ESTERS                                                         Name ______________________________

 

Esters are a class of compounds widely distributed in nature.  They have the general formula

                            O

                                                     ||

                                                R-C-OR’

The simple esters tend to have pleasant odors.  In many cases, although not exclusively so,

the characteristic flavors and fragrances of flowers and fruits are due to compounds with the

ester functional group.  An exception is the case of essential oils.  The organoleptic qualities

(odors and flavors) of fruits and flowers may often be due to a single ester, but more often the

flavor or aroma is due to a complex mixture in which a single ester predominates. 

Some common flavor principles are listed below.

 

            Ester                                        Odor

            isoamyl acetate             banana

            ethyl acetate                             fingernail polish remover

            methyl salicylate                        wintergreen

            ethyl butyrate                            pineapple

            benzyl butyrate             cherry

            ethyl propionate                        rum

            ethyl benzoate                           fruity

            benzyl acetate                           peach

            methyl butyrate             apple

            octyl acetate                             orange

            n-propyl acetate                       pear

            ethyl phenylacetate                    honey

 

Food and beverage manufacturers are thoroughly familiar with these esters and often use them as additives to spruce up the flavor or odor of a dessert or beverage.  Many times odors do not have a natural basis, as is the case with the “juicy fruit” principle, isopentenyl acetate.  An instant pudding that has the flavor of rum may never have seen its alcoholic namesake—this flavor can be duplicated by the proper admixture, along with other minor components, of ethyl formate and isobutyl propionate.  The natural flavor and odor are not exactly duplicated, but most people can be fooled.  Often only a trained person with a high degree of gustatory perception, a professional taster, can tell the difference.

            A single compound is rarely used in good-quality imitation flavoring agents.  A formula for imitation pineapple flavor that might fool an expert includes 10 esters and carboxylic acids that can easily be synthesized in the laboratory, and 7 essential oils that are isolated from natural sources.

            Flavor is a combination of taste, sensation and odor transmitted by receptors in the mouth (taste buds) and nose (olfactory receptors).  There are four different tastes (sweet, sour, salty, and bitter).  The perception of flavor, however, is not so simple.  The human actually possesses 9000 taste buds and odor plays a big role in the perception of taste.

            Although the “fruity” tastes and odor of esters are pleasant, they are seldom used in perfumes or scents that are applied to the body.  The reason for this is that the ester group is not as stable to perspiration as the ingredients of the more expensive essential oils.  The later are usually hydrocarbons (terpenes), ketones and ethers extracted from natural sources.  Esters are only used for the cheapest toilet waters, since on contact with sweat, they hydrolyze, giving organic acids.  These acids, unlike their precursor esters, generally do not have a pleasant odor.

 

                            O                                 O

                             ||                                  ||

                        R-C-OR’  +  H2O  à  R-C-OH  +  R’OH

 

Butyric acid, for instance, has a strong odor like that of rancid butter (of which it is an ingredient) and is a component of what we normally call body odor.  Ethyl butyrate and methyl butyrate, however, are esters that smell like pineapple and apple, respectively.

 

            In this experiment you will note the odor of five carboxylic acids and four alcohols.  Then, you will prepare seven esters using various combinations of these carboxylic acids and alcohols.  From the odor of the esters and the list above you will identify the ester and then the carboxylic acid and alcohol from which it was made.

 

Materials

Each student will need:

 

21  drops acid #1                                             13 drops alcohol #1

0.4 gram acid #2                                              64 drops alcohol #2

21  drops acid #3                                             21 drops alcohol #3

21  drops acid #4                                             16 drops alcohol #4

0.1 gram acid #5                                                7 drops concentrated H2SO4

7   10 x 75 mm test tubes

1    test tube rack

1    test tube holder

1    stirring rod

14  pieces of filter paper (or strips of paper towel)

1    250 mL beaker (to be use as a hot water bath)

      boiling chips

 

Procedure:

1.     Put on goggles.

2.     Put one drop of carboxylic acid and one drop of alcohol on opposite sides of a piece of filter paper.  Waft the vapor toward your nose and describe the odor of the acid and alcohol.  If the acid is solid, open the cap of the container and waft the vapors toward your nose.  Record the odor is the appropriate space in the following data table.

3.     Repeat until the odor of all carboxylic acids and alcohols has been recorded in the following data table.

4.     Read the label on the bottle and add the indicated number of drops/quantity of each reagent to a micro test tube.  Swirl gently to mix the contents.

5.     Add a drop of concentrated sulfuric acid to the test tube.  (Sulfuric acid acts as a catalyst.)  Add a boiling chip.

6.     Using the test tube holder, place the test tube in a boiling water bath for one minute.  Watch the contents carefully to avoid boiling over.  If the reaction mixture begins to boil too quickly, remove it from the water bath for a few seconds and slowly return it.  Each test tube must be in the water bath for one minute.

7.     Use a stirring rod to transfer a drop of the reaction mixture to a clean piece of filter paper (or strip of paper towel).  Waft the vapors toward your nose and record the odor of the new compound.  If the mixture solidifies, waft the vapors from the solid material on the end of the stirring rod.

8.     Identify the ester by the odor of the ester produced.  Use the identification of the ester to identify the carboxylic acid and alcohol used in the reaction to produce the ester.

9.     Repeat steps 4 through 8 until each of the seven esters has been prepared.

 

Data Table

 

 

Larger data table is shown on next page 

(Print out larger table in excel titled “Esters Lab Report”)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Questions

 

1.     Write equations for each of the esters formed above:

      A.

 

 

 

      B.

 

 

 

      C.

 

 

 

      D.

 

 

 

      E.

 

 

 

      F.

 

 

 

      G.

 

 

 

 

2.     Pick one of the equations above and write the mechanism for the formation of the

      ester.