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TECHNIQUES IN ORGANIC CHEMISTRY, 2 ND EDITION ANSWERS FOR QUESTIONS PART 1—BASIC TECHNIQUES Technique 6 Heating and Cooling Methods 1. First, the student should remove the heat and allow the flask to cool down below the boiling point of the liquid. If a boiling stick or boiling stones are added to the superheated liquid, the liquid could erupt, possibly causing a serious accident. 2. A heating mantle does not have a voltage control knob. Using it without a variable transformer would mean there was no way to reduce the line voltage to control the rate of heating. 3. A quantity of steam at 100 ° C has much more energy than the same quantity of water at 100 ° C. 4. Because Denver, Colorado is at a much higher elevation than New York City, the atmospheric pressure at Denver is lower and water boils at a lower temperature. So, in Denver the temperature of the steam bath will be lower and the rate of the reaction will be slower. Technique 8 Extraction and Drying Agents 1. Diethyl ether is less dense than water, so ether forms the upper layer of the extraction. Adding ether to an ether layer will of course produce only one phase. 2. The best way to remove acetic acid from the ether solution is to convert the acetic acid to its conjugate base, the acetate anion, by extracting the ether solution with a 5% NaHCO 3 solution. Sodium acetate is very soluble in water but not soluble in diethyl ether, whereas acetic acid itself is soluble both in water and in diethyl ether. 3. Reaction of Na 2 CO 3 with acid produces gaseous CO 2 by an acid-base reaction. Unless the funnel is vented periodically, the gas will build up pressure in the separatory funnel and could blow out the stopper, allowing the liquids in the funnel to spray out into the laboratory. 4. The easiest way would be to turn to Table 8.1 on page 78 and read the density of diethyl ether in the table of common extraction solvents. One could also remove a few drops of the questionable liquid and, using a small test tube, ascertain if it is soluble in water or not. -1-
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5. After the first 20-mL diethyl ether extraction, the calculation (10 = 0.30 – x/20 divided by x/100) shows that 0.100 g of benzoic acid would remain in the water. After two, three, and four extractions, 0.033 g, 0.0011 g, and 0.0037 g of benzoic acid would remain in the water layer. After one 80-mL extraction with ether, 0.033 g of benzoic acid would remain in the water. Clearly, the four 20-mL extractions with diethyl ether are more efficient than one 80-mL extraction (some 11 times more efficient) in removing the benzoic acid from water. 6. Potassium carbonate would be a better choice as a drying agent for an amine. If it were used with a carboxylic acid, potassium carbonate would also act as a base and deprotonate the acid. Technique 9
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This note was uploaded on 04/11/2010 for the course OCHEM 140A OCHEM 140A taught by Professor Ternansky during the Winter '10 term at UCSD.

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