Chemistry of Amides

This exercise investigates the acid-base properties of amides and the products formed by hydrolysis of amides under either acidic or basic conditions (technically "saponification"). In the first part, students place small scoops of acetamide and benzamide in separate tubes and note physical properties. Solubility and reaction with litmus are tested after adding two dropperfuls of water to each tube.

In the second part, 0.2 g (a small scoop) of acetamide is dissolved in 1 mL of 3 M HCl and heated in a boiling water bath. After 10 minutes, students note the odor of vinegar coming from the tube and test the vapors with moist blue litmus paper. They then add 1.5 mL of 3 M NaOH and return the tube to the boiling water bath.  After two minutes they note the odor of ammonia coming from the tube and test the vapors with moist red litmus paper.

In a separate tube, 0.2 g (a small scoop) of benzamide is suspended in 1 mL of 3 M NaOH and heated in a boiling water bath. The initial insolubility of benzamide is noted. After 10 minutes, vapors from the tube are smelled and tested with moist red litmus paper to identify the evolved ammonia. Distilled water is added as needed to keep it from going dry. After about a half hour, the tube is cooled and it is noted that the solution is clear. Concentrated HCl is added (7-10 drops) and the precipitation of benzoic acid is noted.  (No smelling here!)

In their write-up, students are asked to compare the acid/base behavior of amines and amides and to identify the vapors produced by the acid and base hydrolysis reactions. They are to write equations for the reactions involving (1) acetamide, water, and HCl, (2) ammonium ion and NaOH, (3) benzamide and NaOH, and (4) benzoate ion and HCl. In each case, they are to name reactants and products and indicate whether each reactant and product is soluble in water; for the acid-base reactions, they are to label reactants as acid or base.

An important take-home lesson for this exercise is, as in the acid-base exercise, the relation between observable changes in the materials being examined on the one hand, and chemical changes and the equations that describe them on the other hand.

 

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Revised 9/11/06