Esters

Introduction

The perception of flavor is a phenomenon that incorporates the senses of taste, smell and texture. One cannot taste the flavor of spearmint chewing gum, or raw onion, if the nostrils are held closed. (Try it!) Esters are a class of compound that can help to illustrate the correlation between smell and flavor. Many esters occur naturally and can be derived from a variety of plants. Many esters are prepared synthetically and are on the "GRAS" list (Generally Recognized as Safe). Besides being used as food additives to enhance flavor, they are used in synthetic fragrances, industrial solvents, and as starting materials for plastics such as plexiglass.

Esters can be prepared by reacting a carboxylic acid -- R-COOH, and an alcohol -- R'-OH. A couple of drops of concentrated hydrochloric or sulfuric acid is used to catalyze the reaction.

Esters are named essentially as derivatives or salts of carboxylic acids, with the regular acid ending changed to -ate. The alkyl or aromatic group of the alcohol is named first. Example:

acetic acid + methyl alcohol --> methyl acetate + water

Usually, the cheaper of the two reagents is used in excess to force the equilibrium to the right and favor ester production.

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1. Obtain two test tubes, 10-mL capacity; two one-holed rubber stoppers, #2; and two disposable Pasteur pipets, 2-mL vol, to act as air condensers.

   

2. After deciding what two esters you would like to make, obtain the appropriate acids and alcohols (refer to reactants list).
3. Add the listed amount of alcohol to the test tubes.
4. Add the listed amount of carboxylic acid.
5. Add two drops of concentrated hydrochloric acid and one small piece of boiling chip (not marble chip!) to the test tubes.
6. Stopper the test tube with a pre-assembled air condenser.
7. Place both test tubes, with condensers attached, into a 250-mL beaker 1/3 full of water. Heat the water to boiling (hot plates should be used due to the flammability of organics). Continue to heat for 10 to 15 minutes.
8. Cool for 10 minutes or place in an ice bath. Use a wafting technique to note the odor (if any) of the product.

Formulations:

propionic acid (1.8 mL) + ethanol (1.5 mL)
butyric acid (1.0 mL) + isoamyl alcohol (1.5 mL)
acetic acid (2.0 mL) + isoamyl alcohol (2.8 mL)
acetic acid (2.0 mL) + n-butyl alcohol (2.8 mL)
acetic acid (1.0 mL) + octyl alcohol (2.0 mL)
salicylic acid (1.8 g) + methyl alcohol (1.0 mL)
butyric acid (1.0 mL) + methyl alcohol (0.8 mL)
trans-cinnamic acid (1.8 mL)+ ethyl alcohol (1.0 mL)

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1. What are the names and formulas of the esters you synthesized?
2. Describe the odors of the esters.
3. Compare the odors of the esters to those of the real fruits and flavorings displayed on the lab table.

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• Concentrated hydrochloric acid is corrosive to flesh and causes burns. Wear goggles and an apron at all time. Wash spills immediately with large amounts of water.
• Most alcohols and acids are toxic. Dispose of them in separate waste containers in the hood.

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Odors:

ethyl propionate: (fruity)

isoamyl butyrate: (pear)

isoamyl acetate: (banana)

n-butyl acetate: (banana)

octyl acetate: (orange-like)

methyl salicylate: (winter-green)

methyl butyrate: (pineapple)

ethyl cinnamate: (cinnamon)

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Collect the alcohols and acids in separate containers in the hood for controlled disposal.

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• 30 250-mL beakers
• 60 10-mL test tubes
• 60 one-hole cork stoppers, size #2 (or rubber stoppers)
• 60 2-mL disposable pipets
• 30 hot plates (can set up hot plate stations)
• 60 mL propionic acid
• 60 mL butyric acid
• 150 mL acetic acid
• 60 g salicylic acid
• 60 g trans-cinnamic acid
• 90 mL ethyl alcohol
• 150 mL isoamyl alcohol
• 90 mL n-butyl alcohol
• 70 mL octyl alcohol
• 60 mL methyl alcohol

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The original version of this lesson was developed by:

Wayne L. Michaelchuck
Camden High School
Park Ave. & Baird Blvd.
Camden, New Jersey

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