Expt. 1, Part B. Recrystallization and Melting Point Determinations.

Procedures

1. Melting Point of 4-Chloroaniline.

Take a few milligrams of your crude sample of 4-chloroaniline obtained from the evaporation of the ether and determine its melting point. Comment on the range of your melting point. Use the weight of your crude sample to determine the approximate percentage of 4-chloroaniline present in the original mixture. Place the remainder of your sample into the "4-chloroaniline collection" bottle provided.

2. Melting Point of p-Dibromobenzene:

Take a few milligrams of your crude sample of 4-dibromobenzene obtained from the evaporation of the ether and determine its melting point. Comment on the range of your melting point. Use the weight of your crude sample to determine the approximate percentage of 4-dibromobenzene present in the original mixture. Place the remainder of your sample into the "4-dibromobenzene collection" bottle provided.

3. Recrystallization of Benzoic Acid:

Before starting the recrystallization, set aside a small amount of crude material for determination of its melting point.

To a 125 mL Erlenmeyer flask containing the isolated benzoic acid, add 7 - 10 mL of cold water and one or two boiling chips. Place the flask on a hot plate and boil the solution gently.

Recall: DO NOT use a Bunsen burner or a free flame of any kind in a laboratory in which flammable solvents are being used.

If the solid does not dissolve completely add hot water in SMALL portions until all the benzoic acid dissolves. After each addition of water bring the solution back to the boiling point; this process takes a lot less time if you already have water at or near the boil in a flask on the hot plate. If the solution is clear, set the flask aside to cool to room temperature slowly for crystallization.

If necessary (eg if your hot solution has pieces of dirt or filter paper fibres floating around in it!) filter the hot solution (Figure 1.1) through a funnel which has been pre-heated on a steam bath into a clean 125 mL Erlenmeyer flask which has been warmed on the hot plate. Ensure that the funnel rests on the lip of the Erlenmeyer flask. Wash out the flask with a further 5-10 mL of hot water, boiling the solution as before, to ensure that no benzoic acid remains in the flask, and filter this second portion into the first filtered solution. You now have a warm aqueous solution of benzoic acid, which must be concentrated by boiling on the hot plate to remove approximately 10 mL of water. This solution should then be removed from the hot plate and set aside to cool to room temperature, allowing crystals of the pure solid to form in the flask.

After crystallization is complete at room temperature, cool your flask in an ice-bath for a few minutes (do this particularly if you have few benzoic acid crystals at room temperature) remove the crystals by vacuum filtration on a Buchner funnel, wash them with a small amount of cold water and dry them on a sheet of filter paper before weighing. Determine its melting point.

Place the remainder of your sample into the "benzoic acid collection" bottle provided.

In your lab book you should:

(i) calculate the approximate percentages of 4-chloroaniline, 4-dibromobenzene, and benzoic acid in the mixture provided

(ii) calculate your percent total recovery of the three components

(iii) give the melting points of crude 4-chloroaniline, p-dibromobenzene, and benzoic acid; the melting point of recrystallized benzoic acid.

Questions

1. The solubility of 4-cresol in water at 25 ^oC is 56.0 g/L and in ether at the same temperature 310.0 g/L. Calculate the value of K_p. If you were given 2.5 g of 4-cresol in 50 mL of water, calculate the weights of p-cresol which would be extracted into 150 mL of ether by a single extraction, and the total weight extracted by three sequential extractions with 50 mL of ether each.

2. Why is sodium benzoate more soluble in H₂O than in ether?

3. By extraction techniques, how would you separate a mixture of phenylacetic acid (pK_a = 4.28), 2-cresol (a phenol of pK_a = 10.2), 2-dimethylbenzene (a neutral compound) and quinoline (a base, pK_a of its conjugate acid 4.90)?

04sep97; wjl