Synthesis of Benzoic Acid

Transcription

1 E x p e r i m e n t 5 Synthesis of Benzoic Acid Objectives To use the Grignard reagent in a water free environment. To react the Grignard reagent with dry ice, CO 2(s). To assess the purity of the product by determining its melting point. To determine the molar mass of the product via titration with standardized NaOH. In the Lab Students work in pairs After Lab Complete the Chem21 assignment Waste Place aqueous solutions (from the separatory funnel and filter flask) in the waste container labeled Lab 5 - Aqueous Waste located in the Instructor s hood. Place organic solutions (ether from separatory funnel) in the waste container labeled Organic Waste located in the Instructor s hood. Safety Students must wear goggles for this experiment. Students must wear gloves when handling the Grignard reagent.

2 Mechanism Uses of Benzoic acid Benzoic acid (as its sodium salt) is used as a preservative in foods, fats, fruit drinks, and oral pharmaceutical liquids. Check the ingredient list on your favorite snacks. The antibacterial and antifungal properties are also utilized to retard plant and fungal growth on the soles of feet. It has found use in the curing and seasoning of tobacco, as a plasticizer in the polymer industry, in the flavor and perfume industry, in dentifrice (tooth paste), as an analytical chemistry standard, and to control black rot on pineapples. Finally, benzoic acid (and its derivative benzoyl chloride and benzoyl peroxide) along with salicylic acid have proven to be effective agents in combating acne and other dermatological problems. Table of Physical Constants Chemical Name Chemical Formula Molecular Weight Melting Point Boiling Point Density 20 n D Carbon Dioxide CO Hydrochloric Acid HCl Sodium hydroxide NaOH Benzoic Acid C 6 H 5 CO 2 H Ethanol C 2 H 5 OH Ether (C 2 H 5 ) 2 O Table 1

3 Day 1 Making Benzoic acid 1. Place g crushed dry ice in a dry 600 ml beaker and immediately add 20 ml of the Grignard Reagent to the dry ice. 2. Add 10 ml ether to the graduated cylinder, swirl, and add the ether to the 600 ml beaker. 3. Stir thoroughly with a glass stirring rod for 2 minutes. 4. Prepare 20 ml of cold 3 M HCl by adding 10 ml 6 M HCl to 10 g of ice. Once the dry ice has sublimed away, add 5 ml ether. Add 1 ml cold 3 M HCl to the beaker. Continue to add the cold 3 M HCl 1 ml at time until you have added all 20 ml. The HCl will protonate the Grignard addition product, C 6 H 5 COO - MgBr +, and produce benzoic acid. 5. Place this beaker in the back of the hood, cover it with a watch glass and label it as Lab 5 / Your Name. Isolating the Benzoic acid 6. If there are two distinct layers in the beaker, transfer both layers to a separatory funnel. However, if any solids remain, add 5 ml 3M HCl and 5 ml ether. Set the separatory funnel in a ring on a ring stand. Make sure the stopcock is closed before pouring in the solution! Use a stopper that fits snugly and doesn t leak. Make sure the funnel is pointed away from people when it is vented. When draining, the stopper must be out, or it will not drain. Always keep all layers in labeled beakers until the end of lab! 7. Remove the lower aqueous layer, place it in a labeled beaker (discard it at the end of lab). 8. Add 15 ml 5% sodium hydroxide solution to the separatory funnel and extract the ether layer. This is done by shaking the funnel well, being sure to vent with stopcock a few times in between shaking. (This removes benzoic acid [YOUR PRODUCT] from ether layer and causes it to go into the aqueous layer). 9. After shaking the funnel, allow the layers to separate completely and then drain off the lower layer (basic aqueous layer) into a labeled 250-mL beaker. 10. Repeat steps 8 & 9 two more times and combine these aqueous layers in the labeled beaker you will have a beaker that contains ~ 45 ml of an aqueous solution of sodium benzoate. 11. Discard the ether layer remaining in the separatory funnel into the appropriately labeled waste container in the hood (this layer contains the byproduct biphenyl). Day Add two boiling stones to the graduated 50 ml beaker and note (i.e. write it down) the volume of the liquid in the beaker (read the volume markings on the beaker). Use a hot plate in the hood to evaporate off any dissolved ether. This is accomplished by removing 10% of the volume - if the volume was 50 ml, evaporate 5 ml. Swirl the beaker 3

