Question

Answer ALL parts a)-e). You are given an experimental procedure in which aniline reacts with acetic anhydride to give acetanilide as shown below. The procedure uses 5.9 g of aniline and 6.3 g of acetic anhydride. aniline, M, 93 acetic anhydride, M, 102 acetanilide, M, 135 Calculate which of the reagents is limiting, and hence determine the maximum possible yield of acetanilide in grams. Show your working a) Based on your answer to part a), describe how you would remove the excess reagent from the reaction mixture by solvent extraction. You should explain how the method works. b) c) In the solvent extraction described in part b), how would you determine which is the organic phase? The melting point of acetanilide produced by this method is determined to be 98- 103 °C, compared to a literature value of 114-115 °C d) Explain what can be deduced from this measurement. e) Describe a method for the purification of the product described in part d) explaining how this method leads to purification of the compound.

Answer 1

(a) According to the stoichiometry given, 1 mole of aniline reacts with 1 mole of acetic anhydride to give 1 mole of acetanilide.

Number of moles of aniline used for the experiment= $\frac{5.9}{93}=0.0634\: mol$

Number of moles of acetic anhydride used for the experiment= $\frac{6.3}{102}=0.0618\: mol$

Thus, acetic anhydride is the limiting reagent since it would get consumed completely before aniline.

Now, the maximum possible yield of acetanilide will be obtained when acetic anhydride (limiting reagent) gets consumed completely. In that case,

Number of moles of acetanilide produced = 0.0618 moles= (0.0618 x 135) grams = 8.338 grams

(b) A solvent that dissolved aniline selectively more than it would dissolve acetanilide should be used. The solvent should also have a different density than acetanilide. Add an appropriate amount of extracting solvent in the reaction mixture. Stir the reaction mixture so that aniline gets dissolved in the extracting solvent. Separate the extract and the raffinate using a separating funnel. Repeat the process with fresh solvent to increase the degree of separation.

(c) The organic phase would be insoluble in a polar solvent like water. Hence, the phase that is not miscible with water would be identified as the organic phase.

(d) The acetanilide produced through this process contains some impurities in the form of by-products (Acetic acid) and unreacted reagent (aniline) which have a lower melting point than acetanilide, and thus the product mixture also has a lower melting point as compared to the melting point of acetanilide as mentioned in the literature.

(e) Recrystallization can be used to purify the acetanilide. The product mixture is added to a small amount of solvent such as water or ethanol. The solution is then heated to completely dissolve all the solid. Once the solid mixture is dissolved completely, the solution is cooled down. As it cools down, the compounds start crystallising out according to their solubilities which can then be separated, since they would be in different phases.