Materials Included In Kit

Additional Materials Required

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. All aqueous solutions can be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. Give the isopentyl acetate product back to the instructor for disposal.

Lab Hints

Glass Reaction Vial

• Glass reaction vials used in this kit are the small, or 1-dram size (see Figure 6). The vials are made from borosilicate glass and are extremely durable. When dropped, they will usually resist breaking and can be reused if cleaned.
  {13555_Hints_Figure_6}
• 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. This 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 it.
• 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.

• This Vial Organic laboratory procedure require heating and this 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 7). 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.
  {13555_Hints_Figure_7}
• 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 moreexpensive. It is a good practice to start the hot water bath as soon as the laboratory begins so the bath isboiling 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.

Teacher Tips

• Students enjoy this experiment because the product has such a wonderful odor. The formation of esters can be easily related to the real world by informing students of the role esters play in nature and how they are used to provide flavors and fragrances for many products we use. Most flavors and fragrances are either esters, aldehydes, ketones or mixtures of several of these types of compounds.

• There are many examples of fragrant esters. The aroma of a ripe banana is due to the presence of the ester, isoamyl acetate. The combination of several different acids and alcohols give other esters which have familiar odors. Here are some examples.
  {13555_Discussion_Figure_8}

• You might be tempted to have students make some of these esters. Be aware that valeric and butyric acids, which are used to prepare esters with wonderful aromas, have horrible odors themselves. Use caution and only use these acids in a fume hood.
• This laboratory procedure produces isopentyl acetate that has a banana odor. The lab can easily be accomplished within a 50-minute lab period. Crude yields of 60 to 70% are possible. Heating the reaction mixture in the hot water bath for 25 or 30 minutes will result in higher yields.
• Do not allow students to leave the room with samples of their esters. The materials prepared in the lab are not pure enough for purposes other than for this investigation.
• Heating the reaction mixture longer than 20 minutes will result in higher yields. If possible, heating for 30 minutes will result in very good yields (>70%).
• If performing a boiling point determination, dry the ester with a small amount of anhydrous magnesium sulfate first.
• Resist the temptation to try other combinations of alcohols and acids unless you have researched the hazards and odors of both the reactants and possible products.

Answers to Questions

1. Why are artificial flavors less expensive than natural flavors?

   Natural flavors are extracted from actual fruits or plants. The quantity of active ingredient in the fruit or plant is usually very small and difficult to isolate. For example, hundreds of pounds of bananas are required to extract out a pound of banana extract. However, one pound of isopentyl alcohol reacts with a pound of acetic acid to easily produce a pound of the active ingredient, isopentyl acetate. The synthetic material can also be easily purified by distillation to produce a very pure ester with properties identical to those of the natural products.

2. What is a polyester?

   A polyester is an artificial fiber. The fibers of a polyester material are long chains of carbon-to-carbon and carbon-to-oxygen bonds. Polyester fibers are strong and resistant to wear, water and chemicals. Poly- is a prefix that means many. A polyester is a molecule containing many ester bonds.

3. Why is the crude product “washed” with sodium carbonate solution?

   The ester product needs to be washed to remove impurities, such as the starting materials. The sodium carbonate solution will convert the acid and alcohols into their salts and increase the solubility in the aqueous layer. Sulfuric acid acts as a catalyst in the Fischer esterification. The sulfuric acid protonates the acetic acid in the first step of the mechanism. The final step releases the protein which can then catalyze another reaction.

Background

Carboxylic acids are very different from inorganic mineral acids. One important difference is the vast array of reactions that carboxylic acids undergo to form new classes of organic compounds. The primary reaction for inorganic acids is a reaction with a base to form an inorganic salt. Organic carboxylic acids react with bases to produce salts, but they also react with alcohols to form esters, amines to form amides, thionyl chloride to form acid chlorides and undergo dehydration reactions to form acid anhydrides (see Figure 1). Carboxylic acids are a starting point for several classes of organic compounds and play an important role in organic chemistry.

A very important acid derivative is the ester. Organic esters are widely distributed in nature in the form of waxes, oils, flavors and fragrances. Most of the edible vegetable oils (e.g., corn, soybean, olive oil) are triesters of glycerol (see Figure 2). The acid groups of the edible oils are C₁₂–C₁₆ saturated acids frequently called fatty acids. Waxes are high molecular weight esters where the acid and alcohol portions of the ester may contain 25–36 carbon atoms.

