Chem. 262 Notes 8

Chem. 262 Notes 8 - 10:1 hexane:ethyl acetate solvent, the...

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Experiment VIII (Week 1) The electrophilic substitution of arenes is one of the most versatile ways of preparing functionalized aromatic compounds. The purity of the product can be ascertained by chromatography. Specifically, thin-layer chromatography (TLC) on silica gel plates provides a simple method for determining the purity of a product. 2. Why do you expect the 4-methyl-2-nitroacetanilide to be the major product? The expected major product is 4-metyl-2-nitroacetanilide because the nitrogen group with a lone pair para to the methyl group has an additional resonance structure where the lone pair on nitrogen pushes down and creates a double bond with the benzene ring. This structure has a complete octet on each atom, causing the preferred structure of the nitrogen oxide group in the ortho position. 3. You are asked to rapidly assay the purity of a sample, but do not have access to instrumentation. You decide to use TLC and melting point to check its purity. With
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Unformatted text preview: 10:1 hexane:ethyl acetate solvent, the sample gives one spot by TLC (Rf = 0.15) and has a melting point range of 115 125 C. (a) Do you think the sample is pure? Why or why not? The sample is not pure because the melting point range is very wide, indicating that impurities exist in the sample. Also, the one spot given by TLC could be a combination of different compounds, which would explain the wide melting point. (b) Using the tools you have at hand (a melting point apparatus and TLC setup), what other data could you collect to test your hypothesis from part a? Give two possibilities. The melting points of the pure substance could be compared to the melting point of the literature value to determine the presence of the starting material in the crude product. Also, a TLC plate could be prepared with the pure product and the crude product to compare the Rf values and resolution points....
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This note was uploaded on 05/20/2011 for the course CHEM 262 taught by Professor Forbes during the Spring '08 term at UNC.

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