EXPERIMENT 3. Copper: Its Chemical Transformations. Preparation of Copper(II) Compounds with Glycine. Figure 1. Peptide Structure

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1 EXPERIMENT Introduction Copper: Its Chemical Transformations. Preparation of Copper(II) Compounds with Glycine Copper, second in commercial importance only to iron, was one of the first metals to be isolated from its ores. The process of obtaining pure copper from its ores was known as early as 4500 B.C. It is a ductile, malleable metal, being easily pounded into various shapes for use as wire, ornaments, and implements of various types. Alloys of copper (bronze, an alloy with tin and brass, an alloy with zinc) were discovered quite early in history. Pure copper is the best electrical conductor of the more abundant metals. It has good thermal conductivity and is corrosion- resistant. This experiment deals with the solution chemistry of copper, and the formation of a complex or coordination compound with glycine. Glycine is a member of a biologically important group of compounds called amino acids. Amino acids consist of at least one amino group (NH 2 ) and a carboxylic acid group (-COOH). They are the basic structural units of proteins and therefore constitute the building blocks of living cells. Proteins are polypeptides or polyamides formed by the repeated linking of the -NH 2 group of one amino acid to the -COOH group of another: Figure 1. Peptide Structure Proteins play extremely diverse roles in human biochemistry. One class of proteins, the enzymes, functions as catalysts for various biological reactions. A simple protein, aspartame (NutraSweet L-aspartyl-L-phenylalanine methyl ester, (Fig 2) is increasingly used as an artificial sweetener. The hydrolysis of a typical protein produces as many as 20 different amino acids. The simplest amino acid is glycine, H 2 NCH 2 COOH. It exists as a zwitterion (a covalent compound containing a separated positive and negative charge): H 3 N + CH 2 COO -. Figure 2. Aspartame

2 3-2 Glycine contains two atoms capable of donating a pair of electrons (the N of the amine group and an O of the carboxylic acid group) and can simultaneously form two bonds with copper(ii) cation. It therefore functions as a chelating ligand (from the Greek word chele for claw) and results in the formation of Cu(gly) 2. This compound exists in two isomeric forms: the cis isomer (like donor atoms on the same side of the copper) and the trans isomer (like donor atoms on opposite sides (Figure 3a and 3b). Figure 3. (a) cis-bis(glycinato)copper(ii) (b) trans-bis(glycinato)copper(ii). In this experiment, a given amount of copper metal is transformed through a series of reactions into the two isomeric copper complexes. Cu(s) + HNO 3 (aq) Cu(NO 3 ) 2 (aq) + N0 2 (g) + H 2 O(l) (l) Cu(NO 3 ) 2 (aq) + 2NaOH(aq) Cu(OH) 2 (s) + 2 NaNO 3 (aq) (2) Cu(OH) 2 (s) CuO(s) + H 2 O(l) (3) CuO(s) + CH 3 COOH(aq) Cu(CH 3 COO) 2 (aq) + H 2 O(l) (4) Cu(CH 3 COO) 2 (aq) + 2H 2 NCH 2 CO 2 H(aq) Cu(H 2 NCH 2 CO 2 ) 2 + 2CH 3 COOH (5) The reactions involved are acid-base (with gas formation), decomposition, complex formation, and oxidation-reduction. The sequence of reactions also shows the wide variety of colours often observed for inorganic compounds. Materials Reagents: Cooper wire 6M nitric acid 6M sodium hydroxide solution 6M acetic acid solution 95% Ethanol Glycine

3 Glassware: 3-3 Equipment: 50 ml Erlenmeyer flask 1 x 25 ml Beaker 2 x 100 ml Beaker 2 ml, 5 ml Pipettes Pasteur pipettes 10 ml Graduated cylinder Glass rod Wide-stemmed filter funnel Clock glass Buchner funnel Capillary tubes for melting point determination FT-IR instrument Melting point apparatus Thermometer Stir/Hotplate Stirring plate Stirring bar Weighing dish Spatula Whatman filter paper ( 9 cm, 42.5 mm ) Clamps Rubber tubing Stand Experimental Procedure Note: This experiment is fairly long, especially towards the end. Be sure be well organized, and to work quickly. Note down your observations. Conversion of Metallic Copper to Copper(ll) Acetate Weigh (accurately, so that you can calculate your yield later) approximately 250 mg of copper wire cut into small pieces and place it in a 50 ml Erlenmeyer flask. Working in a fumehood, add 4 ml of 6M nitric acid to the flask and heat on the hot hotplate until all the wire has dissolved. (If necessary only, add 1 to 2 ml of additional acid) Remove the flask from the hot plate, and cool it, and its contents, carefully under running water until it is only warm. Then add 10 ml of distilled water and a stirring bar.

