Lab 7 Conclusion Reduction Reactions Rewrite

Lab 7 Conclusion Reduction Reactions Rewrite - Lab...

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Lab Experiment 7 – Synthesis of Alcohols via Reduction Reactions In this reduction reactions lab, I attempted to reduce my carbonyl compound, ‘Unknown A,’ utilizing sodium borohydride reduction, but through post-reaction TLC analyses, realized that my reaction did not work and my compound was Methyl Benzoate, which is not reducible via sodium borohydride reduction. Unknown A was a clear liquid. Because the options for the unknowns were methyl benzoate (liquid), benzaldehyde (liquid), benzoic anhydride (solid), and benzophenone (solid), I knew my unknown was either methyl benzoate or benzaldehyde, both clear liquids at room temperature. Consequently, I knew that I would either see a product of methyl benzoate or benzyl alcohol, the reduced form of benzaldehyde. Sodium borohydride is a mild reducing agent, and can only reduce ketones and aldehydes, but not esters (such as methyl benzoate). In contrast, lithium aluminum hydride (LAH) is a stronger reducing agent than sodium borohydride, and will also reduce esters, amides, or carboxylic acids. LAH is a stronger reducing agent than sodium borohydride because the B-H bond is weaker than the Al-H bond. Using sodium borohydride is much better for the purposes of our reaction in determining our unknowns, so that we can distinguish compounds based on their functional group (aldehyde or ketone vs. ester). From TLC I could get a good idea of what my unknown A was. I spotted my product, benzyl alcohol (reduction product), methyl benzoate, and benzaldehyde. The R F values were calculated after the TLC procedure finished, and were as follows: Product, 0.53; benzyl alcohol, 0.04; methyl benzoate, 0.49; benzaldehyde, 0.39. From the TLC data, the product had an R F closest to that of the methyl benzoate standard, hinting that the product could be methyl benzoate. The product was not benzyl alcohol (R F of 0.04), which had a lower R F than the product, at 0.53. However, benzaldehyde had an R F of 0.39, which is not too far off from 0.53 (product R F ), indicating that the product has a similar polarity to benzaldehyde and there may even be benzaldehyde in the product. Though TLC hints as to what the product may be, it does not provide absolute evidence to what the product actually is because it only demonstrates polarities and purity, and different compounds could have similar polarities and look the same on TLC. Using TLC, however, we can understand the relative polarities of the compounds we analyzed. Because more polar compounds react with the silica plate more, we expect more polar products to not run as high on the TLC plate, or have a lower R F value. On the other hand, the more nonpolar products should have a higher R F value, and run higher up on the plate more towards the solvent front. Based on the R F values I received, the order of increasing polarity of the compounds tested via TLC was: Product < methyl benzoate < benzaldehyde < benzyl alcohol. The relative polarity of these compounds by TLC makes sense due to the compounds’
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This note was uploaded on 02/04/2012 for the course CHEM 310 taught by Professor Smith during the Spring '11 term at University of Texas at Dallas, Richardson.

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Lab 7 Conclusion Reduction Reactions Rewrite - Lab...

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