Laboratory 1 CSI Meadville: Preparative purification of a mixture

Transcription

1 CHEM 231: rganic Form and Function Laboratory 1 CSI Meadville: Preparative purification of a mixture Specific Practical Goals to effect physical separation by leveraging chemical properties to master the technique of aqueous extraction using a separatory funnel to separate a solid from liquid using vacuum filtration to analyze samples using TLC and HPLC The Premise Benzocaine, or ethyl p-aminobenzoate (4), is a widely used inexpensive local anæsthetic. As part of a homicide investigation, your forensics lab receives a 2 gram sample of impure benzocaine seized from a basement laboratory. The sample is believed to contain significant amounts of benzoic acid (1), most probably the result of low-quality starting materials. Your job is to use the chemical properties of benzocaine and benzoic acid to separate the two components in high purity and recovery (yield), to estimate the purity of the isolated compounds using analytical tools (balance, TLC, HPLC), and to determine the percent composition of the original sample. H pk a = 4.2 1, benzoic acid 2, benzoate pk a = 4.8 H 3 N H 2 N 3, ethyl p-ammoniobenzoate 4, ethyl p-aminobenzoate Scheme 1. Proton transfer reactions of benzoic acid and ethyl p-aminobenzoate Week 1 verview of Extraction The basis of extractive techniques is the "like dissolves like" rule. Water typically dissolves inorganic salts (such as lithium chloride) and other charged species, while solvents (ethyl acetate, methylene chloride, diethyl ether, etc.) dissolve neutral organic molecules. It is important to remember that this method of separation relies on partitioning that is, the preferential dissolution of a compound into one solvent over another. For example, 2-pentanol is somewhat soluble in water (ca. 17 g / 100 ml H 2 ), but infinitely soluble in diethyl ether. Thus, 2-pentanol can be preferentially partitioned into ether. This technique is usually carried out using two immiscible solvents that is, that do not mix with each other one of which is almost always water. Typical organic solvents include ethyl acetate, hexane, chloroform, methylene chloride, and diethyl ether. All of these form crisp delineations between phases.

2 The two layers are commonly referred to as the aqueous phase and the organic phase. It is important to keep track of these phases, as their positions are solvent dependent. For example, diethyl ether is lighter than water, so the organic phase will rest on top in the separatory funnel, whereas methylene chloride is heavier than water and so will sink to the bottom (Figure 1). Many are the students who have realized only too late that they have discarded the wrong layer from an extraction. methylene chloride extraction diethyl ether extraction H 2 aqueous layer Et 2 organic layer CH 2 Cl 2 organic layer H 2 aqueous layer Figure 1. Two possibilities for aqueous extraction By way of vocabulary, actually two operations are encountered in the separatory funnel. When components are removed from an organic layer by shaking with an aqueous solution, the organic phase is said to be washed (e.g., The combined ether extracts were washed with aqueous sodium bicarbonate solution. ). n the other hand, when components are removed from water by treatment with an organic solvent, the aqueous phase is said to be extracted (e.g., The aqueous layer was extracted with three portions of ethyl acetate. ). Thus, aqueous layers are extracted, and organic layers are washed although these two terms are sometimes (erroneously) used interchangeably. Usually low-boiling organic solvents are used for extractions because they are easily removed by evaporation, leaving behind the organic compound of interest. Many of these, however, carry over a considerable amount of residual water from the aqueous washes. Ethyl acetate and diethyl ether both dissolve large quantities of water (3.3% and 1.2%, respectively). Therefore, it is advantageous to wash these organic layers with brine (saturated NaCl solution) at the end of the extraction sequence the brine draws out the dissolved water through an osmotic-like effect. For methylene chloride and chloroform a brine wash is unnecessary, since the solubility of water in these solvents is quite low. nce freed from the bulk of residual water, the organic layer is dried over a desiccant, such as sodium sulfate, calcium chloride, or magnesium sulfate, and then decanted or filtered before evaporation. Review of Acid-base Chemistry You will recall from freshman chemistry that the pk a of an acid corresponds to the ph at which it is exactly half ionized. Therefore, if hydrofluoric acid (HF) has a pk a of 3.2, and you make an aqueous solution of HF at ph 3.2, then the solution will consist of exactly 50% HF and 50% F -. Also bear in mind that the ph (and pk a ) scale is logarithmic, so that by changing the ph by one unit, we change this balance tenfold. In other words. an HF solution at ph 4.2 would consist of 90% F -, and one at ph 5.2 would be 99% F -. Thus, ph can be a powerful tool for modifying the charge properties of a compound. If this feels a bit rusty, you should brush off the freshman chemistry text and review the section on acid-base chemistry, particularly the passages dealing with the acidity constant.

