CHEMISTRY 241
Organic Chemistry Laboratory 1
Lab Manual
Lab 1 – Acid-Base Extractions and Recrystallization
Professor Lawrence Goldman
Professor Tomi Sasaki
Department of Chemistry
University of Washington
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Organic Chemistry Laboratory 1: Acid-Base Extractions and Recrystallization, Study Guides, Projects, Research of Chemistry
A lab manual for a chemistry 241 course at the university of washington, focusing on separating and identifying three compounds (an amine, a carboxylic acid, and a neutral compound) through acid-base extractions and recrystallization. The lab includes safety information, procedures for each step, and questions for further analysis. The goal is to purify the compounds and determine their melting points for identification.
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2023/2024
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CHEMISTRY 24 1
Organic Chemistry Laboratory 1
Lab Manual
Lab 1 – Acid-Base Extractions and Recrystallization
Professor Lawrence Goldman
Professor Tomi Sasaki
Department of Chemistry
University of Washington
Chemistry 24 1 Lab 1: Acid-Base Extractions and ReCrystallization , and Melting Point 2023 - 2024
LAB 1 : ACID-BASE EXTRACTIONS AND RECRYSTALLIZATION, AND
MELTING POINT
From this point onward, you are responsible for knowing the information contained within the Introduction to this manual, including the “Organic Chemistry Laboratory Safety,” “Laboratory Notebooks,” “Chemistry Stockroom,” and “General Chemistry Review Material” sections. Scenario: You are a brand new employee working in a research lab at ChemCorp. On a seemingly ordinary Tuesday, you have a vet appointment for your pet labradoodle (Chaser) that evening. To save yourself a trip home, you bring Chaser to the lab. But unexpectedly (or maybe completely predictably, in hindsight), Chaser gets loose and starts running around the lab. He jumps onto the lab bench and knocks 3 bottles of white powder onto the floor. The bottles all smash and the powders get mixed together. To add insult to injury, Chaser slobbers all over the labels, obscuring most of the name of the compounds so you can’t even order replacement bottles. All you know is that the first compound is an amine, the second compound is a carboxylic acid, and the third compound is neither acidic nor basic. Based on what you could make out on the label, you narrowed down each compound to two different candidates. Your task is to separate the 3 compounds from each other, make sure that each compound is pure, and put them back into bottles before your boss finds out. You’ll also need to determine the identity of each compound so you can relabel the bottles. You will separate the 3 compounds from each other by taking advantage of the fact that certain compounds have different solubility based on the pH of the solution. Once the compounds are separated from each other, they will each be further purified using other techniques such as crystallization. Finally, the purity of the compounds will be ascertained using melting point, an analytical technique. Theory : Most organic carboxylic acids (with more than 5 carbons) are not very soluble in neutral water. Treatment of a carboxylic acid with dilute aqueous NaOH produces the corresponding sodium carboxylate salt. Due to its ionic character, the sodium carboxylate salt is soluble in water but not very soluble in organic solvents. If the basic aqueous solution containing the carboxylate salt is then made acidic by addition of aqueous HCl, the sodium carboxylate salt will be converted back to the carboxylic acid, which is not water soluble and will precipitate. Likewise, many organic amines are not soluble in neutral water. Treatment of organic amines with dilute aqueous HCl produces the corresponding ammonium salt. The ammonium salt is soluble in water but not very soluble in organic solvents. If the acidic aqueous solution containing the ammonium salt is then made basic by the addition of aqueous NaOH, the ammonium salt will be converted back to the original amine, which is not water soluble and will precipitate. By taking advantage of these solubility properties, it is possible to separate a mixture containing acidic, basic, and neutral components using sequential extractions with acidic and basic water.
Chemistry 24 1 Lab 1: Acid-Base Extractions and ReCrystallization , and Melting Point 2023 - 2024
PROCEDURE
You will work in pairs for experiment 1, but will work individually for most other experiments. Ensure that each partner does roughly half of the work. Each individual student will write and submit their own lab report. Melting point practice: Obtain a melting point capillary tube. Dip it into the jar of azelaic acid so that a small amount of solid — about the thickness of a nickel — enters the capillary tube. Pack the tube down and obtain a melting point using the Mel-Temp devices. Your melting-point should be within 5 degrees of the literature value of azelaic acid (see Appendix II for more information). If you are further away from the correct value, you must repeat the melting point determination. Accurate determination of melting points will be critical to analyzing several of the experiments in Chem 241 and 242. The picture below gives the approximate heating curves for the Mel-Temp. These curves are also posted around the Mel- Temp devices. Do not add 6M HCl or 6M NaOH into the separatory funnel, they should only be used to neutralize compounds after the extractions. As a guide, the volumes of the organic and aqueous layers in the separatory funnel should be roughly similar. During this lab you will be working with a lot of colorless solutions and it is easy to get them mixed up. Many students find it helpful to consult their flow chart throughout the experiment to help them avoid mixing up their solutions. Label all of your glassware throughout the experiment.
