Materials Included In Kit

Additional Materials Required

Safety Precautions

Benzaldehyde is a mildly toxic irritant and may catch fire when heated. Acetone is a flammable liquid and mildly toxic by ingestion and inhalation. Keep away from open flames and sparks. Sodium hydroxide is a corrosive solid; skin burns are possible. Considerable heat is evolved when sodium hydroxide pellets are added to water. It is very dangerous to eyes; wear eye protection plus gloves when handling and using sodium hydroxide. Ethanol is a flammable liquid and a dangerous fire risk. Addition of denaturant makes the product poisonous—it cannot be made nonpoisonous. Dibenzalacetone is considered nonhazardous according to GHS classifications, however unpredictable reactions among chemicals are always possible. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron or lab coat. Remind students to wash their hand thoroughly with soap and water before leaving the lab. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Excess benzaldehyde may be disposed of according to Flinn Suggested Disposal Method #2. Excess sodium hydroxide may be neutralized according to Flinn Suggested Disposal Method #10. Excess acetone may be disposed of according to Flinn Suggested Disposal Method #18A. Excess ethyl alcohol may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b. Dibenzalacetone may be handled of according to Flinn Suggested Disposal Method #26a.

Lab Hints

• This laboratory activity was specifically written, per teacher request, to be completed in one 50-minute class period. It is important to allow time between the Prelab Homework Assignment and the Lab Activity. Prior to beginning the homework, show the students the chemicals and equipment that will be available to them on lab day. Alternatively, you could provide the students with a list of the chemicals and equipment. For more advanced groups, you could include additional supplies that are not required to successfully complete the lab.
• The most common problem encountered in this lab is that the acetone has evaporated prior to adding the benzaldehyde. Encourage students to have the benzaldehyde ready to add before the addition of any acetone to the mortar.
• Higher quality crystals can be obtained by recrystallizing from pure ethanol. To do this, dissolve the crude product in the minimal amount of hot ethanol and then let the flask slowly cool to room temperature before placing it into an ice bath. This method results in much slower crystal formation and takes longer than the method suggested in the student section.

Teacher Tips

• You can demonstrate the UV absorption properties of dibenzalacetone by dissolving the purified product in ethanol. Place the solution and a reference sample of ethanol under UV light; the dibenzalacetone solution should appear much darker. You will need to ensure that no other sources of visible light are present, otherwise both solutions will appear transparent.
• In addition to being an example of the application of green chemistry principles, this experiment gives students a taste of the types of organic chemistry reactions that they will encounter in college.
• This experiment provides a good talking point for rates of reactions. Since both reactants are pure liquids and no solvent is being used, the concentration of the reactants is at a maximum. However, a catalyst needs to be present in order for the reaction to proceed at an appreciable rate.

Further Extensions

Alignment to the Curriculum Framework for AP^® Chemistry 

Enduring Understanding and Essential Knowledge
Atoms are conserved in physical and chemical processes. (1E)
1.E.1: Physical and chemical processes can be depicted symbolically; when this is done, the illustration must conserve all atoms of all types.
1.E.2: Conservation of atoms makes it possible to compute the masses of substances involved in physical and chemical processes. Chemical processes result in the formation of new substances, and the amount of these depends on the number and the types and masses of elements on the reactants, as well as the efficiency of the transformation.

Chemical changes are represented by a balanced chemical equation that identifies the ratios with which reactants react and products from. (3A)
3.A.2: Quantitative information can be derived from stoichiometric calculations that utilize the mole rations from the balanced chemical equations. The role of stoichiometry in real-world applications is important to note, so that it does not seem to be simply an exercise done only by chemists.

Chemical and physical transformations may be observed in several ways and typically involve a change in energy. (3C)
3.C.1: Production of heat or light, formation of a gas, and formation of a precipitate and/or a color change are possible evidence that a chemical change has occurred.

Many reactions proceed via a series of elementary reactions. (4C)
4.C.1: The mechanism of a multistep reaction consists of a series of elementary reactions that add up to the overall reaction.
4.C.3: Reaction intermediates, which are formed during the reaction but not present in the overall reaction, play an important role in multistep reaction.

Reaction rates may be increased by the presence of a catalyst. (4D)
4.D.1: Catalysts function by lowering the activation energy of an elementary step in a reaction mechanism and by providing a new and faster reaction mechanism.
4.D.2: Important classes of catalysis include acid–base catalysis, surface catalysis, and enzyme catalysis.

