Aldo Condensation

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Date: 19th February 2011

Topic: Lab report 03

Synthesis of organic Sun Screen

Introduction:

Aldol Condensation, a useful carbon-carbon bond-forming is the an example of a reaction named based on the type of product formed. This product formed involves two aldehydes or ketones in the presence of dilute base, yields a molecule having both aldehyde and alcohol. The aldol products are β-hydroxyaldehydes or β- hydroxyketones. Condensation reaction is the transformation that result from combination of large molecules to produce another larger molecule. In an aldol condensation, β-hydroxyaldehyde or a β-hydroxyketone is produced in but upon heating the α,β-unsaturated aldehyde or ketone with water is produced. In double mixed aldo condensation of acetone and benzaldehyde, the α-hydrogen’s on both sides of the acetone deprotonates and a nucleophilic enolate anion arises as a result. In an aldol condensation many products are formed there reactant are more than one thus the aldehyde reacts with itself to yield a product. The product formed in an aldol condensation contains and adehydes and an alcohol. Claisen-Schmidt condensation is a crossed aldol condensation that involves an aromatic aldehydes dehydration to yield double bond conjugated aromatic ring and the carbonyl group. Aldol products can dehydrate under either acidic or basic conditions to give, α, β-unsaturated aldehydes and ketones. Although there is even balance in the equilibrium in aldo condensation, the dehydration is usually exothermic. (Wade, 1998) In crossed aldol condensation careful selection of compound is important to avoid unwanted product mixtures. This is because a crossed aldol condensations is most effective if only one of the reactants can form an enolate ion and the other reactant is more electrophilic toward enolates. Thus Benzaldehyde is a good reagent for crossed aldols because it has no α-hydrogens, so it cannot form an enolate. In this experiment, acetone was used as the enolate-forming compound, adding to benzaldehyde followed by dehydration to form a benzal group. Acetone has enolizable α-hydrogens on both sides of the carbonyl group that allows it to add to two molecules of benzaldehyde (Wade, 1998). The ability of dibenzalacetone to absorbs sunlight and UV rays is a result of conjugated molecule thus this conjugation allows it to absorb sunlight when in sunscreen. Therefore preventing sun burns from occurring before it reaches the body. The purpose of this experiment was to synthesisd dibenzalacetone a yellow precipitate, using a base catalyst to condense acetones with benzaldehyde. The final product of the synthesized benzalacetone after filtration and weighing was 0.442g, the theoretical yield was 0.233g and the percent yield was 190%.

Chemical Structures and Equation:

Figure 1: The equation for the synthesis of dibenzalacetone from acetone and benzaldehyde using a base catalyst.

Figure 2: Mechanism for the aldo condensation of dibenzalacetone using of based catalysed reaction to condense acetone with benzaldehyde.

Table 1: Chemical Structures used in the synthesis of dibenzalacetone by aldol condensation of acetone with benzaldehyde using a base catalyst | | | | | |

Equation 1: Percent yield

(Actual yield/ Theoretical yield) * 100= percent yield

Material and Methods:
Weighed a clean empty test tube, added 2mL of 3M NaOH and 1.6mL of 95% of ethanol. Added 0.212g of benzaldehyde and 0.058g of acetone, to do this zero the test tube and add the acetone from the bottle. Capped the tube immediately with a septum, and shake the mixture vigorously for 30 minutes. The initial insoluble benzaldehyde, goes into solution, and a water clear, pale...
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