Chemistry 24 1 Lab 1: Acid-Base Extractions and ReCrystallization , and Melting Point 2023 - 2024 Extracting the Amine: Record the exact mass of your unknown, which will be approximately 1.0 g. Dissolve the unknown mixture containing 33% each of an acid, base and neutral in 25 mL of diethyl ether. Transfer the ether solution to a separatory funnel and add 5 mL of 1M aqueous HCl. Gently shake the separatory funnel for several minutes, venting it frequently to avoid pressure buildup. Place the separatory funnel in a ring stand and allow the ether and aqueous layers to separate. Uncap the funnel and drain the aqueous (lower) layer into a flask. Use pH paper to monitor the pH and ensure that the pH of this layer is at least 2 pH units lower than the pKa of the possible amines to ensure that they are protonated. Repeat the extraction with another 5 mL of 1M aqueous HCl. Save the ether layer in the separatory funnel for later use. In a beaker, combine the acidic aqueous extracts and cool them in an ice-tap water bath. Slowly add 6M aqueous NaOH (~2 mL) until you see a substantial amount of precipitate form. Collect the precipitate on a Büchner funnel by vacuum filtration (PLKE Figure 8.5, p. 672). Make sure the filter paper fits snugly into the funnel and wet the filter paper slightly before you pour your material onto it. Wash the solid with cold DI water (~2 mL) on the funnel and allow it to air dry. Prepare a melting point capillary tube and obtain the mass and melting point of your amine. Extracting the Carboxylic Acid: Extract the ether solution (left over from above) with two 5 mL portions of 1M aqueous NaOH. Again, save the ether layer in the separatory funnel. Combine the basic aqueous extracts and cool them in an ice-water bath, ensuring that the pH is at least 2 pH units more basic than the pKa of the carboxylic acids. Add 6M aqueous HCl (~2 mL) until you see a substantial amount of precipitate form. Collect the precipitate that forms in a Büchner funnel by vacuum filtration. Make sure the filter paper fits snugly into the funnel and wet the filter paper slightly before you pour your material onto it. Rinse the solid with cold DI water (~2 mL) to remove any water-soluble impurities, and then allow it to air dry. Prepare a melting point capillary tube and obtain the mass and melting point of your crude carboxylic acid. ( HINT: To save time, you may want to take the melting point of your crude and crystallized carboxylic acid at the same time, but you can save a capillary tube’s worth of crude carboxylic acid at this time). Drying the Ether Solution and Isolating the Neutral (Non-Ionizable) Compound: Add 10 mL of saturated aqueous sodium chloride solution (brine) to the ether remaining in the separatory funnel and repeat the shaking and venting procedure from above. Allow the layers to separate and discard the brine layer. Pour the ether layer into a beaker containing 0.8-1.2 g of anhydrous Na 2 SO 4 and allow it to stand for about 5 minutes, swirling occasionally. While your ether solution is drying, put a piece of tape on a beaker, weigh it, and record the weight both in your notebook and on the tape. If all of your ether evaporates, you will need to add more in order to separate the solid Na 2 SO 4 from your solid neutral compound. You will know that you have removed all traces of water when some of the sodium sulfate remains “powdery” while the rest is “clumpy”. Pour the ether solution into a beaker, taking care to leave behind all of the solids. This is called decanting. Evaporate the ether by using a stream of air directed into the beaker by rubber tubing. When all of the ether has evaporated, prepare a melting point capillary tube and obtain the mass and melting point of your crude neutral compound. (Note: If an oil forms instead of a solid, cool it on ice until it solidifies). ( HINT: To save time, you may want to take the melting point of your crude and recrystallized neutral compound at the same time, but you can save a capillary tube’s worth of crude neutral compound at this time). This is the suggested stopping point for Day 1.
Chemistry 24 1 Lab 1: Acid-Base Extractions and Recrystallization 2023 - 2024
LAB 1: ACID-BASE EXTRACTIONS AND RECRYSTALLIZATION
Name: __________________________________________ Section:_______ GRADING – each section below is worth 1 point. You must earn 3 out of 4 points to get credit for this lab report. You do not need perfect scores on each section to get credit for that section. SECTION 1 – Notebook Include all of your notebook pages for this experiment. Each of the following sections will be graded 0/1: Goal Safety information Procedure Data/observations Conclusion If 4 of the 5 notebook parts are successfully completed, you will get credit for this section. SECTION 2 – Data. Answer the following questions. If you get 5 of 7 pieces, you will get credit for this section. Unknown codename__________________________ Mass of unknown____________g 1 ) (2 pieces) Carboxylic Acid identity __________________________ mp after extraction _______ mp after crystallization________ Mass recovered from extraction _______ mass of pure compound _______ % recovery from original mixture________ (show calculations) 2 ) (2 pieces) Amine identity __________________________ mp after extraction _______ Mass recovered from extraction _______ % recovery from original mixture________ (show calculations)
Chemistry 24 1 Lab 1: Acid-Base Extractions and Recrystallization 2023 - 2024 3 ) (2 pieces) Non-Ionizable Compound identity __________________________ mp after extraction _______ mp after crystallization________ Mass recovered from extraction _______ mass of pure compound _______ % recovery from original mixture________ (show calculations) 4 ) (1 piece) Considering the exact mass that you started with, what mass of material was not recovered from the original sample (show calculations): SECTION 3 – Analysis questions. Answer the following questions. If you get 4 of 6 pieces, you will get credit for this section.
- (1 piece) Consider the melting points of the crude and pure neutral compound. Did your recrystallizations improve its purity? Explain your reasoning in 1 sentence. Your explanation should quote specific melting point values.
- (1 piece) Consider the melting points of the crude and pure acidic compound. Did your recrystallizations improve its purity? Explain your reasoning in 1 sentence. Your explanation should quote specific melting point values.