4 continually during this process to avoid spattering the boiling liquid and loss of product. 13. Allow the remaining alkaline solution to cool to room temperature. 14. Precipitate the benzoic acid by slowly adding 5 ml of conc HCl hydrochloric acid to the solution over 2 minutes. Swirl the beaker to mix the contents. If a large amount of crystals do not form, add another ml of conc HCl. 15. Place the beaker / crystals in an ice bath for 5 minutes to allow complete crystallization. During this time, obtain 50 ml of ice-cold water. 16. Collect the crystals by vacuum filtration on a Büchner funnel. Place the Büchner funnel with its rubber stopper on top of the filter flask. Secure the top of the filter flask with a clamp. Attach the hose from the vacuum trap to the side of the filter flask. Place an appropriately sized filter paper in the Büchner funnel and turn the vacuum on. Transfer the crystals to the filter paper - use 15 ml of the 50 ml cold water to aid in the transfer. As soon as the filtrate has collected in the filter flask, rinse the crystals with the remaining 35 ml of cold water. 17. Allow the crystals to air-dry for 3 minutes. 18. Place the crystals on a labeled, weighed watch glass and place them in the 50ºC oven for 20 minutes and then place in your lab drawer. Day 3 Analyzing the Benzoic acid 19. Determine the mass of the dry crystals. 20. Pack a melting point tube containing your Benzoic Acid and one containing Pure Benzoic Acid (get this from your instructor). 21. At the same time, determine the melting points of these two Benzoic acids. 22. Place the used melting point tubes in the glass trash. 23. Place the Benzoic Acid you synthesized in a labeled vial (use the label at the end of this lab guide) and turn in this vial with the Instructor Data Sheet. The next part of the lab will be performed with Benzoic acid obtained from the Instructor. Standardizing the NaOH solution 24. Obtain ~250 ml of a sodium hydroxide solution that is ~ 0.05 M in a clean beaker. Label the beaker either mark on the beaker or write NaOH on a piece of paper and set the beaker on top of the paper. 25. Obtain a clean buret and buret clamp. Make sure the stopcock on the buret is closed. 26. Add ~ 5 ml of the NaOH solution (from Step 24) to the buret (rotate the buret to wash down the sides with this NaOH solution) and empty it into a beaker designated for waste. 27. Repeat Step 26. 4

5 28. Fill the buret to the top with the NaOH solution. Place the beaker containing the waste NaOH under the stopcock and fully open the stopcock allowing ~1 ml of base to exit. Repeat opening and closing the stopcock until no more air bubbles exit the tip of the buret. Remove Air Bubble In Buret Tip. 33. Record the volume of NaOH in the buret [Data Sheet] and begin to slowly add the base to Flask As base drops onto the acid solution, a pink color appears that rapidly disappears with stirring. As time passes, the color remains longer you should add the base dropwise at this point. Eventually, one drop of base will change the colorless solution to a persistent (for at least 1 minute) light pink. Light pink for 1 minute 29. Obtain three 200 (or 250 or 300) ml Erlenmeyer flasks. Tare a weighing paper (make sure the balance is not fluctuating due to air currents). Using a spatula, place g Potassium hydrogen phthalate (KHP) on the weighing paper and close the lid to the balance to minimize air currents. Record exactly the mass of KHP [Data Sheet]. 30. Add the KHP, ml of distilled water, and three drops of the phenolphthalein indicator to Flask Place a stir bar in the flask and place the flask on a stirring plate. 32. Adjust the buret so that it is directly above the flask and the stirring plate. 35. Record the final volume of base used. 36. Empty the contents of the flask in the sink and rinse the flash with water. 37. Repeat Steps for Flasks 2 & 3 record the data. Titrating the Benzoic acid 38. Accurately weigh (to three significant figures) approximately 0.2 g of Benzoic acid (obtained from your Instructor) using weighing paper [Data Sheet]. 5

6 39. Place the weighed Benzoic acid into the Erlenmeyer flasks used earlier (it is fine if the flasks contain some water droplets). 40. Dissolve the acid in each flask with 50 ml of 50% aqueous ethanol (the Benzoic acid does not need to completely dissolve since it will dissolve as it is titrated). 41. Add 3 drops of a phenolphthalein indicator. 42. Record the starting buret volume [Data Sheet]. 43. Titrate the Benzoic acid solution to a light pink endpoint (pink color should persist for 1 minute to allow any undissolved benzoic acid to completely dissolve). 44. Record the final buret volume [Data Sheet]. 45. Repeat for Flasks 2 & 3 record the data. 46. Calculate the molar mass of benzoic acid for each titration [Online Lab Sheet]. The equation below describes how the molar mass is calculated if the Molarity of the NaOH solution is 0.05 M. 47. Determine the average molar mass of Benzoic acid [Online Lab Sheet]. Lab Report Once you have turned in your Instructor Data Sheet, lab attendance will be entered and you will have access to enter your lab data online and begin the lab submission process. Enter you lab data before exiting the lab. ( grams of acid) MM Benzoic Acid = 0.05 moles NaOH 1 mole acid liters base used 1 liter solution 1 mole NaOH Name: Mass: Lab 5 Benzoic acid g 6

7 Synthesis of Benzoic Acid Student Data Sheet Mass of Benzoic Acid obtained g Titration 1 Titration 2 Titration 3 KHP (g) Titration 4 Titration 5 Titration 6 Benzoic Acid (g) Melting Point Range of Your Benzoic Acid - ºC Melting Point Range of pure Benzoic Acid ºC Name: Partner: Synthesis of Benzoic Acid Instructor Data Sheet Mass of Benzoic Acid obtained g Titration 1 Titration 2 Titration 3 KHP (g) Titration 4 Titration 5 Titration 6 Benzoic Acid (g) Melting Point Range of Your Benzoic Acid - ºC Melting Point Range of pure Benzoic Acid ºC 7