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Question: Could you use IR alone to determine if there was any
biphenyl contaminating your benzoic sample? Explain. What other
spectra test could you run to determine whether or not there was
any biphenyl in your product? Why?
INTRODUCTION: Preparation of Benzoic Acid In organometallic compounds, such as Grignard reagents, the carbon directly attached to the metal has the nature of a carbanion. Organometallic compounds behave as strong bases and, as expected, they react with acids to form a C- H bond. On the other hand, organometallic compounds also react as good nucleophiles especially in addition reactions to compounds with carbonyl groups to form secondary or tertiary alcohols. Since organometallic compounds are also able to react with oxygen, and water in the air, it is important to protect them from air and moisture when they are used or stored. Dens! acid I. Today, you will synthesize a Grignard reagent (phenyl magnesium bromide) and then use that Grignard reagent in a reaction with carbon dioxide to form the desired product, benzoic acid. Notice in the procedure that we are never isolating the Grignard reagent; we must observe its presence before continuing to the second step of the synthesis. Gringard reagent MgBr Mgr Mg metal .COOH 1) CO, (sclid) Rr diethyl ether 35 °C 2) H.O' Electoplih Step 1 Formation of phenyl magnesium bromide Step 2: Formation of benzoic acid Cots Mg Br. Due to the possible formation of side products such as biphenyl, the reaction has to be done at a well-controlled temperature. If any formation of biphenyl occurs, it can be separated from the desired product during the extraction step Mg Er MgBrz biphenyl Potential side reaction The purification process for the final product, often called the reaction work-up, includes the use of acid-base chemistry in combination with liquid-liquid extraction. Review the separation process that was first introduced in experiment 2
glass pipet Mag. Strips 6,170 . 10-3 mol (requiv) Bromobenzene 6.687. 100 mil (requir) CO2 (dryice) 9.089 . 10-2 mol (14,7 equir) water out condenser water in Stir bar clamp stirring plate with 1. heater (add waterbath) Place 0.15 g of shiny magnesium strips in a dry 50 mL round bottom flask. Transfer about 10 mL of anhydrous diethyl ether into the flask, add a magnetic bar and assemble the remainder of the reflux apparatus as shown in the figure below. Although not shown, the round bottom should be placed in a 100 mL beaker water bath to ensure even heating. Place this glassware on a magnetic stirrer equipped with a heater 2. Weigh 0.70 mL of bromobenzene in a 5 mL tared conical vial. 3. Add 4.0 ml. of anhydrous diethyl ether - could be slightly more and dissolve the bromobenzene. Keep this solution capped. 4. Using a Pasteur pipet, transfer 0.8 mL of the bromobenzene solution into the flask with magnesium through the condenser. Start stirring and heating gently the flask. Bubbling should start developing. (Diethyl ether boils at 35 °C, heat the water bath to no more than 50 to 60 °C) *Note: If no visible reaction has occurred add a tiny crystal of iodine. 5. If the reaction has started, you should observe the formation Egyre 1. Reflux apparatus set up brownish-gray, cloudy solution. Keep adding more bromobenzene solution with your pipet for a period of 15 minutes. *Note: If the reaction is too vigorous, slow the addition of bromobenzene and remove the flask from the hot plate. If the volume of the reaction solution gets low or it becomes difficult to stir, add 2 or 3 mL more ether 6. Once most (or all) the magnesium has reacted, cool the reaction to room temperature. Formation of benzoic acid 7. When phenylmagnesium bromide has cooled at room temperature, weigh as quickly as possible about 40 g. of crushed dry ice in a 25 mL beaker. *Note: Dry ice has no need to be weighed precisely. Time is more important here because it condenses water from the air. Water will destroy your phenylmagnesium bromide. Be sure you have everything prepared to proceed with the reaction as soon as you obtain the dry ice from your TA. 8. Transfer with a Pasteur pipet the solution of phenylmagnesium bromide into the beaker with dry ice. Rinse the round bottom flask with 2-3 mL of ether and transfer that into beaker 9. Cover the beaker with a watch glass and let stand until most of dry ice has disappeared. The product will be a viscous glassy mass. 10. Place your reaction beaker in an ice water bath. Slowly add about 10 mL of 6 M hydrochloric acid while stirring the solution *Note: At this point the solution may become a dense liquid or even a solid. Try to dissolve it by adding a bit more ether. If not dissolved, add a little 6 M HCI. The result will be two distint ai a lavere in the hotel 257-
- 7.02 MIS ure UtaACI WIU 110 SUNU CIL, CALEP peinaps a VIL OL liaglicsuIIL 11. Transfer these two layers into a separatory funnel. Discard the aqueous phase and keep the ether layer. ether (organic phase) aqueous phase Figure 2. Separatory funnel with the 12. two immiscible solvents Add to the funnel, 4 mL of 5% NaOH cap the funnel and shake it thoroughly. Let the layers to separate. Separate the aqueous phase and keep it! Benzoic acid is in water now! (why?) 13. Extract from the ether layer again with 4 mL of 5% NaOH, separate phases and combine the two aqueous phases. Repeat this step for a third time, keeping the basic aqueous layer. Discard the ether layer after the third extract. 249.2°C 14. Combined basic aqueous extracts and heat this aqueous phase to the boiling point with stirring for 5 minutes to remove excess of ether. *Note: Ether is partially soluble in water. If any ether is present, the precipitation of benzoic acid will produce a waxy product instead of crystals. 15. Cool the alkaline solution and precipitate the benzoic acid by adding 5 mL of 6M hydrochloric acid with stirring. Cool the mixture in ice bath. 16. Collect the solid by vacuum filtration on a Hirsch funnel. 17. Wash the solid with small portions of ice cold water (only a couple drops needed) 18. Let the solid to dry thoroughly at room temperature at least overnight on a watch glass. ma lelles timely