Materials Included In Kit

Additional Materials Required

Lab Hints

• It is important that the proper stoichiometry of benzaldehyde and potassium permanganate be used. Both reagents should be present in about equimolar amounts. Always check for the presence of excess potassium permanganate after removing the reaction from the hot water setup and destroy any excess permanganate with sodium sulfite before preceding with the isolation steps.
• Benzaldehyde is a pleasant-smelling material with the odor of almonds. It is moderately toxic by inhalation and ingestion; LD₅₀ 1300 mg/kg. Limit student exposure to this compound by storing and dispensing the benzaldehyde in an operating laboratory fume hood.
• Have students dispose of the aqueous filtrate from the benzoic acid filtration in a waste beaker located in the fume hood. Some unreacted benzaldehyde may still exist in aqueous solution. Leave the aqueous solutions in an operating fume hood overnight to allow the benzaldehyde to evaporate before disposing of the remaining solution down the drain.
• If a lab hood is not available, store and dispense the benzaldehyde supply near a strong exhaust fan to minimize the level of vapor in the lab.
• Vacuum filtration using a Büchner funnel works best to isolate the manganese dixide and the crude benzoic acid. Gravity filtration will also work but will require additional time.
• Benzoic acid is recrystallized from water. The solubility of benzoic acid is 2.2 g/100 mL H₂O at 75 °C and 0.27 g/100 mL at 18 °C. To maximize the yield of pure benzoic acid, use only 1 mL of H₂O, and cool the solution to around 5–10 °C.

Teacher Tips

• Glass Reaction Vial: Glass reaction vials used in this kit are the large, or 2-dram size (see Figure 5). The vials are made from borosilicate glass and are extremely durable. When dropped, they will usually resist breaking, and can be reused if cleaned.
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• The most important contributor to the success of Vial Organic labs is the vial cap which contains a PTFE seal. The vials and PTFE seals are designed to resist the organic solvents and contain the pressure generated as a reaction progresses.
• It is important that you tighten the cap of a reaction vial very well. If the seal fails, the reactants will leak out into the water bath. To obtain a tight seal, hold the cap and vial in opposite hands and tighten as hard as possible. Never use pliers to tighten the cap. Rubber or latex gloves or a wet towel are sometimes useful to provide extra grip for tightening the cap. After a little practice you will be able to “feel” the seal set without using too much force.
• When the vial is heated, the glass vial and plastic cap expand differently and the cap may no longer be sealed tightly. This will lead to a leak and may contaminate the reaction. A leak is evidenced by a small stream of tiny bubbles coming from the vial cap. If this happens, simply remove the vial from the water bath with tongs, allow it to cool for a few seconds and carefully tighten the vial cap. The vials cool down rapidly and the plastic caps do not retain heat so they will not be excessively hot. However, care must be taken not to burn oneself; use rubber gloves and carefully test the temperature of the vial before attempting to handle.
• Always use tongs to place the reaction vial into the boiling water bath or remove it from the bath. Test tube, utility, or crucible tongs work best. Do not drop the vial in the beaker; the beaker or vial may crack. The reaction vial does not have to stand upright in the hot water bath.
• Hot Water Bath and Immersion Heating Coil: This Vial Organic laboratory procedure requires heating which is safely performed in a hot water bath utilizing an immersion heating coil. The hot water bath is simply made by adding 300 mL of deionized water to a 400- or 600-mL Pyrex^® beaker (using deionized water greatly extends the life of your heater) and placing the immersion heater into the water (see Figure 6).
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  The size of the water bath is not critical to Vial Organic. If 400-mL beakers are not available, 250-mL or a beaker larger than 400-mL will also work. Make sure the immersion heater coil is always submerged. If the beaker is too small, there will not be much room for the reaction vial. If the beaker is too large, it will take too long to heat up the water. Always make sure the beaker is Pyrex or borosilicate glass. The immersion heater does not have to be attached to the side of the beaker; it can rest on the bottom. There are five simple rules that must be strictly observed whenever the immersion heater is used:
  1. Place the coil into the water before plugging it in.
  2. Unplug the coil before removing it from the water.
  3. Always unplug the immersion heater and remove it from the water bath as soon as the reaction is complete.
  4. Only heat water with the coil. Do not heat oils or organic solvents.
  5. Never drink water heated by the immersion heater.

