The Cinnamon Clock Reaction

Introduction

A clock reaction is conducted using a solution of cinnamaldehyde, the organic molecule extracted from cinnamon sticks and used to create artificial cinnamon flavoring. The effects of temperature and concentration changes are shown through dramatic color changes in the reacting solution.

Concepts

• Kinetics
• Reaction rates
• Aldol condensation

Materials

Safety Precautions

Cinnamaldehyde solution is flammable and toxic by ingestion and inhalation. Sodium hydroxide solution is a corrosive liquid; skin burns are possible; very dangerous to eyes. Acetone is a dangerous fire risk, flammable and toxic by ingestion and inhalation. Keep away from open flames and sparks. Avoid contact of all chemicals with skin and eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Perform this demonstration in a well-ventilated room. 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.

Prelab Preparation

1. Prepare a cold water bath by pouring 200 mL of water into a 400-mL beaker and adding ice cubes until the volume is approximately 300 mL.
2. Prepare a warm water bath by placing a 400 mL beaker containing 300 mL of water on a hot plate. Heat the water to approximately 40 °C. 

Procedure

1. Place a large test tube in a 400-mL beaker.
2. Add 20 mL of the cinnamaldehyde solution to the test tube.
3. Add 12 mL of the sodium hydroxide solution to the same test tube.
4. Stir the mixture to form a clear homogeneous solution. Record the solution temperature.
5. Quickly add about 15 drops of acetone; mix thoroughly with a stirring rod.
6. Start timing the reaction immediately following the addition of acetone.
7. Record the time required for the bright yellow precipitate to form. Label results Reaction 1.
8. Repeat the procedure at a lower temperature. Mix the cinnamaldehyde solution and the sodium hydroxide in a large test tube and set the tube in the cold water bath.
9. Allow the solution to equilibrate at the lower temperature (about five minutes) before adding acetone. Record the solution temperature.
10. Add the acetone, mix, and time the reaction until the bright yellow precipitate first forms. Record the time. Label results Reaction 2.
11. Repeat the procedure again at a higher temperature. Set the large test tube with cinnamaldeyde and sodium hydroxide in the warm water bath at approximately 40 °C.
12. Allow the solution to equilibrate at the higher temperature. Record the temperature.
13. Add 15 drops of acetone, mix, and time the reaction until the bright yellow precipitate first forms. Record the time. Label results Reaction 3.
14. Repeat steps 1 through 7 using 6 mL of sodium hydroxide solution and 6 mL of deionized water instead of 12 mL of sodium hydroxide solution. Label results Reaction 4.

Student Worksheet PDF

12542_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration seven times.
• Consistency is important in comparing reaction times. Record the time when the bright yellow crystals first appear. The reaction will continue producing precipitate for some time.
• Use a hot plate or hot water bath to heat the reactants. Do not use an open flame.
• The reaction rate is directly related to the hydroxide ion (OH^–) concentration. Step 14 should take twice the time as step 7. Have the students count down from the predicted time (two times step 7).

Answers to Questions

1. What is the relationship between temperature and reaction rate?

   Temperature and reaction rate are directly proportional. In other words, as temperature increases, the reaction rate also increases, causing the reaction time to decrease.

2. What is the relationship between solution concentration and reaction rate?

   At a constant temperature, the reaction rate is directly proportional to the hydroxide ion concentration. In this demonstration, hydroxide ion concentration was decreased. As the concentration was cut in half, the reaction rate time was also cut in half, causing the reaction time to double.

Discussion

A condensation reaction occurs when two molecules are joined together and a molecule of water is produced. In an aldol condensation, the two molecules contain carbonyl groups (>C=O) that react together to form a new C—C bond. The reaction mechanism involves five steps. The first three involve the formation of an aldol (from aldehyde alcohol). The final two steps involve dehydration of the aldol to produce the final products, an unsaturated ketone and water. The reaction mechanism is shown.

The reaction of cinnamaldehyde with acetone is essentially a “double” aldol condensation, catalyzed by hydroxide ions. The product of the first step, a ketone, can itself react like acetone with cinnamaldehyde in a second aldol condensation reaction to form the final product. The reaction process is summarized. The rate of the reaction is a function of temperature and hydroxide concentration. As temperature increases, the rate of the reaction increases, which reduces the reaction time. When the temperature is lowered, the rate of the reaction decreases, which increases the reaction time.

At constant temperature, the reaction rate is directly proportional to the hydroxide ion concentration.

Rate = k[OH^–]

If the hydroxide ion concentration is doubled, the reaction time is cut in half. If the hydroxide ion concentration is cut in half, the reaction time doubles.

Listed are typical results for the various reaction conditions.

References

Special thanks to Jim and Julie Ealy, The Peddie School, Hightstown, NJ, who provided us with the instructions for this activity. Hathaway, Bruce A. J. Chem. Ed., 1987, 64, 367. Hawbecker, B. L., et al. J. Chem. Ed., 1978, 55, 540. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry: University of Wisconsin: Madison: 1985; Vol. 4, pp 65–69.