4 With constant stirring on the stirring plate to avoid bumping, add 6M sodium hydroxide solution dropwise to the solution until it is basic ( ph ~ 10) to universal test paper. (Use a glass rod which has been dipped into the solution for this test: do not put the test paper into the solution.) A precipitate of copper(ii) hydroxide is formed. Add an additional 0.5 ml of the sodium hydroxide solution to ensure complete precipitation. Place the flask back on the hotplate, and heat with stirring, to the boiling point. The precipitate will be converted to black copper(ii) oxide. Continue the heating for an additional 5 minutes to coagulate the precipitate. Remove the flask from the heat and cool it under running water until it is only warm. Filter the slurry by gravity using a wide-stemmed filter funnel. Rinse out the flask and stirring bar with 1 to 2 ml of distilled water. Discard the filtrate, leave the precipitate on the filter paper in the funnel. In a 25 ml beaker containing a stirring bar, heat 10 ml of 6M acetic acid on the hot hotplate. Put the Erlenmeyer flask back under the filter funnel. Using a Pasteur pipette, begin to transfer the hot acetic acid to the filter. The copper oxide will begin to dissolve. Make a small hole in the filter paper with a spatula to allow the acid and precipitate to wash through. Use the pipette recycle the acid back onto the filter paper until all the undissolved copper oxide has been washed into Erlenmeyer flask. Finally, use 2 to 3 ml of hot distilled water to wash the filter paper into the flask. If necessary heat the contents of the flask on the hot plate to complete the dissolution of the copper oxide. Divide the solution into two equal parts and use one portion for the preparation of the glycinato complex. (The other portion will not be needed unless something goes wrong) Each portion should be about 15 ml. 3-4 Preparation of the Glycinato Complexes To a 100 ml beaker, add one of the two portions collected in the previous step, and then add about 2 ml of 95% ethanol. Heat gently on the hot plate, with stirring, to a constant temperature of ~70 o. Weigh out approximately g of glycine and add it to the solution. Continue heating for about 6 minutes. Remove the solution from the heat and allow it to cool. Upon cooling the cis-bis(glycinato)copper(ii) will begin to precipitate as the hydrate, Cu(H 2 NCH 2 CO 2 ) 2.xH 2 O. Complete this process by cooling the beaker in ice for at least 30 minutes.

5 3-5 Buchner funnel Water aspirator Sidearm Clamp Filter flask Filter trap Stand Thick-walled vacuum tubbing Vacuum filter the product using a Buchner funnel (see diagram above) and wash the precipitate on the filter using a Pasteur pipette with about 10 ml of 95% ethanol. Continue to draw air through the filter, allowing the product to dry on the filter paper for half an hour before scraping it off onto a tared* weighing dish which you will re-weigh in order to determine the yield. Obtain the infrared spectrum of the product, and its melting point. Tare a clock glass. Place about half of the cis-product on it, and re-weigh it. Heat the sample in an oven set at 220 o C for 10 to 15 minutes to convert it to the anhydrous transproduct. Allow the watch glass to cool to room temperature and weigh it once again in order to determine the amount of water that has been lost. Obtain the infrared spectrum and melting point of the trans-product. * This means put the weighing dish on the balance, and zero it by pressing the tare button. The next weighing will give the mass of product on the dish, provided the balance has not zeroed again in the meantime!

6 Experiment W3 - Report 3-6 Name Date Results Initial mass of copper wire: Colour of the NO 2 evolved when Cu dissolves in HNO 3 : Colour of the solution of copper(ii) nitrate: Colour of the copper(ii) hydroxide precipitate: Colour of the copper(ii) acetate solution: Colour of the cis-glycinato complex: Mass of cis-glycinato complex: Percentage yield (show your calculation): Melting point of cis-glycinato complex: Mass of the cis-glycinato complex used to make the trans-product: Colour of trans-glycinato complex: Mass of the (anhydrous) trans-glycinato complex obtained: Value of x in cis-cu(h 2 NCH 2 CO 2 ) 2.xH 2 O (show your calculation): Melting point of trans-glycinato complex: Attach the IR s of the cis- and trans- complexes Questions: 1. Balance equation (1) in the introduction.

7 2. Write a balanced equation for the reaction of copper with nitric acid to produce nitric oxide (NO). Predict whether this reaction would occur with more, or less concentrated nitric acid than equation (1) How many isomers might you expect to make if you did the analogous preparation with racemic DL-alanine? Sketch them. (Actually, it would not be possible to separate them all because of rapid ligand exchange.)