• A hot plate can also be used to heat the hot water bath but it will take longer and is considerably more expensive. It is a good practice to start the hot water bath as soon as the laboratory begins so the bath is boiling by the time the reaction vial is ready. If the above rules are followed and deionized water is used, the immersion heater will last a long time.

Answers to Questions

1. What other organic compounds can be oxidized to produce benzoic acid?

   Toluene is oxidized commercially to benzoic acid. Acetophenone and benzyl alcohol are also easily oxidized to benzoic acid.

2. Why is it important that all the permanganate is consumed in this oxidation reaction?

   If the permanganate ion is not completely reduced to the insoluble manganese dioxide species, the final product will be contaminated with the very brightly colored permanganate ion. Additional oxidation reactions may also occur if the permanganate is not completely destroyed.

3. Potassium permanganate is a very powerful oxidizing agent that will even oxidize carbon–carbon double bonds to diols or carboxylic acids. Why are the double bonds in the benzene ring not attacked?

   Benzene is inert to most chemical reagents due to the stability of the aromatic pi orbital system. The six carbon atoms equally share the six electrons from the three double bonds in six overlapping p orbitals that make up an electron cloud above and below the ring called the pi system. This electron sharing is also called delocalizing and greatly increases the stability of the benzene ring.

4. Why is this oxidation performed under basic conditions?

   The base converts the product, benzoic acid, into sodium benzoate. Sodium benzoate is more soluble in water than benzoic acid. The insoluble manganese dioxide is then easily removed from the aqueous solution of sodium benzoate.

5. Find three products at the store that contain benzoic acid or sodium benzoate.

Discussion

The oxidation of aldehydes to carboxylic acids is one of the easiest oxidation reactions and is often performed in introductory organic chemistry laboratories. Potassium permanganate is a common oxidizing agent and used for oxidizing a wide variety of functional groups to alcohols or carboxylic acids. For example, permanganate will oxidize alkenes to diols or dicarboxylic acids. It also oxidizes alcohols, aldehydes and aromatic side groups to carboxylic acids. It is one of the more versatile reagents used in organic oxidation reactions. 

The preparation of benzoic acid is always a great lab because the product is easily isolated as fluffy white crystals. Benzoic acid can be recrystallized from water and has a relatively low melting point for analysis. This lab is a great introduction to organic oxidations. The preparation and isolation of benzoic acid is easily accomplished in a 50-minute lab period. The yield of crude benzoic acid is usually in the 70–80% range and after recrystallization, the final yield is around 50%.

Introduction

To explore the use of permanganate as an oxidizing agent in the oxidation of an aldehyde to an acid.

Background

Oxidation is a widely used process in organic chemistry. It is principally used to form carbonyl groups in aldehydes, ketones and carboxylic acids. For example, primary alcohols are easily oxidized to aldehydes or carboxylic acids and secondary alcohols to ketones. Some oxidizing agents are even strong enough to cleave alkenes into two sets of carboxylic acids or to oxidize aromatic side chains to carboxylic acids.

Oxidation is also an important commercial process. Many synthetic fabrics are made from carboxylic acids which are prepared by the oxidation of simple organic starting materials. For example, p-xylene is oxidized to terephthalic acid which is a starting material for polyester (Figure 1). To produce nylon, cyclohexane is initially oxidized to a mixture of cyclohexanol and cyclohexanone and then further oxidized to adipic acid. Both of these processes require highly reactive catalysts that use the oxygen in air for oxidation of the starting materials.

In the laboratory, it is difficult and dangerous to perform oxidations under the high pressures required to use atmospheric oxygen as an oxidizing agent. Therefore, oxidizing agents that contain oxygen atoms are frequently used in small scale organic synthesis. The two most common oxidizing agents used in organic chemistry are chromium(VI) oxide compounds and potassium permanganate. Both are very strong oxidizing agents and are used in a variety of reactions.

