Solventless Aldol Condensation, an Example of Green Chemistry
Guided-Inquiry Wet/Dry Kit
Materials Included In Kit
Acetone, CH3COCH3, 50 mL
Benzaldehyde, C6H5CHO, 100 mL
Ethanol (ethyl alcohol), CH3CH2OH, 500 mL
Sodium hydroxide, NaOH, 45 g
Additional Materials Required
(for each lab group)
Water, distilled or deionized
Büchner flask, 250-mL
Büchner funnel and adapter, 63 mm
Erlenmeyer flask, 125-mL
Filter paper, 5.5 cm, 2
Graduated cylinders, 10-mL, 2
Graduated cylinder, 50-mL
Mortar and pestle, porcelain
Watch glass, 65 mm
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.
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.
- 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.
- 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.
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.
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.
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.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Planning and carrying out investigations
Obtaining, evaluation, and communicating information
Constructing explanations and designing solutions
Using mathematics and computational thinking
Analyzing and interpreting data
Disciplinary Core Ideas
HS-PS1.A: Structure and Properties of Matter
HS-PS1.B: Chemical Reactions
HS-PS4.A: Wave Properties
HS-ESS3.C: Human Impacts on Earth Systems
HS-ETS1.C: Optimizing the Design Solution
Energy and matter
Structure and function
Stability and change
HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
HS-ESS3-4: Evaluate or refine a technological solution that reduces impacts of human activities on natural systems.
Answers to Prelab Questions
- Read through the following experimental procedure for the synthesis of chalcone (C15H12O) (see Figure 3) and then answer the questions.
- 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.
- The density of acetophenone is 1.028 g/mL; how many moles of acetophenone were added to the mortar?
- The density of benzaldehyde is 1.044 g/mL; how many moles of benzaldehyde were added to the mortar?
- Which species was the limiting reactant?
Benzaldehyde was the limiting reagent.
- If the researcher isolated 1.67 g of product, what is the percent yield for the reaction?
- 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.
- 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.
- 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?
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.
- 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.
This photon absorption corresponds to an electronic transition within the molecule.
- 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).
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
- Set up a vacuum filtration apparatus with a Büchner funnel.
- Obtain a porcelain mortar and pestle.
- Pour 3 mL of benzaldehyde into a 10 mL graduated cylinder.
- Place one piece of NaOH in the bottom of the mortar.
- Add 1 mL of acetone to the mortar and grind briefly to break up the NaOH.
- Add the benzaldehyde to the mortar and grind for 2–3 minutes.
- Add another 0.5 mL of acetone to the mortar and grind for another 2–3 minutes.
- Add 20 mL of distilled water to the mortar and grind for 2 minutes to dissolve the NaOH.
- Remove gloves, then wash and dry hands thoroughly before putting on a new pair of gloves.
- Filter the slurry and wash the precipitate with another 10 mL of distilled water.
- Transfer the crude product into a 125-mL Erlenmeyer flask.
- Clean and dry the vacuum filtration apparatus and set it up again for a future filtration.
- Set up an ice bath.
- Add 25 mL of an 80:20 ethanol:water solution to the Erlenmeyer flask.
- Heat the solution on a hot plate until it comes to the boil.
- Transfer the Erlenmeyer flask to the ice bath.
- Once cooled filter off the pale yellow crystals.
- Wash the precipitate twice with 5 mL of ice cold ethanol.
- Dry the recrystallized product and record the weight.
Mass of recrystallized dibenzalacetone: 2.20 g
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.