Chem 35.1 – TEG Espiritu, Walter Aljhon Silong, Rafaelle Tumimbang, Glenn Vincent Experiment 5: Aldol Condensation February 18, 2014 I. Abstract Some organic compounds like ketones, carboxylic acids and esters contain a carbonyl group (C=O). In these compounds, Greek letters are used to approximate each carbon atom to the carbonyl group (Klein, 2012). The labeling starts after the carbonyl group. Also, hydrogen atoms are named with the Greek letter of the carbon in which they are attached, for example for a methyl group in alpha position, there are α-hydrogens. In this experiment, one of the most important reactions at the alpha position is observed, which is aldol condensation. II. Keywords: α-carbon, aldol condensation, nucleophilic addition, base-catalyzed reaction III. Introduction Aldol condensations are important in organic synthesis, providing a good way to form carbon–carbon bonds. The Robinson annulation reaction sequence features an aldol condensation; the Wieland-Miescher ketone product is an important starting material for many organic syntheses. Aldol condensations are also commonly discussed in university level organic chemistry classes as a good bond-forming reaction that demonstrates important reaction mechanisms. In its usual form, it involves the nucleophilic addition of a ketone enolate to an aldehyde to form a βhydroxy ketone, or "aldol" (aldehyde + alcohol), a structural unit found in many naturally occurring molecules and pharmaceuticals. IV. Methodology In a 50mL Erlenmeyer flask, combine 0.80 mL (0.83 g, 8mmol) of benzaldehyde, 0.30 mL acetone and 2 mL 95% ethanol. Then add 2 mL of 10& NaOH and swirl for 20 minutes. After swirling, collect the solid by suction filtration then break up the solid and remove the suction. Add the gathered residue to 3 mL water then apply suction, repeat this process twice. Finally, recrystallize the solid using 95% ethanol and determine its melting point and %yield. V. Results and Discussion In an acidic or basic medium followed by heating, an aldol addition will yield an elimination product, an α,β-unsaturated product. This product is achieved under higher temperature to yield the product. Figure 1. Aldol condensation Condensation is used to refer to any to a reaction in which two molecules undergo addition which is followed by a loss of a small molecule such as water. There are two parts in aldol condensation. The first is the addition of a ketone with either a ketone or an aldehyde (nucleophilic attack), an example is the attack of acetone to benzaldehyde. The product is a protonated alkoxide. The next step is the loss of water molecule. The hydroxyl group is eliminated by the hydroxide ion, resulting to the elimination of water molecule. When the reaction is proceeded under low temperature, the aldol addition product is Chem 35.1 – TEG obtained (β-hydroxy alkane) but has poor yield. In the experiment, acetone is used and the equilibrium does not favor formation of an aldol addition product. On the other hand, in higher temperature, the aldol condensation product is obtained because the equilibrium favors its formation as well as the formation of a conjugated π system. Experiment 5: Aldol Condensation 2. Write the reactions that may take place during the synthesis. How can these reactions be minimized or avoided? VI. Guide Questions 1. Write a mechanism for the base-catalyzed aldol condensation of dibenzalacetone. Do the same for the acid-catalyzed aldol condensation. a. base-catalyzed b. acid-catalyzed One of the side reactions that may occur is the formation of α,β-unsaturated ketone, which limits the concentration of ketone in the solution. To minimize this, one should add enough acetone in the solution. 3. Why is the α-hydrogen in a carbonyl compound more acidic than any other hydrogen in the alkyl chain? The reason is related with the stability of the anion and the possibility of resonance stabilization via delocalization of the negative charge to the more electronegative oxygen atom like in the case of the carbonyl group. In the case of alkyl chain, you cannot delocalize the negative charge of the anion, and it is always "focused" on the carbon atom. In short, the alpha-hydrogen atoms are more acidic because the resulting anion is resonance stabilized. 4. Why is the γ-hydrogen in an α,β-unsaturated carbonyl compound also acidic? Because the conjugated double bonds of the C=C-C=O part of the molecule allow the electron-attracting effect of the carbonyl group to be extended weakening the bond to the gamma-hydrogen. Chem 35.1 – TEG 5. Ketones with non-equivalent α-hydrogens are not usually made to undergo aldol condensation. Explain the rationale behind this. If a ketone is to undergo aldol condensation, there must be a presence of equivalent hydrogen that will both be used to form two products. One of the alpha hydrogen is required in the formation of the enolate and the other is for the dehydration reaction. Experiment 5: Aldol Condensation I hereby certify that I substantially contribute to this report. _____________________ Walter Aljhon Espiritu _____________________ Rafaelle Silong VII. Conclusion and Recommendation Aldol condensation is an important reaction in organic chemistry due to the formation of C-C bonds. The experiment showed that the reaction proceeds in two steps, the first involving the attack of acetone to benzaldehyde which produces an alkoxide. The second part of the reaction is the loss of water molecule. It shows that high temperature favors the formation of the product in the experiment. It is recommended to use pure acetone to have better yield. The use of ethanol must also be kept at a minimal to have better recrystallization of the product. References Klein, D. (2012). Organic Chemistry. Chapter 19. (n.d) Retrieved from: http://cas.umkc.edu/chemistry/Faculty/cours es/Kilway/Ch19.pdf Enols and enolates. (2000) Retrieved from: http://www.mhhe.com/physsci/chemistry/car ey/student/olc/ch18acidity.html Aldol condensation. Retrieved from: http://research.cm.utexas.edu/nbauld/teach/ emphtopB2.html _____________________ Glenn Vincent Tumimbang
Report "CHEM35.1 E5 Aldol Condensation: Synthesis of Dibenzalacetone"