Learning Objectives
1.17 The student is able to express the law of conservation of mass quantitatively and qualitatively using symbolic representations and particulate drawings.
1.18 The student is able to apply conservation of atoms to the rearrangement of atoms in various processes.
3.3 The student is able to use stoichiometric calculations to predict the results of performing a reaction in the laboratory and/ or to analyze deviations from the expected results.
3.4 The student is able to relate quantities (measured mass of substances, volumes of solutes, or volumes and pressures of gases) to identify stoichiometric relationship for a reaction, including situations involving limiting reactants and situations in which the reaction has not gone to completion.
4.9 The student is able to explain changes in reaction rates arising from the use of acid-base catalysts, surface catalysts, or enzyme catalysts, including selecting appropriate mechanisms with or without the catalyst present.

Science Practices
1.1 The student can create representations and models of natural or manmade phenomena and systems in the domain.
1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
2.2 The student can apply mathematical routines to quantities that describe natural phenomena.
4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question.
5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.

Answers to Prelab Questions

1. Read through the following experimental procedure for the synthesis of chalcone (C₁₅H₁₂O) (see Figure 3) and then answer the questions.
   {12371_PreLab_Figure_3}
   • 1.20 mL of acetophenone (C₈H₈O) was pipetted into a porcelain mortar.
   • A single pellet of NaOH was then added to the mortar.
   • Using a pestle, the NaOH was carefully ground into a fine powder.
   • 1.00 mL of benzaldehyde (C₇H₆O) was pipetted into the mortar.
   • The mixture in the mortar was carefully ground with the pestle for 5–10 minutes, at which point the contents of the mortar had become a thick yellow paste.
   • 5 mL of distilled water was added to the mortar, and the mixture was ground for an additional five minutes.
   • The solid contents of the mortar were collected using vacuum filtration with a Büchner funnel.
   • The collected solids were washed twice with ice-cold distilled water and then left to air dry.
   1. The density of acetophenone is 1.028 g/mL; how many moles of acetophenone were added to the mortar?
      {12371_PreLabAnswers_Equation_1}
   2. The density of benzaldehyde is 1.044 g/mL; how many moles of benzaldehyde were added to the mortar?
      {12371_PreLabAnswers_Equation_2}
   3. Which species was the limiting reactant?

      Benzaldehyde was the limiting reagent.

   4. If the researcher isolated 1.67 g of product, what is the percent yield for the reaction?
      {12371_PreLabAnswers_Equation_3}
   5. Why was only a single pellet of NaOH added to the mortar rather than a specific quantity?

      NaOH is a catalyst for the reaction. Catalysts lower the activation energy of a reaction but are not consumed in the reaction. For this reason the catalyst doesn’t need to be present in any specific stoichiometric quantity.

   6. Why do you think water was added to the mortar before filtering?

      Water is added to dissolve the NaOH. NaOH is a strong base that readily dissolves in water, whereas the product is insoluble in water.

2. One way to increase the purity of a crude product is through recrystallization. Recrystallization involves dissolving the crude product in a minimal amount of hot solvent and then cooling the solvent to precipitate out the purified product. With reference to the solubility chart below (see Figure 4), if a solution of 70 g K₂Cr₂O₇ dissolved in 100 g of boiling water is cooled to 5 °C, how many grams of K₂Cr₂O₇ will precipitate out?
   {12371_PreLab_Figure_4}

   From the graph, the solubility of K₂Cr₂O₇ at 5 °C is 6 g in 100 g of water. Since the solution initially contained 70 g of K₂Cr₂O₇, 64 g will precipitate out.

3. Dibenzalacetone is sometimes used as an ingredient in sunscreen because it absorbs light in the UV region, with a maximum absorption at 320 nm. Calculate the energy of a photon with a wavelength of 320 nm, and describe what type of transition corresponds to the absorption of a photon at this wavelength.
   {12371_PreLabAnswers_Equation_4}

   This photon absorption corresponds to an electronic transition within the molecule.

4. The 2nd principle of green chemistry, atom economy, is measured by dividing the mass of the desired product by the mass of all reactants and expressing the result as a percentage. Use this definition to determine the atom economy when two moles of benzaldehyde react with one mole of acetone to produce one mole of dibenzalacetone (see Figure 2 in the Background section).
   {12371_PreLabAnswers_Equation_5}

5. The 6th principle of green chemistry, design for energy efficiency, calls for reactions to be done at ambient temperature whenever possible. Ethylene glycol is an organic solvent with a very high boiling point (197.3 °C) and a heat capacity of 2.41 J g^–1 K^–1. How much energy does it take to heat 125 g of ethylene glycol from an ambient temperature of 21.0 °C to its boiling point?

