Indigo lab (Aug 9_10) - Aldol Condensation Synthesis of...

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Aldol Condensation: Synthesis of Indigo. Vat Dyeing. An extremely important class of carbon-carbon bond forming reactions are those which involve reaction of a compound which has relatively acidic hydrogens (i.e., alpha hydrogens on an aldehyde, ketone, ester, nitrile, or nitro compound) with a compound having a carbonyl group. When the reactants involved are either aldehydes or ketones, the reaction is called an “aldol condensation.” For example, two molecules of acetaldehyde condense to form the four-carbon compound known as aldol: The above reaction may be either acid or base catalyzed; however, base catalysts are generally preferred. The mechanism of the base catalyzed condensation reaction of acetaldehyde to form aldol is given below. The first step is removal of an alpha proton to give a resonance-stabilized enolate anion. In the second step, the enolate anion attacks the carbonyl carbon of a second molecule of acetaldehyde to form a carbon-carbon bond and an alkoxide anion. In the third step, the alkoxide anion abstracts a proton from water to regenerate the hydroxide catalyst. Since all of these steps are reversible, a successful synthesis is contingent on the product being more stable than the reactants, or on some technique for driving the equilibrium towards products. An aldol condensation between two different aldehydes or ketones is called a crossed- aldol condensation . Such condensations generally result in an undesirable mixture of products, although in some cases a single product may be obtained in good yield. An example of this is the crossed aldol condensation between acetone and benzaldehyde to give dibenzalacetone.
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Crossed condensation is successful in this case because benzaldehyde has no α-protons, so it cannot undergo condensation with itself and because the carbonyl group of benzaldehyde is far more reactive than that of acetone. As is generally true of aldol condensations involving aromatic aldehydes, the aldol product readily undergoes dehydration to give an α,β-unsaturated carbonyl compound in which the double bond formed is conjugated with both the carbonyl group and the aromatic ring. This step is
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