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10-WilliamsonF07

10-WilliamsonF07 - Application of the SN2 Reaction...

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Application of the S N 2 Reaction: Williamson Ether Synthesis of 2-Ethoxynaphthalene Reading assignment: Fessenden, R.J., Fessenden, J.S., Feist, P. Organic Laboratory Techniques 3 rd ed.; Brooks/Cole: Pacific Grove, 2001, pgs 154-156 (reflux) Introduction An example of a synthetically useful S N 2 reaction is the Williamson ether synthesis. This reaction between an alkoxide ion and an alkyl halide or alkyl tosylate (often primary) is named after Alexander Williamson. The products of the reaction are an ether, ROR’, and an inorganic salt. In lab today we will synthesize the ether 2-ethoxynaphthalene from 2-naphthol, potassium hydroxide and ethyl iodide (figure 1). 2-ethoxynaphthalene, also known as Nerolin, is a commercial odor enhancer used in the perfume industry. OH KOH O K + 2-naphthol CH 3 CH 2 I OCH 2 CH 3 2-ethoxynaphthalene 2-naphthoxide CH 3 OH + KI CH 3 OH (not isolated) Figure 1. The Williamson ether synthesis of 2-ethoxynaphthalene. The alkoxide ion (RO ) has increased nucleophilicity as compared to the parent alcohol (ROH) and therefore increases the rate of this S N 2 reaction. Correct choice of base is important to ensure high yields of the desired alkoxide ion. Aliphatic alcohols such as methanol and ethanol are relatively weak acids (pKa ~15-17) and react completely (high K eq ) only with strong bases. Often, a metal hydride (NaH or KH) or sodium or potassium metal are used for this purpose because H 2 gas is formed as a by-product, which bubbles out of the reaction mixture therefore rendering the reaction irreversible .
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