Question

EXPERIMENT #4 – PREPARATION OF o-CHLOROBENZOIC ACID

1. PREPARE A SOLUTION OF CuCl AS FOLLOWS: Dissolve CuSO4∙5H2O (2.34 g) and pure NaCl (0.62 g) in water (8 mL) in a 50 mL Erlenmeyer flask. Warm on a hot plate to dissolve. Add a solution of sodium bisulphite (0.56 g) in water (6 mL) to the hot solution, with constant shaking over a period of about 5 minutes. Cool to room temperature in an ice bath, and decant the supernatant liquid from the colourless CuCl. Wash the precipitate twice by decantation with water containing a little (approximately 1% (v/v) solution) sulphurous acid (HSO3 ‐ + H+ →H2SO3), to prevent oxidation. Dissolve the CuCl in concentrated HCl (4 mL). Stopper the flask and cool in an ice‐salt mixture (0‐5oC) while the diazonium salt is being prepared.

2. PREPARE A DIAZONIUM SALT AS FOLLOWS: In a 125 mL conical flask, dissolve o‐ aminobenzoic acid (anthranilic acid, 0.94 g) in a solution containing conc. HCl (1.4 mL) and water (8.2 mL). Cool the solution to 0 oC. Slowly add a cold solution of NaNO2 (0.46 g) in water (1.6 mL). Do not allow the temperature to rise above 10 oC during this procedure. Add a small amount of urea (a few crystals) to the mixture and cool the resulting solution in an ice bath.

3. PREPARE o-CHLOROBENZOIC ACID AS FOLLOWS: Transfer the cold CuCl solution to a 150 mL beaker. Slowly add, with stirring (use long glass rod), the cold diazonium salt solution to the cold CuCl solution. Watch for foaming. Allow the mixture to stand for 20 minutes at room temperature with frequent stirring. Vacuum filter the precipitated o-chlorobenzoic acid and wash the product with a small amount of cold water.

4. RECRYSTALLIZE AS FOLLOWS: Clean the Buchner funnel and place it along with a 500 mL vacuum flask into the oven at 105 oC. In a 150 mL beaker, add the crude product, a small amount of charcoal, water (~20 mL) and ethanol (~2.6 mL). Heat to a gentle boil, stirring the mixture periodically. When the product has dissolved, quickly filter through the hot Buchner funnel (PRERINSE FILTER PAPER WITH BOILING WATER) in order to remove the charcoal before the o‐chlorobenzoic acid is allowed to 26 crystallize. Allow to cool to room temperature. Vacuum filter the recrystallized product. Dissolve a few crystals of the product in ethanol and spot it on a TLC plate and label it “R”. Likewise dissolve a few crystals of anthranilic acid in ethanol and spot that on a TLC plate as well, labelling it as “SM”. Develop the TLC plate as usual, taking note of the solvent used. Determine the m.p. and percent yield of the dried product next week while doing experiment #5.

If the experiment were to be performed exactly this way, how would one determine the limiting reagent of the reaction for the percent yield calculation? What should be the limiting reagent of this reaction? When I ran this experiment, I was able to form 0.43 g of product.

Thanks for your help!

Answer 1