   q = mcΔT = 125 g x 2.41J g^–1 K^–1 x (197.3–21.0) K = 53100 J = 53.1 kJ

Sample Data

Example Procedure

1. Set up a vacuum filtration apparatus with a Büchner funnel.
2. Obtain a porcelain mortar and pestle.
3. Pour 3 mL of benzaldehyde into a 10 mL graduated cylinder.
4. Place one piece of NaOH in the bottom of the mortar.
5. Add 1 mL of acetone to the mortar and grind briefly to break up the NaOH.
6. Add the benzaldehyde to the mortar and grind for 2–3 minutes.
7. Add another 0.5 mL of acetone to the mortar and grind for another 2–3 minutes.
8. Add 20 mL of distilled water to the mortar and grind for 2 minutes to dissolve the NaOH.
9. Remove gloves, then wash and dry hands thoroughly before putting on a new pair of gloves.
10. Filter the slurry and wash the precipitate with another 10 mL of distilled water.
11. Transfer the crude product into a 125-mL Erlenmeyer flask.
12. Clean and dry the vacuum filtration apparatus and set it up again for a future filtration.
13. Set up an ice bath.
14. Add 25 mL of an 80:20 ethanol:water solution to the Erlenmeyer flask.
15. Heat the solution on a hot plate until it comes to the boil.
16. Transfer the Erlenmeyer flask to the ice bath.
17. Once cooled filter off the pale yellow crystals.
18. Wash the precipitate twice with 5 mL of ice cold ethanol.
19. Dry the recrystallized product and record the weight.

Data

Mass of recrystallized dibenzalacetone: 2.20 g

References

AP^® Chemistry Guided-Inquiry Experiments: Applying the Science Practices; The College Board: New York, NY, 2013.

Anastas P. T. and Warner J. C. Green Chemistry: Theory and Practice, New York: Oxford University Press, 1998. Print.

Introduction

Capture the concepts and hit the ground running on exam day with this lab! Encompassing multiple Big Ideas, this lab involves applying the principles of green chemistry to an organic synthesis reaction. A prelab homework assignment guides you through the necessary concepts to ensure success on lab day. You will find it fun, engaging and challenging!

Concepts

• Green chemistry
• Stoichiometry
• Catalysis
• Synthesis

Background

The green chemistry approach uses 12 principles that help evaluate the production and use of chemical products so that the generation of hazardous substances can be reduced or eliminated. These principles are listed.

1. Prevention—It is better to prevent waste than to treat or clean up waste after it has been created.
2. Atom Economy—Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
3. Less Hazardous Chemical Syntheses—Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
4. Designing Safer Chemicals—Chemical products should be designed to affect their desired function while minimizing their toxicity.
5. Safer Solvents and Auxiliaries—The use of auxiliary substances (e.g. solvents, separation agents) should be made unnecessary wherever possible and innocuous when used.
6. Design for Energy Efficiency—Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.
7. Use of Renewable Feedstocks—A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.
8. Reduce Derivatives—Unnecessary derivatization (use of blocking groups, protection/deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.
9. Catalysis—Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
10. Design for Degradation—Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.
11. Real-Time Analysis for Pollution Prevention—Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
12. Inherently Safer Chemistry for Accident Prevention—Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions and fires.

In this lab, you will be conducting a solventless base catalyzed aldol condensation as an example of green chemistry. An aldol condensation is an important organic synthesis reaction, as it results in the formation of a new carbon–carbon bond. The reaction takes place between an aldehyde and a ketone in the presence of a catalytic amount of sodium hydroxide (see Figure 1). There are three main steps for this reaction. The first step is the reaction of the ketone with the hydroxide to form an intermediate called an enolate. In the second step, the enolate reacts with the aldehyde. The product of this reaction contains both a ketone and an alcohol group (in the original aldol experiment an alcohol and an aldehyde were present at this point, the name aldol is a portmanteau of aldehyde and alcohol). The third and final step is the condensation; water is removed from the aldol affording the final product. This reaction already aligns with several of the principles of green chemistry. Because the only unwanted byproduct of this reaction is water, it exhibits excellent atom economy (principle 2). The use of a catalyst (principle 9) means that the reaction can be done at standard room temperature and pressure (principle 6). Finally, to make this reaction greener still, you will conduct it without the use of a solvent (principle 5).

The specific reaction you will be conducting is the synthesis of dibenzalacetone from benzaldehyde and acetone (see Figure 2). Dibenzalacetone is a pale yellow solid with applications as both a sunscreen and in the synthesis of palladium(0) catalysts.

Experiment Overview

The purpose of this activity is to complete the homework assignment prior to lab to promote understanding of various aspects of chemical synthesis and green chemistry. You will need to consider the equipment and chemicals that are being made available to you, and then using the pre-lab questions as a guide, design an experiment that will result in the formation of dibenzalacetone from benzaldehyde and acetone.

