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message7 - From Subject Date To Cc [email protected]

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From: [email protected] Subject: Message #7 Date: February 6, 2009 12:00:31 PM EST To: [email protected], [email protected], [email protected] Cc: [email protected] Sorry for this lengthy message.. * Quiz #1 will be given next week in the lab (25 minutes for 25 points). It will include questions on stoichiometry, TLC, and recrystallization. You really don't need to prepare or study for the quiz. But if you want to practice stoichiometry questions, try those on pp 25 - 28 of the lab manual. Answers to these questions are now posted on the course web site. * I will post five sets of previous 216 exams for the first half of the course this weekend. Answer keys to these exams will also be posted. ==================================================================== Starting next Tuesday, 2/10, most of you will carry out Experiment 3. This esterification reaction requires about 1.5 hrs of refluxing. During this 1.5 hr-reaction time, please go through the pre-lab for Experiment 4 (see the bottom of page 10 of the lab manual). Experiment 3 . Esterification of p-aminobenzoic acid 1. You are going to use a large excess of anhydrous ethanol in order to obtain the ester product in high yield. You should do mole ratio calculations of p-aminobenzoic acid, conc sulfuric acid, and ethanol (for pre-lab). You may assume conc sulfuric acid is essentially 100% sulfuric acid (unlike conc HCl) and its density is 1.84 g/mL. The density of ethanol is 0.789 g/mL. 2. There are two kinds of ethanol in the lab; 95% ethanol and absolute ethanol (i.e., 100% ethanol). Make sure to use the absolute ethanol. You might as well think why it is important to use absolute ethanol rather than 95% ethanol. 3. Even though you need to use only a catalytic amount of conc sulfuric acid for the esterification step, in this experiment we will be using more than one H+ equivalent of conc sulfuric acid. Speculate as to why you need to use more than one H+ equivalent (relative to p-aminobenzoic acid). 4. Please make sure to dissolve p-aminobenzoic acid in ethanol completely before you add conc sulfuric acid. As soon as you add conc sulfuric acid, a white solid should form. This solid is the H2SO4 salt of p-aminobenzoic acid. This solid should eventually go into solution as the ester product is formed. So, keep heating to maintain refluxing of the reaction mixture. You need to make sure to keep the reaction mixture refluxing well. It seems you need to continue good 20 minutes of refluxing after the solid disappears in the reaction mixture.
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