The Splashy Spectrum

Demonstration Kit

Introduction

A spectrum of colors is observed as a halogenated hydrocarbon is added to an alcoholic sodium hydroxide solution containing universal indicator.

Concepts

  • Organic chemistry (nucleophilic substitution)
  • Acids and bases
  • Indicators

Materials Included In Kit

2-Bromo, 2-methylpropane, CH3CBrCH3CH3, 10 mL
2-Chloro, 2-methylpropane, CH3CClCH3CH3, 10 mL
Isopropyl alcohol solution, (CH3)2CHOH, 50%, 1400 mL
Sodium hydroxide solution, NaOH, 0.5 M, 35 mL
Universal indicator solution, 35 mL

Additional Materials Required

Beaker, 150-mL, tall-form
Graduated cylinder, 100-mL
Medicine droppers, 2

Materials

2-Bromo, 2-methylpropane, CH3CBrCH3CH3, 10 mL*
2-Chloro, 2-methylpropane, CH3CClCH3CH3, 10 mL*
Isopropyl alcohol solution, (CH3)2CHOH, 50%, 1400 mL*
Sodium hydroxide solution, NaOH, 0.5 M, 35 mL*
Universal indicator solution, 35 mL*
Beaker, 150-mL, tall-form
Graduated cylinder, 100-mL
Medicine droppers, 2
*Materials included in kit.

Safety Precautions

Sodium hydroxide solutions are corrosive to eyes, skin and other tissue. 2-Bromo, 2-methylpropane and 2-chloro, 2-methylpropane are flammable liquids. t-Butyl alcohol is a moderate fire risk; toxic on prolonged inhalation; eye irritant; absorbed by skin. Isopropyl alcohol is a moderate fire risk; toxic by ingestion and inhalation. Wear chemical-resistant gloves, splash goggles and a chemical-resistant apron. 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 products of this reaction can be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Procedure

  1. Place 100 mL of the 50% isopropyl alcohol solution in the 150-mL tall-form beaker.
  2. Add a magnetic stir bar to the beaker, and place the beaker on a magnetic stirrer at slow speed.
  3. Add 10 drops of the 0.5 M sodium hydroxide solution.
  4. Add 30–40 drops of universal indicator solution (enough to get a deep blue color).
  5. Add 10 drops of 2-chloro, 2-methylpropane.
  6. Observe color change.
  7. After the indicator has changed to pink/orange, add 0.5 M NaOH dropwise to return the indicator to a blue color. The nucleophilic substitution reaction will continue and the colors will “splash” by again. This process can be repeated several times.
  8. Repeat the procedure using 2-bromo, 2-methylpropane.

Student Worksheet PDF

13042_Student1.pdf

Answers to Questions

  1. Describe what happened in this demonstration.

    Three solutions were combined in a beaker. When a universal indicator solution was added, the combined solution became a deep blue color. Several drops of a fourth solution were added. The color of the entire solution in the beaker went through a series of color changes before turning a pink/orange color. When some sodium hydroxide was added back into the beaker, it reverted to blue and began the color changes over again.

  2. In this reaction, hydroxide ions replace chlorine ions in 2-chloro, 2-methylpropane (CH3CClCH3CH3).
    1. Write a chemical equation showing this replacement.

      CH3CClCH3CH3(aq) + OH–(aq) CH3COHCH3CH3(aq) + Cl–(aq) 

    2. Does this reaction make the solution more acidic or more basic? Why?

      This reaction makes the solution more acidic. The concentration of hydroxide ions is decreasing due to the formation of t-butyl alcohol.

    3. Write a chemical equation in which hydroxide ions replace bromine ions in 2-bromo, 2-methylpropane (CH3CBrCH3CH3).

      CH3CBrCH3CH3(aq) + OH–(aq) CH3COHCH3CH3(aq) + Br–(aq)

  3. In this demonstration, the solution experienced changes in pH. When it was acidic, the pH was approximately 3–4. When it was basic, the pH was approximately 11. What color is universal indicator solution at pH 3–4? What color is it at pH 11?

    Universal indicator solution is a pink-orange color at a pH of about 3–4. At a pH of about 11, it is a blue color.

Discussion

In this reaction the OH– ion replaces the halogen of the halogenated hydrocarbon by means of a nucleophilic substitution.

{13042_Discussion_Equation_1}
{13042_Discussion_Equation_2}
As the reaction proceeds, the concentration of the hydroxide ion decreases, which causes a decrease in pH. As the pH of the solution changes from ≈11 to ≈3–4, the color of the solution changes from blue to pink/orange.

Adding more sodium hydroxide increases the concentration of OH– ions in solution, and the pH once again increases. The solution becomes basic (blue), until enough OH– ion is used up in the substitution reaction to again form an acidic (pink/orange) solution.

Comparison of nucleophilic substitution rates based on halogen species can also be accomplished with this demo. Since bromine is a better leaving group than chlorine for nucleophilic substitution, 2-bromo, 2-methylpropane reacts more quickly than 2- chloro, 2-methylpropane and the color change will occur more quickly.

References

Ealy, J. B. and Ealy, J. L., Jr. Visualizing Chemistry, Investigations for Teachers; American Chemical Society: Washington, D.C., 1995.

Riley, J. T. J. Chem Ed. 1977, 54, 29.

Roberts, J. D. and Caserlo, M. C. Basic Principles of Organic Chemistry; W. A. Benjamin: 1964; p 312.

†Next Generation Science Standards and NGSS are registered trademarks of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.