lab 10 - Experiment #10 The Aldol Condensation The aldol...

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Experiment #10 – The Aldol Condensation The aldol condensation is the name given to the reaction in which the conjugate base of an aldehyde or a ketone (ie. the anion derived by removal of a proton from a carbon adjacent to the carbonyl group) adds to the carbonyl group of either the same species or another aldehyde or ketone. It is one of the most powerful methods available for forming a carbon-carbon bond. A characteristic example, illustrated below, is the aldol condensation of propanal catalyzed by hydroxide ion: Figure 1: Aldol condensation of propanal 1. The base removes a proton from the -carbon to form an anion. Although carbanions are unstable, this one gains significant stability by being a resonance hybrid of two contributors, a carbanion and an enolate ion with the negative charge on oxygen. Figure 2: Carbanion and enolate formation 2. The anion generated in step 1 is a nucleophile, and is capable of added to the carbonyl group of aldehydes and ketones. While either the negatively charged carbon of the carbanion or the negatively charged oxygen of the enolate could, in principle, be the nucleophile, a stronger bond is formed if carbon serves as the nucleophile. In this example, a second molecule of propanal provides the carbonyl group that undergoes nucleophilic attack: Figure 3: Formation of the alkoxide intermediate
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3. The alkoxide ion formed in step 2 abstracts a proton from water to form the neutral product called an aldol for the two functional groups it contains, an aldehyde and an alcohol: Figure 4: Protonation of the alkoxide ion The combination of aldol reactions chosen for this experiment differs from the example used above in two respects. First, it is a mixed aldol condensation between two different carbonyl containing compounds, acetone and benzaldehyde (or a close derivative). In a mixed aldol condensation one molecule provides the carbonyl group and the other the carbanion. Benzaldehyde has no -hydrogens and thus is unable to furnish a stable carbanion, so acetone must be the source of the anion. Since aldehyde carbonyl
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This note was uploaded on 10/05/2011 for the course CHEM 2211 taught by Professor Hubbard during the Fall '07 term at University of Georgia Athens.

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lab 10 - Experiment #10 The Aldol Condensation The aldol...

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