In this laboratory procedure, potassium permanganate is used to oxidize benzaldehyde to benzoic acid. The mechanism for a permanganate oxidation is thought to involve a permanganate ester group (see Figure 2). This reaction can be performed under acidic, basic or neutral conditions. Basic conditions are used in this procedure to allow the sodium benzoate product, which is soluble in the aqueous reaction mixture, to be easily removed from the insoluble manganese dioxide by filtration. In the final step, acidification of the sodium benzoate gives benzoic acid in high yield. Benzoic acid is an important commercial product and is produced by oxidizing toluene using a cobalt catalyst under 2–3 atmospheres of air pressure to produce benzoic acid in 90% yields. Benzoic acid and sodium benzoate are important food preservatives and are added to many fruit juices, syrups and salad dressings. Benzoic acid is also used as a precursor to both phenol and caprolactam, two important feedstocks to the plastics industry. Phenol is used to make phenolic resins and caprolactam is used to make Nylon 6 (see Figure 3). Reaction and Physical Properties

Materials

Safety Precautions

Benzaldehyde is flammable and toxic by inhalation, ingestion or skin absorption. Potassium permanganate solution may be a skin irritant. Sodium hydroxide solutions are corrosive; skin burns are possible and these solutions are very dangerous to eyes. Hydrochloric acid is severely corrosive to skin and eyes and is highly toxic by inhalation. Always place the immersion heater in the water before plugging it in. Always wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron.

Procedure

Setup

1. Add approximately 300 mL of deionized water to a 400-mL beaker. Place an immersion heater in the water, plug it in and allow the water to come to a boil. Note: Do not plug in the immersion heater until after it is placed in the water.
2. Place 0.2 mL (0.2 g) of benzaldehyde in a large reaction vial using a graduated glass pipet. If performing percent yield calculations, weigh the benzaldehyde as it is being added to the vial.
3. Measure out 6.5 mL of 0.3 M potassium permanganate solution—using a graduated cylinder—and add to the reaction vial. Add 4 drops of 1 M sodium hydroxide solution to the reaction vial using the E-Z drop bottle.
4. Seal the vial with a PTFE-coated cap. Make sure the cap is on tight. Shake the reaction mixture. The purple solution should begin to turn brown as the permanganate is reduced to MnO₂.
5. Using tongs, place the sealed vial in the beaker containing boiling water and immersion heater. If at any time during the reaction, a small stream of bubbles begins to flow out of the reaction vial cap, remove the vial from the boiling water, allow it to cool and tighten the cap.
6. After 20 minutes, remove the vial from the boiling water bath and place on the counter for a minute to cool.
7. Prepare an ice-water bath.

Isolation of Product

8. Place the vial in a beaker containing ice water for about 3 minutes.
9. Wear chemical-resistant gloves to carefully open the reaction vial because it may be pressurized and some solution may squirt out.
10. Test the reaction mixture for residual potassium permanganate by dipping a glass rod or pipet into the mixture and touching it to a piece of filter paper. If the brown spot is surrounded by a pink or purple ring, there is excess permanganate. Add a few drops of sodium sulfite solution (1.0 M) to destroy the permanganate. Retest the reaction mixture.
11. Vacuum filter the reaction mixture using a Büchner funnel or gravity filtration setup. The brown solid is manganese dioxide and the product, which is sodium benzoate, is in the filtrate (liquid layer). Wash the brown solid twice with 2 to 3 mL of warm water. Discard the brown solid and transfer the tea-colored filtrate to a test tube or 50-mL Erlenmeyer flask.
12. Add 1 or 2 drops of sodium sulfite solution to the sodium benzoate solution. This should further clarify the solution.
13. Slowly add about 1–2 mL of 1 M hydrochloric acid solution or until the solution is about pH 1. Test the solution with pH paper. White benzoic acid will begin to precipitate out of solution.
14. Cool the mixture in an ice bath to maximize precipitation of benzoic acid. Isolate the benzoic acid by vacuum filtration using a Büchner funnel or gravity filtration setup. Wash twice with 2-mL portions of ice water. Dry the product under vacuum and then allow the benzoic acid to dry overnight.

Analysis

15. The benzoic acid may be off-white or even slightly brown due to the presence of manganese dioxide and other impurities. The benzoic acid can be recrystallized from water.
16. Carboxylic acids are detected by their pH and solubility in water. Aqueous solutions of carboxylic acids will be acidic. Carboxylic acids will also react with a dilute (5 wt%) solution of sodium bicarbonate. The formation of gas bubbles (CO₂) verifies the presence of a carboxylic acid group.
17. (Optional) Pure benzoic acid has a melting point of 122 °C. Confirm the melting point of the product.
18. The brown, solid manganese dioxide can be disposed of by throwing it in the garbage. Any aqueous solutions remaining from the filtration of the benzoic acid should be given to your instructor because it may contain some unreacted benzaldehyde. The benzoic acid should also be given to your instructor after analysis.

Student Worksheet PDF

12304_Student1.pdf