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Unformatted text preview: 1 CHM 2123, Quiz #8: Reactions of Aldehydes and Ketones Under Basic Conditions
Assigned: November 5, 2009 Due: November 9, 2009, before 5pm MARKING SCHEME
1. a) Why is benzaldehyde a better electrophile than acetophenone? (2 points) The benzaldehyde carbon is less sterically encumbered than the acetophenone. Also, acetophenone has a weakly electron-donating methyl group, rendering the carbonyl carbon less electrophillic than the aldehyde. b) When carrying out the aldol condensation between acetophenone and benzaldehyde, a student decides to heat up the reaction flask to make it go faster. When she takes a TLC at the end of the reaction, she notices a second spot with similar polarity to the crossed-aldol product. What could this side-product be? Illustrate with a mechanism (hint: the aldehyde is not involved in this process). (4 points). Self-condensation product of acetophenone. (1 Point) Same mechanism as regular aldol. (2 Points for the mechanism. 1 Point for going all the way to the condensation (alkene) product, vs the regular aldol product)
O O O O Me B- + H+ O Me - H 2O O OH Me Δ Aldol Condensation Product Aldol Reaction Product 2 2. The experimental procedure for the haloform test is relatively experimentally simple. Perform a detailed analysis of this reaction. a) Why do we use an NaOCl solution during the experiment and not an NaOH solution and Cl2? Short answers please! (1 point) NaOH is a strong base and Cl2 is a toxic gas. Bleach is commercially available: way more practical! b) At the end of the reaction, it is suggested to add acetone. Why? Explain with the use of a balanced chemical equation. (2 points) Acetone destroys excess reagents, generating products that are easy to eliminate (AcOH : water soluble; CHCl3 : volatile). Excess acetone is also volatile.
O O + 4 NaOH + 3 Cl2 Na
O + 3 H2O + 3 NaCl + CHCl3 3. Why is it important to make sure your compound is completely dry before taking a melting point? Explain clearly. (1 point). The presence of water or other impurities can lower the melting point of the compound by breaking up the intermolecular bonds between the product molecules. ...
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