Experiment 8 - Reactions of aldehydes and ketones under basic conditions

Experiment 8 - Reactions of aldehydes and ketones under basic conditions

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Experiment 8 A. Crossed Aldol Condensation B. Haloform Reaction Lut Ming Cheng, 4957880, CHM2123, Section C TA: Wendy Campbell Due Date: November 24, 2008 1 – Theory and Mechanism In the case of aldehydes and ketones under basic reactions, the α-proton is resonance stabilized by the delocalization of electrons from the carbonyl group. Thus, the α-proton is slightly acidic compared to other hydrogen atoms in the molecule. When the α-proton is deprotonated by a base, enolate is often formed as an intermediate in the condensation reaction. Enolate is resonance stabilized, and also high in nucleophilicity. Therefore, it can react readily with a wide range of electrophiles. An aldol (β-hydroxyaldehyde or β-hydroxyketone) consists of aldehyde and alcohol functional groups. For aldol reaction, at least one α-proton must be present. For the following elimination reaction, another α-proton must also be available. In the experiment, aldol condensation as follow, CH 3 O + H O NaOH O Acetophenone Benzaldehyde Benzalacetophenone Crossed condensation is expected in the product with a mixture of self product. However, in this experiment, students use acetophenone and benzaldehyde to avoid self- condensation as much as possible. The self-condense of acetophenone is slower than benzaldehyde, therefore cross-condensation can be proceeded to form benzalacetophenone. O H H H Na + O H - O - CH 2 CH 2 - O + H 2 O Na + Na + CH 2 O - H O + O - O OEt H OH O H H Na + O H - O - OH C - O OH H O - OH O enolate aldol
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In the second part of the experiment, haloform reaction is conducted. The brief reaction is as follow, CH 3 O NaOCl O - O Na + + HCCl 3 Hypochlorite is in an equilibrium state. Therefore, it can be converted to Cl 2 and sodium hydroxide. The α-proton of acetophenone is first deprotonated by sodium hydroxide. Cl 2 then attacks the carbocation. The mechanism repeats itself as follow, R O H H Cl + Na + O H - O - H Cl R O C - H Cl R Na + + H 2 O O C - H Cl R Cl Cl O H Cl R Cl + NaCl NaOH, Cl 2 R CCl 3 O Nucleophilic attack from hydroxide to the carbonyl group then takes place to form tetrahedral
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This note was uploaded on 11/13/2010 for the course CHM CHM2132 taught by Professor Goto during the Fall '07 term at University of Ottawa.

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Experiment 8 - Reactions of aldehydes and ketones under basic conditions

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