3 Safety Considerations You must abide by all laboratory safety rules Exposure routes for methylene chloride are transdermal and inhalation. Avoid contact with skin and always work in a fume hood. Concentrated hydrochloric acid is corrosive to skin, eyes, and clothing. Handle with extreme caution! Procedural verview Important Note: You will be evaluated on the amount of compound you recover. Therefore, make sure all transfers are quantitative. If you are unsure how to do this, ask your instructor. Likewise, purity counts, so make sure all glassware is clean before using. As soon as you receive the sample, record its weight and ID number in your notebook. Completely dissolve the solid in 20 ml of methylene chloride, and save two drops in a centrifuge vial for a TLC sample. Transfer the remaining solution to a separatory funnel 1 and partition using an equal amount of 3M sodium hydroxide. Separate the phases into two Erlenmeyer flasks, taking care to keep track of which flask contains which phase (aqueous or organic). Extract the organic phase with another portion of 3M sodium hydroxide and separate the phases again into two Erlenmeyer flasks. Dry the organic phase with sodium sulfate 2 and remove the solvent using rotary evaporation. 3 Store the flask until next laboratory period according to your instructor s directions. Notes: 1 always use a ring stand for a sep funnel never use a clamp for holding the funnel. 2 make sure the sodium sulfate container is immediately and tightly resealed! 3 remember to tare the round-bottom flask before you transfer the solution to it. Week 2 verview of Chromatography Chromatography represents the most versatile separation technique readily available to the organic chemist. Conceptually, the technique is very simple there are only two components: a stationary phase (usually silica or cellulose) and a mobile phase (usually a solvent system). Any two compounds usually have different partitioning characteristics between the stationary and mobile phases. Since the mobile phase is moving (thus the name), then the more time a compound spends in that phase, the farther it will travel. Chromatographic techniques fall into one of two categories: analytical and preparative. Analytical techniques are used to follow the course of reactions and determine purity of products. These methods include gas chromatography (GC), high-performance liquid chromatography (HPLC), and thin-layer chromatography (TLC). Sample sizes for these procedures are usually quite small, from microgram to milligram quantities. In some cases, the chromatograph is coupled to another analytical instrument, such as a mass spectrometer or NMR spectrometer, so that the components that elute can be easily identified. Preparative methods are used to purify and isolate compounds for characterization or further use. The most common techniques in this category are preparative HPLC, preparative TLC, and column chromatography. ne popular modification of column chromatography is the flash column, which operates at medium pressures (about 10 psi) and provides very rapid separation (ca. 5 min elution times). Column chromatography is suitable for sample sizes ranging from a few milligrams to several grams. Collectively, these methods fall under the category of LC (liquid chromatography), meaning the mobile phase is some liquid solvent system. ccasionally, this is a single solvent, but more often than not it

4 is a binary mixture of solvents with different polarities. The advantage of the latter is that the bulk polarity can be modulated by varying the ratio of the two solvents. For example, consider a typical TLC plate (Figure 1) developed in a 1:1 mixture of ethyl acetate and hexane, which exhibits two well-separated components. The spots can be characterized by their R f value, which is defined as the distance traveled from the origin divided by the distance traveled by the mobile phase. Generally speaking, the slower moving component (R f 0.29) is either larger, more polar, or both. If we wanted a larger R f value, we could boost the solvent polarity by increasing the proportion of ethyl acetate in the mobile phase. Conversely, more hexane would result in lowerrunning spots. 50% EtAc in hexane solvent front c R f2 = b/c (0.69) b R f1 = a/c (0.29) a baseline (origin) Figure 1. A typical thin-layer chromatography (TLC) plate Sometimes one or more of the components interact so strongly with the silica gel that departure from the origin comes only with great difficulty. ne answer to this problem is to derivatize the silica with non-polar substituents, such as long-chain aliphatic residues. This creates a situation known as a reverse phase system. In normal phase chromatography, the stationary phase (i.e., silica) is more polar than the mobile phase; in reverse phase, the stationary phase is less polar than the mobile phase. The derivatized silica is not only much less polar itself, but it also allows for the use of very polar solvents, such as water and methanol. The reversed phase results in some counterintuitive outcomes. For example, more polar components actually elute faster, and a more polar solvent system results in an increased retention time (i.e., lower R f value). These parameters are summarized in Table 1. In this laboratory, you will be using both types: the TLC plates are normal phase and the HPLC column is reverse-phase. parameter normal phase reverse phase typical stationary phase underivatized silica gel C8-hydrocarbon derivatized silica representative mobile phase ethyl acetate / hexane mixture acetonitrile / water mixture more polar components: have lower R f values have higher R f values increasing solvent polarity: increases R f values decreases R f values Table 1. Comparison of normal and reverse-phase chromatography Safety Considerations You must abide by all laboratory safety rules

5 Exposure routes for methylene chloride are transdermal and inhalation. Avoid contact with skin and always work in a fume hood. Do not look directly into the UV lamp! Protect skin from prolonged exposure. Procedural verview Important Note: You will need to bring a ruler and pencil to lab. Add 1 ml of acetonitrile to the centrifuge tube containing the sample from last week. Then, after weighing the isolated solids, prepare analytical samples of each (1-5 mg in 1 ml acetonitrile) and analyze using TLC and HPLC according to the procedure below. TLC Procedure a. Spot the two analytical samples from last week on a single TLC plate b. Develop the plate in a solvent of methylene chloride containing 3% methanol and 0.5% acetic acid c. Visualize the developed plate with a UV lamp and calculate all R f values HPLC perating Procedure a. Make sure you are operating in the proper software window (Instrument 1 is on the left) b. Ensure that the system is available a green "Ready" box will show in the upper left quadrant c. n the top menu, go to RunControl Sample Info; enter your name under perator Name d. Clean a 50 L Hamilton syringe (handle with utmost care!) by drawing methanol into it to full capacity; purge into a waste container; repeat e. With injector in Load position (counterclockwise), flush out sample loop with methanol 4 or 5 times using a full syringe f. Load sample into injector using at least 3 full injections, avoiding any air bubbles in the syringe; leave the syringe in the injector port after the last injection g. Turn injector to Inject position (clockwise), leaving the syringe in the injector; software will set a zero point on the trace h. Do not make any hardware adjustments while a run is in progress i. When the sample has finished, your trace will automatically print out