Referring to the procedure for the synthesis of substituted benzoic acids, write

Question

Referring to the procedure for the synthesis of substituted benzoic acids, write balanced chemical equations to explain why: A. HCl is added to the product of the reaction of the Grignard reagent with carbon dioxide. B. The ethereal later is extracted with aqueous sodium hydroxide. gloves when h when handling . Dry ice causes severe burns. Wear insulated protective glo causes severe burns. Wear insulated protective gloves pieces of dry ice. Miniscale Grignard Synthesis of Substituted Benzoic Acids All glassware must be dried in put plastic parts in the oven. Dry a 10-mL round-bottom flask, a 5-mL co cool to room temperature. Assemble glassware as pictured. for at least I hour prior to t 1 hour prior to use. Do no sware must be dried in an oven at 110 C for at least 1 ho bottom flask, a 5-mL conical Claisen adapter. Allow the gla vial. a lassware to Weigh 190 mg of magnesium turnings and place in a dry 10-mL round flask containing a spin bar. Add a tiny crystal of iodine to the flask. In a dry 5-ml.c cal vial, dissolve 900 L of the aryl halide in 4.0 mL of anhydr freshly opened can. Cap the vial. Draw 0.2 mL of this solution into a dry syn add it dropwise through the septum into the flask containing the magnesium and iodin Cloudiness and bubbles at the surface of the metal are indicators that the begun. This may take 5 minutes. If no reaction is observed at the end o warm the flask with the palm of your hand or try adding another crystal of iodine. It reaction still hasn't started, consult the instructor -bottom coni- ous diethyl ether from a syringe and reaction has minutes, Once the reaction has started, add the remainder of the bromobenzene/diethyl ether solution dropwise over 5-10 minutes. Rinse the 5-mL conical vial with 1.2 mL of anlhy drous diethyl ether and add the rinse to the reaction flask. Reflux the reaction mixtu gently, with stirring, in a sand bath or heat block for 15 minutes after the addition of the aryl halide is complete. The final Grignard solution should be cloudy and most of the magnesium should be gone. Cool to room temperature and add more anhydrous diethy ether as necessary to maintain the original volume. The Grignard reagent must be uscd immediately for the next step re Transfer the solution of the Grignard reagent as quickly as possible over about of crushed dry ice in a 25-mL beaker. The dry ice sublimes (goes directly from the so to the gas phase), so weigh it immediately prior to use. Rinse the flask with 2 ml.of diethyl ether and add it to the beaker. Cover the beaker with a watch glass until the excess dry ice has sublimed. slowly that the aqucus dissohel Cool the beaker in an ice bath. Then hydrolvze the carboxylate salt by slo adding 1.6 mL of 6 M HCI to the viscous solid while continually stirring. Wo the hood. Any excess magnesium metal will vigorously react. Magnesium p do not react can be removed from the beaker with tongs. Check the pH of the phase to make sure that the solution is acidic. It is important that all solid be ag disson

Explanation / Answer

Grignard reaction to form benzoic acid

Let ArMgX be the grignard reagent, then,

A. Equation for reaction with HCl,

RCOOMgX + HCl ---> RCOOH + MgXCl

HCl neutralized the RCOO- species to form neutral benzoic acid product.

B. Ether layer is washed with NaOH to convert benzoic acid to water soluble sodium benzoate. The other side product diphenyl remained in ether layer and thus separated from product.

ArCOOH + NaOH --> ArCOONa + H2O

C. Sodium benzoate in aqueous layer is neutralized with HCl to reprecipitate benzoic acid from the solution.

ArCOONa + HCl --> ArCOOH + NaCl

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