The Preparation of Fragrant Esters
Esters are the product of reaction of an organic (carboxylic acid) with an alcohol. Many esters are components of the essential oils of flowers and fruits. Several esters with pleasant fragrances will be synthesized in this experiment, and a common fragrant ester will be hydrolyzed to demonstrate the reverse of the esterification reaction.
When an organic ace, R-COOH, is heated with an alcohol, R’-OH, in the presence of a strong mineral acid, the chief organic product is a member of the family of organic compounds known as esters.
The general reaction for the esterification of an organic acid with an alcohol is
R-COOH + HO-R’ ( R-CO-OR’ + H2O
In this general reaction, R and R’ represent hydrocarbon chains, which may be the same or different. As a specific example, suppose acetic acid, CH3COOH, is heated with ethyl alcohol, CH3CH2OH, in the presence of a mineral acid catalyst. The esterification reaction will be
CH3-COOH + HO-CH2CH3 ( CH3-COO-CH2CH3 + H2O
The ester product of this reaction (CH3-COO-CH2CH3) is named ethyl acetate, indicating the acid and alcohol from which it is prepared. Esterification is an equilibrium reaction, which means that the reaction does not go to completion on its own. Frequently, however, the esters produced are extremely volatile and can be removed from the system by distillation. If the ester is not very easily distilled, it may be possible instead to add a desiccant to the equilibrium system, thereby removing water from the system and forcing the equilibrium to the right.
Unlike many organic chemical compounds, esters often have very pleasant, fruitlike odors. Many of the odors and flavorings of fruits and flowers are due to the presence of esters in the essential oils of these materials. The table that follows lists some esters with pleasant fragrances, and indicates from what alcohol and which acid the ester may be prepared.
The esterification reactions shown above are actually equilibrium processes and can be reversed. The reverse of the esterification reaction is referred to as a hydrolysis reaction, because it represents the break down of the organic compound through the action of water.
R-CO-OR’ + H2O ( R-COOH + HO-R’
Table of Common Esters
|Ester |Aroma |Constituents | |n-propyl acetate |pears |n-propyl alcohol/acetic acid | |methyl butyrate |apples |methyl alcohol/butyric acid | |isobutyl propionate |rum |isobutyl alcohol/propionic acid | |octyl acetate |oranges |n-octyl alcohol/acetic acid | |methyl anthranilate |grapes |methyl alcohol/2-aminobenzoic acid | |isoamyl acetate |bananas |isoamyl alcohol/acetic acid | |ethyl butyrate |pineapples |ethyl alcohol/butyric acid | |benzyl acetate |peaches |benzyl alcohol/acetic acid | |methyl salicylate |wintergreen |methyl alcohol/salicylic acid |
Generally a fruit or flower may contain only a tiny amount of ester, giving a very subtle odor. Usually, the ester is part of some complex mixture of substances, which, taken as a whole, have the aroma attributed to the material. When prepared in the laboratory in relatively large amounts, the ester may seem to have a pronounced “chemical odor, and it may be difficult to recognize the fruit or flower that has this aroma.
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