Labs 6-8.docx - Free-radical bromination and E2 reactions...

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Free-radical bromination and E2 reactions with hydrocinnamic acid, followed by analysis of their products Noel Pena, CHEM 301L-008, 17 November 2016 Department of Biology and Chemistry, Liberty University INTRODUCTION This experiment seeks to explore free radical allylic bromination and E2 reactions. Hydrocinnamic acid is reacted in the presence of NBS and other reagents to brominate at the first non-benzylic carbon. KOH and heat cause the brominated acid to undergo an elimination reaction and produce trans-cinnamic acid. 2 Analysis of all products by percent yield, IR, and melting points allow one to indicate that the proper reactions have occurred and produced desired crystals. Recrystallization produces purer crystals that exhibit a narrower melting range. 3 Scheme 1 shows the general reactions taking place throughout the entire experiment. Mechanisms forming the products will be studied in more detail as each part of the experiment is discussed. Scheme 1. Synthesis of brominated hydrocinnamic acid and trans-cinnamic acid. RESULTS AND DISCUSSION Bromination of Hydrocinnamic Acid While stirring the NBS, AIBN, acetic acid, and hydrocinnamic acid solution at room temperature, the solids could be seen only partially dissolved into the liquids, with some clumps and some free-floating particles. The solution turned a clear but bright orange. After the solution was heated via hot water bath, there was a slight lightening of color to yellow-orange and all solids fully dissolved, demonstrating that the desired free radical allylic bromination reaction had taken place. Precipitating the crystals out yielded powdery white solids floating in the water. After reflux and crystal isolation, the crystals had a fine, white powdery appearance, with a few clumps. (KOH) (NBS, AIBN)
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Analysis of Brominated Hydrocinnamic Acid by Melting Point and IR Crystals of the dried brominated product weighed 2.52 grams, and calculations revealed a 78.7% yield from the initial 3.204 grams hydrocinnamic acid used. Atom economy, a measure of how many atoms of reactants have been converted into desired product, was found to be 69.8%, and the reaction efficiency, derived from percent yield and atom economy, was 54.93%. Thus the reaction was successful, but only semi-efficient. The literature melting point of hydrocinnamic acid is 47-50 °C, and that of brominated hydrocinnamic acid is 137°C. The brominated product of the free radical reaction had an observed melting range of 126-131°C; this indicates that all hydrocinnamic acid was fully converted into product and no hydrocinnamic acid was present in
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