Question

Preparation of Benzoic Acid using a Grignard Reagent URGENT

1. During your Grignard formation, a small amount of benzene is formed. Provide a brief explanation and mechanism to explain this observation.

2. During your Grignard formation, a small amount of biphenyl is formed. Provide a brief explanation and mechanism to explain this observation.

3. What mass of water would be required to destroy the phenylmagnesium bromide that you prepared in this experiment? What volume does this represent?

4. Why is the bromobenzene added to your flask in two portions rather than one?

5. If you were to take a TLC of your Grignard solution before you add it to the dry ice, what would your TLC look like?

Lab details

1. The grades for this laboratory session will be determined to a considerable extent by the yield and quality of benzoic acid you produce. You will be supplied with 3.0 mL of bromobenzene. The demonstrators will be required to verify the weight and yield of your preparation. 2. In order to have a chance at a good yield it is necessary for you to work with CLEAN, DRY equipment and use good quality ANHYDROUS DIETHYL ETHER for the reaction. For this reflux, you will need a 50 mL round bottom flask, a condenser, a glass adapter and a drying tube. 3. When you first get to the lab, check your glassware. If dirty, clean and dry it before starting your reaction. Once the glass is dry, place the glassware in an oven for 15 to 20 minutes to remove the surface water layer. DO NOT place glassware upside down. CAUTION: hot glassware is hot. 4. While waiting, prepare the drying tube. The drying tube is prepared by adding a small piece of cotton (or glass wool), then 1-2 cm of CaCl2 and finally another small piece of cotton (see image above). When you remove the glassware from the oven, assemble it immediately and quickly add the drying tube before the glass cools. 5. Preparation of the Grignard Reagent. Place 0.8 g of magnesium turnings and one or two small crystals of iodine in a 50 mL round bottom flask. In a clean DRY beaker, prepare a solution of 3.0 mL of bromobenzene in about 20 mL of ANHYDROUS diethyl ether. Be sure to keep the flask covered to avoid water contamination. Add one-half of this solution to the flask, and quickly re-assemble your reflux apparatus. 6. Swirl gently until the reddish I2 color disappears (indicating that the reaction has begun). Watch for the formation of bubbles rising from the Mg. An exothermic reaction should start within a few minutes that causes the diethyl ether to boil (diethyl ether has a boiling point of 37 °C). If the reaction does not commence within 5 min, carefully break some of the magnesium turnings with a Teflon rod under the surface of the solvent to expose fresh magnesium surface which should help to start the reaction. 7. After the reaction has started, add the remaining bromobenzene solution slowly through the top of the condenser at such a rate that the solvent keeps refluxing gently. SEE FIGURE below to decide whether you should proceed or restart. 8. When the addition is complete, allow the mixture to boil on its own to ensure that the reaction is indeed taking place. As the boiling subsides, use a hot water bath (hot tap water is sufficient) to continue the reflux (boiling) for 20 to 30 minutes. It is very important that the reaction start boiling on its own. Do not warm the flask until this exotherm has subsided. 9. During this period most, but not all, of the magnesium should disappear and your solution will turn brown but remain clear or slightly turbid (cloudy). If your reaction is bright milkywhite, it may have been contaminated with some water. If time permits, you may want to start over (ask your demonstrator to verify your observations). 10. Occasional swirling should help speed up the reaction. At the end of the reflux you should have a dark but clear solution of phenylmagnesium bromide in diethyl ether. 11. Reaction with Carbon Dioxide. Remove the flask from the hot water bath and allow any undissolved residue to settle. 12. Fill a DRY 150 mL beaker approximately 2/3 full of DRY ICE. This ensures you have an excess of dry ice relative to the limiting reagent. Decant the reaction solution carefully with stirring (use a Teflon rod), over a period of 15- 30 seconds. 13. CAUTION: dry ice is very cold (–78 °C) and causes burns if touched for more than a second or two. Never touch dry ice if your fingers are wet, or if there is solvent on the dry ice. 14. Isolation of the Benzoic Acid. To the resulting sticky mass, add approximately 20 – 30 g of ice, 25 mL of 2.5 mol/L HCl, and finally 15 mL ordinary diethyl ether (do not use the anhydrous ether for this). 15. Transfer the mixture to a separatory funnel. Rinse the beaker with 15 mL of ordinary diethyl ether and add the rinsings to the separatory funnel. Shake the mixture and separate the layers, storing them in separate containers. Reextract the aqueous phase with another 15 mL of ether, and combine the organic extracts. 16. Take a TLC of the organic layer, using the starting material as a reference. Solvent system: EtOAc:Hexanes 1:9 17. Extract the combined organic phases twice, each time with 20 mL of aqueous 10 % NaOH solution. 18. Acidify the aqueous extracts by the drop wise addition of conc. HCl until the solution is strongly acidic (litmus) and no more precipitate is formed (adding too much HCl, a huge amount, will eventually cause NaCl to precipitate out.) 19. Cool the mixture in an ice bath and collect the solid product by gravity filtration. Dry your product using suction filtration. 20. Disperse your product on a watch glass or in a clean dry beaker, and allow it to air-dry until the end of the laboratory period. Take the melting point and do a final TLC. 21. Please discard your remaining organic extracts in the containers provided throughout the laboratory for organic waste.

Answer 1