layer (currently both of them are not used in Chem 30BL or Chem30CL due to safety concerns!).
It is not uncommon that a small amount of one layer ends up on top of the other. Mixing with a stirring rod or gentle shaking usually takes care of this problem. Small amounts (compared to the overall volume of the layer) should be discarded here.
This would usually happen if the mixture was shaken too vigorously. Subsequently, an emulsion is formed instead of two distinct layers. In such an event, the mixture can be stirred slowly with a glass rod to bring the small droplets together a little faster, which ultimately leads to the formation of a new layer. In some cases, a careful draining of the existing lower layer can also be helpful because it pushed the bubbles together in the smaller part of the extraction vessel. In cases, where the phases have similar polarity or density, the addition of more solvent can assist the separation. Sometimes, the addition of a salt (or salt solution) can also lead to a better phase separation (“salting out”). In many cases, centrifugation or gravity filtration works as well. When it is known, through experience, that some mixtures may form emulsions, vigorous shaking should be avoided. Instead, gently rocking the separatory funnel back and forth for 2-3 minutes will accomplish sufficient degree of mixing while minimizing the formation of emulsions.
Strictly speaking, hardly ever all of the solute will be extracted since there is finite distribution coefficient for the compound (see also Extraction II). As a general rule, multiple extractions with small quantities of solvent or solution are more efficient than one extraction using the same amount of solvent (see below). The amount of material left behind after two or three extractions is usually very small (less than 5 % in most cases) and does not justify the effort and resources (solvent and time to perform the extractions and to remove the solvent later on). Excessive washing will also lower the yield of the product, if the desired compound dissolves noticeably in the other phase.
This phenomenon will often be observed if sodium bicarbonate is used for the extraction in order to neutralize or remove acidic compounds. The reaction affords carbon dioxide (CO2), which is a gas at ambient temperature. Pressure builds up that pushes some of the gas and the liquid out. The container should be vented immediately before the pressure build-up can cause an explosion, an ejection of the stopper on the top or excessive spillage upon opening. A similar observation will be made if a low boiling solvent is used for extraction. The shaking of the mixture increases the surface area, and therefore the apparent vapor pressure of the solvent. In addition, many extraction processes are exothermic because they involve an acid-base reaction.
Often times the cap is either the wrong cap in the first place or it is not properly placed on the top. If NaHCO3 is used for extraction, the centrifuge tube has to be vented more frequently.
Before using the separatory funnel, the user should check if the stopcock plug and the stopcock fit together well. In addition, the stopper on the top has to fit into the joint on the top to prevent leakage there (for more details at the end of this chapter).
The conical shape of these pieces of equipment makes it easier to collect the solution on the bottom using a Pasteur pipette because of the smaller interface. The task of getting a clean phase separation will be more difficult if the liquids are spread out over a large, flat or curved surface.
The bottom layer is always removed first independently if this is the one of interest or not because it is much easier to do. If a centrifuge tube or conical vial was used, the bottom layer should be drawn using a Pasteur pipette. From this point of view, a solvent with higher density than water would be preferential, especially when very small quantities are used. This will allow to minimize the number of transfer steps required.
This highly depends on the quantity of a compound that has to be removed. For most washing processes, 10-20 % of the volume of the solution to be washed will do an adequate job. If a large amount of a compound has to be transferred or neutralized, more concentrated solutions and larger quantities might be needed. Multiple extractions with smaller quantities are preferred over one extraction with the same quantity of solution/solvent. If solutions with higher concentrations are used, extra caution is advised because neutralization reactions are exothermic. This can pose a serious problem when using low boiling solvents i.e., diethyl ether, dichloromethane, etc. because a pressure build-up will be observed in the extraction container.
How to Tackle Organic Chemistry Synthesis Questions
For instance, if the target compound was the base in the system, the extraction with HCl should be performed first. Whatever remains in the organic layer is not of interest anymore afterwards, unless one of the other compounds has to be isolated from this layer as well. If the target compound was an acid, the extraction with NaOH should be performed first. This strategy saves steps, resources and time, and most of all, greatly reduces waste.
Practical Aspects of an Extraction
An extraction can be carried out in macro-scale or in micro-scale. In macro-scale, usually a separatory funnel (on details how to use it see end of this chapter) is used. Micro-scale extractions can be performed in a conical vial or a centrifuge tube depending on the quantities. Below are several problems that have been frequently encountered by students in the lab:
Research in organic chemistry at MIT addresses a broad spectrum of important problems of current interest and includes investigations at the frontier of bioorganic chemistry, organic synthesis, and materials science. Specific areas of research include protein glycosylation and protein design, chemosensors, continuous flow synthesis, liquid crystals, supramolecular catalysis, the design of new organometallic reagents and catalysts, the invention of new methods for asymmetric catalysis, engineering and pharmaceutical manufacturing, and the development of new strategies for the total synthesis of a wide array of biologically important natural products. A central theme in many projects is the study of structure-reactivity relationships of biological, organic, and organometallic molecules. Much of the current research in the department takes place at the interface of organic chemistry with other areas such as biology, medicine, materials science, and nanotechnology.