Lower molecular weight esters are also found in nature and are responsible for many of the pleasant aromas or fragrances of fruits, flowers and even perfumes and fragrances. Many organic esters are also manufactured and used as artificial flavors, fragrances and polymers. For example n-pentyl acetate has a strong pineapple flavor and isopentyl acetate has a strong banana flavor. These esters may be added to a fruit flavored drink, or to an air freshener, or used for fruit flavoring in candy. They can be either extracted from their natural sources or manufactured. Polyester is another ester that is an important fiber used in clothing, carpeting, and soda bottles. It is made from the reaction of terephthalic acid with ethylene glycol.

In this experiment, isopentyl alcohol reacts with acetic acid to form isopentyl acetate and water. This type of reaction, where an alcohol is reacted with an acid, is the most common route to prepare esters and is called the Fischer esterification. The Fischer esterification is an equilibrium reaction because each step is reversible. If there is excess water present, the equilibrium will shift back to acetic acid and isopentyl alcohol. In some procedures, the water is removed from the reaction to shift the equilibrium toward the products. The general reaction mechanism is shown in Figure 3.

Esters are named by naming the alcohol part first, then the acid with an -ate replacing the -ic ending. In the example given below, isopentyl alcohol reacts with acetic acid to form the ester, isopentyl acetate (see Figure 4).

The presence of the ester in your product is identified by its characteristic and pleasant odor. Isopentyl alcohol has a medicine smell, while acetic acid has the odor of vinegar. Both starting materials, acetic acid and isopentyl alcohol are also very soluble in water and the product is insoluble in water.

Reaction and Physical Properties

Experiment Overview

To explore the acid catalyzed reaction between an alcohol and a carboxylic acid to produce esters.

Materials

Safety Precautions

Glacial acetic acid is corrosive to skin and tissue, moderate fi re risk and moderately toxic by ingestion, LD₅₀ 3310 mg/kg. Isopentyl alcohol is slightly toxic by ingestion and inhalation; moderate fire risk; may form explosive peroxides, do not distill to dryness. TLV 361 mg/m3. LD50 1300 mg/kg. Sulfuric acid is severely corrosive to skin and eyes. 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. Do not plug in the immersion heater until after it is placed in the water.
2. Add 1.2 mL of isopentyl alcohol (using a graduated glass pipet) and 2.5 mL of glacial acetic acid (using a graduated cylinder) to a small reaction vial. If performing yield calculations, weigh both reactants as they are being added to the vial.
3. Using a plastic Beral-type pipet, add 5 drops of concentrated sulfuric acid to the reaction vial. Be sure to wear chemical-resistant gloves.
4. Seal the vial with a PTFE-coated cap. Make sure the cap is on tight. Mix the reaction mixture by inverting the reaction vial several times.
5. Using tongs, place the sealed vial in the hot water bath. Watch for a small stream of bubbles coming out of the vial cap. If this occurs, the cap is no longer sealed and the vial must be removed and cooled before the cap is retightened.
6. Heat the vial for 20 to 25 minutes in boiling water.
7. Using tongs, remove the vial from the hot water and place it on the table top. Allow the vial to cool for a minute.
8. Place the vial in an ice-water bath for 3 minutes to cool.
9. Add 10 mL of ice water to a small test tube for step 11.

Isolation of Product

10. Carefully open the reaction vial. Wear chemical-resistant gloves because the reaction vial may be pressurized and some solution may squirt out.
11. Pour the contents of the reaction vial into a small test tube or beaker containing ice water. Gently stir to dissolve all unreacted starting materials. Rinse out the reaction vial with water. Transfer the organic (upper) layer containing the ester product back to the reaction vial using a Beral-type pipet.
12. Wash the crude ester product twice with 2 mL of 10% sodium carbonate solution and once with distilled water. (2 mL is about 1.5 cm in the vial) To wash the crude ester, first add the sodium carbonate solution slowly to prevent the mixture from bubbling out of the vial from the generation of CO₂. Then remove the lower aqueous layer using a Beral-type pipet.
13. Remove as much of the lower aqueous layer as possible from the solution using a pipet.

Purification and Analysis

14. Isopentyl acetate has a strong banana- or pear-like odor and is called banana oil. The boiling point of isopentyl acetate is 142 °C.