Preparation of Esters

This is a short lab with minimal write-up. Esters are prepared from various mixtures of alcohol and acid, the odor is identified, and the structural equation is written. The pairs we generally use are as follows: methanol and salicylic acid; ethanol and propionic acid; butanol and acetic acid; methanol and anthranilic acid; 3-methyl-1-butanol (isopentyl alcohol) and acetic acid; and 1-pentanol and propionic acid. The procedure is to mix 20 drops of the alcohol with either 6 drops (liquid) or 0.2 g (solid) of acid in a 18 x 150 mm test tube. Three drops of concentrated sulfuric acid are added and mixed well. A 13 x 100 mm test tube with a rubber band around its top is half filled with cold water and dropped into the larger tube to serve as a condenser. When the assembly is heated in a water bath at about 70 degrees Celsius (160 degrees Fahrenheit), ester forms, distills, and collects on the bottom of the smaller tube, which can be removed to detect odors.

Students find odor identification challenging. Methyl salicylate (wintergreen) is pretty easy to identify.  Methyl anthranilate (grape) is not especially volatile; longer heating and especially treatment with a couple mL of 10% sodium bicarbonate make it easier to detect. According to lore, butyl acetate smells like pear, isopentyl acetate like banana, and pentyl propionate like apricot. Those identifications aren't as obvious as I would like. Ethyl propionate is pretty easily recognized as butterscotch. To make identification a little easier, I give the students the above list as suggestions. Other possibilities I have seen are isobutyl formate (raspberry), octyl acetate (orange), methyl butyrate (apple), and ethyl butyrate (pineapple). We have chosen to avoid butyric and isobutyric acids because although their esters are delightful, the acids smell wretched.

Students finish the exercise by writing the equations for each reaction. When I have included in the course a unit on cosmetics and personal care, this exercise fits in fairly well in the context of fragrances. Otherwise, it fits well with the topic of soap as examples of ester chemistry.

 

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Revised 8/31/06