Organic Rainbow

Introduction

Observe the hydrolysis reaction of an organic compound as the solution undergoes a series of vivid color changes.

Concepts

• Organic chemistry
• Hydrolysis reaction
• pH

Materials

Safety Precautions

Acetone is flammable and a moderate fire risk. It is slightly toxic by ingestion and inhalation. tert-Butyl chloride is a moderate fire risk. It is moderately toxic by ingestion and inhalation. It is an eye irritant and is absorbed by the skin. Sodium hydroxide solution is a corrosive liquid and skin burns are possible. It is very dangerous to the eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. 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. The final solution may be flushed down the drain with excess water according to Flinn Suggested Disposal Method 26b. Any excess sodium hydroxide solution may be neutralized according to Flinn Suggested Disposal Method 10. Any excess tert-butyl chloride may be treated according to Flinn Suggested Disposal Method 27j.

Procedure

1. Add 4 mL of acetone to the 10-mL graduated cylinder.
2. Using the Beral-type pipet, add 1 mL of tert-butyl chloride to the acetone in the graduated cylinder. Stir with the stirring rod.
3. Add approximately 500 mL of distilled or deionized water to a clean 500-mL graduated cylinder or hydrometer cylinder.
4. Add the magnetic stir bar to the cylinder and place the cylinder on a magnetic stirrer. Stir the water.
5. Add the tert-butyl chloride/acetone solution to the cylinder.
6. Using the 10-mL graduated cylinder, add 4 mL of universal indicator solution to the solution in the 500-mL graduated cylinder.
7. When the solution turns red, add 0.5 mL (8–9 drops) of 1.0 M sodium hydroxide solution to the graduated cylinder. continue stirring.
8. Observe the color changes as the pH of the solution changes.
9. Continue adding sodium hydroxide solution in 0.5 mL increments to repeat the cycle of color change.

Student Worksheet PDF

13886_Student1.pdf

Teacher Tips

• The kit contains enough chemicals to perform the demonstration seven times using the amounts listed.
• The kinetics of this reaction is studied in the demonstration kit, Kinetics of tert-Butyl Chloride Hydrolysis, Flinn Scientific, Catalog No. AP6615.
• An optional student worksheet for testing student understanding of the demonstration is included. If using the worksheet, pass out copies before starting the demonstration.
• The initial color changes should occur about 60 seconds after adding the sodium hydroxide solution.
• The solution may be purple, pH ≥ 10, rather than blue, pH = 9, when the sodium hydroxide is initially added.
• The amount of distilled water added in step 3 needs only to be around 500 mL.

Answers to Questions

In this demonstration, the progress of the reaction between tert-butyl chloride and water is followed by adding an indicator and base to the reaction solution and observing the results.

1. What are some observations of the solution as the reaction proceeded?

   The original red color of the reaction solution became a blue color when the sodium hydroxide solution was added. The solution changed to green, followed by a yellow color, then orange, and finally a red color. When additional sodium hydroxide solution was added, the color cycle was repeated.

2. Universal indicator, in solution, changes color based on the solution pH. Colors range from red, pH 4, orange, pH 5, yellow pH 6, green, pH 7, dark green, pH 8, blue, pH 9, and purple, pH 10. What do your observations tell you about changes that occurred in the solution? 

   After each addition of sodium hydroxide solution, the pH of the solution changed progressively over time from a pH of 10 (basic) to a pH of 4 (acidic).

3. When the tert-butyl chloride was added to the distilled water, the following reaction, called hydrolysis, occurred.
   {13886_Answers_Equation_3}
   Universal indicator solution and sodium hydroxide solution were then added to the reaction mixture. Use this information to formulate an explanation for the color changes of the solution.
   
   Before the addition of sodium hydroxide solution, the reaction started producing hydrochloric acid. This acidic condition caused the solution color to change to red. When sodium hydroxide solution was then added, the hydrochloric acid was neutralized, leaving only sodium hydroxide in solution. The solution was now basic, as indicated by its blue color. As the reaction proceeded, the hydrochloric acid produced neutralized the hydroxide ions in solution.

   H₃O⁺(aq) + OH^–(aq) → 2H₂O

   As the concentration of hydroxide ions decreased, the pH of the solution also decreased as indicated by the color changes in the solution. Once all the hydroxide ions were neutralized, the concentration of hydronium ions, H₃O⁺, started to increase. This was indicated by the color changes of the solution from green, pH 7, through to red, pH 4.

4. What would the color changes be if only universal indicator solution and tert-butyl chloride were added simultaneously to the solution?

   The solution would start out possibly green, pH 7, or yellow, pH 6 (from dissolved carbon dioxide in solution). As the reaction proceeded, the hydrochloric acid produced would cause the solution colors to progress to red.

Discussion

tert-Butyl chloride, (CH₃)₃CCl, is an organic molecule that, while polar, is only slightly soluble in water. It is, however, soluble if it is first mixed with acetone, CH₃COCH₃. When this tert-butyl chloride/acetone mixture is added to water, the following reaction, called hydrolysis, occurs:

In this demonstration, tert-butyl chloride and universal indicator solution are added to distilled water. As the tert-butyl reaction proceeds, a small amount of hydrochloric acid is produced, turning the solution from yellow to red, indicating a decrease in pH to 4. When sodium hydroxide is added, the small amount of acid produced is neutralized and the solution turns basic. The color changes to blue, indicating a change to a pH of 9. As the reaction proceeds, the hydroxide ions are consumed by the hydrochloric acid, lowering the pH of the solution.

The pH continues to fall as the reaction proceeds, causing the universal indicator in solution to change to green, pH 7, to yellow, pH 6, to orange, pH 5, and finally red, pH 4. Adding additional sodium hydroxide will cause the reaction to start again and the color cycle to be repeated until all the tert-butyl chloride has reacted.

References

Special thanks to Penney Sconzo, Westminster Academy, Atlanta, GA, for bringing this demonstration to our attention.