Prelab Questions

Complete the following homework set and write a lab procedure to be approved by your instructor prior to performing the lab. When writing your procedure, be mindful of the chemicals, quantities and equipment that will be available to you on lab day. Along with your procedure, you will turn in any graphs or figures you were asked to create in this homework set and the answers to the questions on a separate piece of paper, if needed.

1. Read through the following experimental procedure for the synthesis of chalcone (C₁₅H₁₂O) (see Figure 3) and then answer the questions.
   {12371_PreLab_Figure_3}
   • 1.20 mL of acetophenone (C₈H₈O) was pipetted into a porcelain mortar.
   • A single pellet of NaOH was then added to the mortar.
   • Using a pestle, the NaOH was carefully ground into a fine powder.
   • 1.00 mL of benzaldehyde (C₇H₆O) was pipetted into the mortar.
   • The mixture in the mortar was carefully ground with the pestle for 5–10 minutes, at which point the contents of the mortar had become a thick yellow paste.
   • 5 mL of distilled water was added to the mortar, and the mixture was ground for an additional five minutes.
   • The solid contents of the mortar were collected using vacuum filtration with a Büchner funnel.
   • The collected solids were washed twice with ice-cold distilled water and then left to air dry.
   1. The density of acetophenone is 1.028 g/mL; how many moles of acetophenone were added to the mortar?
   2. The density of benzaldehyde is 1.044 g/mL; how many moles of benzaldehyde were added to the mortar?
   3. Which species was the limiting reactant?
   4. If the researcher isolated 1.67 g of product, what is the percent yield for the reaction?
   5. Why was only a single pellet of NaOH added to the mortar rather than a specific quantity?
   6. Why do you think water was added to the mortar before filtering?
2. One way to increase the purity of a crude product is through recrystallization. Recrystallization involves dissolving the crude product in a minimal amount of hot solvent and then cooling the solvent to precipitate out the purified product. With reference to the solubility chart below (see Figure 4), if a solution of 70 g K₂Cr₂O₇ dissolved in 100 g of boiling water is cooled to 5 °C, how many grams of K₂Cr₂O₇ will precipitate out?
   {12371_PreLab_Figure_4}
3. Dibenzalacetone is sometimes used as an ingredient in sunscreen because it absorbs light in the UV region, with a maximum absorption at 320 nm. Calculate the energy of a photon with a wavelength of 320 nm, and describe what type of transition corresponds to the absorption of a photon at this wavelength.
4. The 2nd principle of green chemistry, atom economy, is measured by dividing the mass of the desired product by the mass of all reactants and expressing the result as a percentage. Use this definition to determine the atom economy when two moles of benzaldehyde react with one mole of acetone to produce one mole of dibenzalacetone (see Figure 2 in the Background section).
5. The 6th principle of green chemistry, design for energy efficiency, calls for reactions to be done at ambient temperature whenever possible. Ethylene glycol is an organic solvent with a very high boiling point (197.3 °C) and a heat capacity of 2.41 J g^–1 K^–1. How much energy does it take to heat 125 g of ethylene glycol from an ambient temperature of 21.0 °C to its boiling point?
6. Write a detailed lab procedure for synthesizing dibenzalacetone. Once synthesized, you will need to calculate the pecent yield of the reaction. Please note the following hints and safety warnings before writing your procedure.
   1. Solid NaOH is a corrosive chemical that can cause chemical burns and blindness. Care should be taken when working with this compound and it is highly recommended that you remove your gloves and thoroughly wash your hands after grinding the reaction mixture.
   2. Acetone is a highly volatile solvent and the amount remaining in the mortar after briefly grinding with NaOH will most assuredly be less than you started with. For this reason, it is recommended that the benzaldehyde is added as soon as possible. You can also add an additional measure of acetone after the formation of the yellow paste; you will need to continue grinding after this until all the additional acetone has either reacted or evaporated off. Do not add extra acetone at the start of the reaction as an excess of acetone can result in the formation of unwanted byproducts.
   3. The crude yellow product that you isolate by filtration is wet and contains a number of impurities. The product can be purified and more efficiently dried by recrystallization. To recrystallize the crude product, place it in a 150 mL Erlenmeyer flask and add 25 mL of an 80:20 ethanol:water mixture. Heat the solution to boiling and then place the flask in an ice bath. Once cooled, filter off the pale yellow crystalline product. Wash the precipitate twice with 5 mL of ice-cold ethanol. Continue drawing air over the product until dry. Do not attempt to dry the solid in a hot oven as the melting point of dibenzalacetone is ~110 °C.