Aldol Condensation - Lab report PDF

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Aldol Condensation

Writer: Emily Norwood Reviewer: Miklynn Gebhart Editor: Erin Bishop

Introduction: Russian chemist Alexander Borodin discovered the Aldol reaction in 1869 and again by Christian-Adolphe Wurts in 1872. Aldol condensation reactions are used to make carbon-carbon bonds. It makes a nucleophilic reaction between a ketone enolate ion and an aldehyde to form an aldol. Even though this is called a condensation reaction this process does not lose a small molecule like other condensation reactions.1 Acetone has αh y d r o g e n so ne a c hs i d ea n dc a nb ee a s i l yd e p r o t o n a t e dt og i v ea ne n o l a t e a n i o n .Th ea l d e h yd ec a r b o n y li smo r ee l e c t r o p h i l i ct h a nt h a to fak e t o n e , s ot h ea l d e h y d er e a c t s wi t ht hee no l a t e .The alkoxide produced is protonated by NaOH, forming β-hydroxyketone. The β-hydroxyketone then undergoes a base-catalyzed dehydration. The elimination process is particularly fast in this case because the alkene is stabilized by conjugation to not only the carbonyl but also the benzene. Two moles of benzaldehyde are used so that the aldol condensation can occur on both sides of the ketone.3

Figure 1: 2 moles of Benzaldehyde reacting with one mole of acetone in NaOH to dibenzalacetone. Table 1. Reagents Used to Complete an Aldol Condensation Reaction2 Reagent Molecular Boiling Melting Density

Weight

Point ( C) 

Point ( C)  (g/mL)

Acetone Benzaldehyde dibenzalacetone

(g/mol) 58.08 106.12 234.298

56 178.1 130

-95 -26 110-112

0.784 1.04 1.1

Ethanol (EtOH)

39.997

78.37

-114.1

0.789

Sodium Hydroxide (NaOH)

46.07

1338

318

2.13

Experimental: In a small (10mL) Erlenmeyer flask, 0.1 mL of acetone and 3 mL aqueous ethanolic NaOH were combined. After swirling for 3-4 minutes, 0.3 mL of benzaldehyde and a cap was added to let stir for 30 minutes. After 30 minutes of stirring, a yellow precipitate formed. A 25ml side arm flask and a Hirsch funnel was used to vacuum filter the yellow precipitate. During filtration the solid was washed three times with 1-ml portions of cold ethanol. The final wash was tested with litmus paper to determine a nearly neutral pH. If it was not nearly neutral, then more ethanol wash was applied until a neutral pH was reached. The solid was dried and weighed to determine a percent yield and take a melting point.2

Results/ Calculations: Below is the data that was collected during the experiment. Equation 1. Percent yield = Actual yield x100 Theoretical yield Percent yield =

0.073 g ×100=31.2 % 0.234 g

Table 2: Data for Aldol Condensation reaction.

Actual Yield

Percent Yield

Melting Point

Possible Melting Points

From Product

0.073g

31.2

110

Cis, Cis-below 25°C Cis, Trans-60°C Trans, Trans-110-111°C

After using acetone (0.058g) and benzaldehyde (0.212g) to make dibenzalacetone (234g) the actual yield received from the reaction was 0.073g of dibenzalacetone. The molar ratio for each of the reagents and product was calculated to find the theoretical yield. The theoretical yield was found to be 0.234g. By using Equation 1. percent yield was calculated by taking our actual yield=0.073g, dividing it by the theoretical yield=0.234g, multiplied by 100, and found to be 31.2%.

Discussion: In this experiment of Aldol Condensation, one mole of acetone was combined with two moles of benzaldehyde to form one of the three isomers of dibenzalacetone. The precipitate formed during this step was weighed and the melting point was recorded. The melting point that was observed was 110°C which is equal to the Trans, Trans isomer.

Conclusion: This Aldol experiment yielded a melting point of 110°C giving the Trans, Trans isomer of dibenzalacetone. This happened when the enolate ion attacks the benzaldehyde instead of the acetone because there is less steric hinderance. While our product had a melting point consistent

with the isomers melting point given, our percent yield was low. This could be due to unideal lab conditions or product lost during filtration as there was some precipitate in the filtered solution. When preforming the reaction in the future maybe I higher percent yield could be reached by a second filtration of the solution to regain some of the lost product.

Questions: 1). Draw all three possible isomers of dibenzalacetone. Which one is likely the major product and why? The major product would likely be the (E,E) or cis, cis conformation because it is more stable due to less steric hinderance.

2). In the synthesis as described, why does the enolate of acetone react with benzaldehyde rather than another molecule of acetone?

In this reaction the enolate of acetone reacts with benzaldehyde rather than acetone because acetone has more steric hinderance.

3). A student carries out the synthesis but instead of reacting 2 moles of benzaldehyde with 1 mole of acetone, they react 1 mole of benzaldehyde with 2 moles of acetone. What product is most likely to form? Instead of dibenzalacetone forming like the reaction would carry out with 2 moles of benzaldehyde it would form just one benzalacetone.

References: 1. Aldol Condensation https://en.wikipedia.org/wiki/Aldol_condensation (accessed March 9, 2019). 2. Aldol Condensation https://uab.instructure.com/courses/1496160/files/60590724? module_item_id=15007352 (accessed March 9, 2019). 3. Aldol Condensation Reaction http://web.mnstate.edu/jasperse/chem365/Aldol%20Reaction.doc.pdf (accessed March 9, 2019)....