Esters

Type of physical science: Chemistry

Field of study: Chemical compounds

An ester is a compound in which the acidic proton of an acid has been replaced with a group containing one or more carbon atoms. Esters derived from carboxylic acids are the most common and are widely distributed in nature.

Overview

When a carboxylic acid molecule reacts with a molecule of alcohol, a new carbon-containing compound called an ester is formed, along with a molecule of water. This ester-forming reaction is termed an "esterification reaction." When the carboxylic acid and the alcohol combine, each loses the portion of its molecule called the functional group, which gives each of these molecules characteristic properties. The functional group lost by the alcohol is the hydroxy group, while a hydrogen ion is lost by the carboxylic acid. Thus, an ester has chemical and physical properties that are quite different from its carboxylic acid and alcohol precursors.

An example of the esterification reaction is the reaction between acetic acid (the two-carbon carboxylic acid found in vinegar) and ethyl alcohol (the two-carbon alcohol found in alcoholic beverages). This reaction forms water and an ester named ethyl acetate. Esters' names are derived from the names of the alcohol and carboxylic acid precursors. The first word of the name comes from the alcohol, and the second word denotes the carboxylic acid. The "-ate" suffix on the acid part of the name indicates that a hydrogen ion has been lost from the carboxylic acid.

Ethyl acetate is very different from its parents. While acetic acid is an acidic compound capable of losing one of its hydrogen atoms as an ion, ethyl acetate is a neutral compound which does not lose hydrogen ions. Esters also differ in odor and taste from the parent carboxylic acid and alcohol compounds. Carboxylic acids tend to have unpleasant odors and sour tastes, while pure aqueous solutions of alcohols are almost tasteless and have a faintly sweet odor. Esters of low and intermediate molecular weight are typically the pleasant-smelling compounds responsible for the fragrances of flowers as well as the aromas and flavors of fruits. The ester isoamyl acetate, which is formed from isoamyl alcohol (a five-carbon alcohol) and acetic acid, has the odor of bananas. Although the odor and flavor characteristics of fruits and flowers may be caused by a single ester, a complex mixture in which a single ester predominates is more typical. The odors and flavors of bananas, grapes, peaches, apples, pears, pineapples, oranges, and rum result, at least in part, from the presence of esters.

Although esters have pleasant fragrances, these compounds are rarely used in perfumes that are applied to the body. This results from the fact that the esterification reaction is reversible.

When warmed, an ester will react with water to reform its parent carboxylic acid and alcohol.

This reverse reaction is called hydrolysis, meaning a reaction with water. Thus, if a person applied a pineapple-scented perfume whose odor came from the ester ethyl butyrate and then participated in a strenuous activity such as dancing, the warmth and perspiration of the skin would cause the ester to be broken down into ethyl alcohol and butyric acid. The hydrolysis reaction would be readily detected because butyric acid is the foul-smelling compound that is responsible for the odor of rancid butter and is a component of body odor. Esters are used as fragrances only in inexpensive toilet waters.

Another disadvantage of using esters as perfumery compounds is their attractiveness to insects. Fruit flies and other insects find food by detecting the compounds that are responsible for the odors of the fruits and flowers. Isoamyl acetate is the alarm pheromone for the honeybee. A pheromone is a compound that is secreted by a particular organism to evoke a particular response in another member of the same species. When the honeybee injects its venom into a victim, it also secrets isoamyl acetate. Detection of this ester by other honeybees causes them to become aggressive, swarming and attacking the intruder.

Long-chain carboxylic acids containing ten or more carbon atoms can be combined with long-chain alcohols to form esters called waxes. Waxes are typically solids instead of liquids at room temperature and do not give off easily detectable fragrances. Beeswax, earwax, and carnuba wax are examples of these types of esters. Because waxes do not dissolve well in water, surfaces that are covered with them are impervious to water, or waterproofed.

If a molecule contains a carboxylic acid group at one of its ends and an alcohol group at its other end, then these two groups can combine into an ester group and cause the molecule to form a ring. The resulting cyclic compounds are called "lactones." Many of these cyclic esters also possess distinctive odors. For example, a cyclic ester called coumarin is responsible for the distinctive odor of freshly cut hay. Felines of all types--from the house cat to the lion--are attracted to the catnip plant. The active ingredient in catnip is a lactone called nepetalactone.

One lactone that is very important to human beings is ascorbic acid, which is commonly known as vitamin C. Unlike most animals, primates are not capable of synthesizing their own ascorbic acid and must rely on their diets to supply this important vitamin. Vitamin C is obtained from sources such as fresh fruits and vegetables. Unfortunately, it is quickly destroyed by heating; therefore, most of it does not survive the cooking process. Vitamin C is a water-soluble compound. When vegetables are cooked in large quantities of water, the vitamin C that survives the heating process will end up in the water and not in the vegetable.

A molecule containing more than one hydroxy group or more than one carboxylic acid group can form esters at more than one place in the molecule. Fats and oils are special types of esters formed from the reaction of three molecules of long-chain carboxylic acids called fatty acids with one molecule of glycerol, a three-carbon alcohol that has a hydroxy group on each carbon. The resulting molecule contains three ester groups per molecule. The molecule is termed a "fat" if it is a solid and an "oil" if it is a liquid at room temperature. The first step in digesting fats and oils in the body is the enzyme-aided hydrolysis of the ester groups. Esters undergo a saponification reaction that is similar to the hydrolysis reaction. In saponification, an ester reacts with water in the presence of a base such as sodium hydroxide to yield the alcohol parent and the carboxylic acid parent, whose acidic hydrogen has been removed. This form of the parent carboxylic acid, called a carboxylate ion, carries a negative charge. Saponification is an important reaction in fats and oils. If a fat is heated in an aqueous solution of sodium hydroxide (commonly called lye), then the resulting carboxylate ions possess interesting properties toward both water and substances such as dirt and grease. These carboxylate ions are known as soaps.

Applications

Low- and intermediate-molecular-weight esters are useful commercially because of their organoleptic qualities (odors and flavors). Although esters are generally not satisfactory ingredients for body perfumes, they are widely utilized by food and beverage manufacturers for enhancing the odor and flavor of their products. High-quality imitation flavoring agents are rarely composed of a single component, but rather are a complex mixture of compounds.

Although these mixtures do not duplicate the natural flavor and odor exactly, the average consumer cannot distinguish between the real and imitation flavoring. Not all products mimic naturally occurring flavors and odors. For example, isopentenyl acetate is an artificial flavoring agent which is used in some chewing gum. Most of the ester-flavoring additives used in commercial products have been synthesized in the chemical laboratory because it is generally less expensive to synthesize these compounds than to extract the natural products.

Willow bark has been used as a pain-killing remedy for centuries. Medical experimentation has found that extracts of willow bark exhibit analgesic (pain-relieving), antipyretic (fever-reducing), and anti-inflammatory (swelling-reducing) properties. The active ingredient in willow bark extracts is salicylic acid. This acid, whose name comes from the Latin word for willow (salix), contains both a hydroxy group and a carboxylic acid group.

Although salicylic acid possesses marvelous therapeutic properties, it also often causes severe irritation of the mucous linings of the mouth, throat, and stomach, which limits its utility for routine medical problems. Modifying salicylic acid by esterifying its hydroxy group with acetic acid forms an ester (acetylsalicylic acid, or aspirin) that possesses all the desirable medicinal properties of salicylic acid while diminishing the mucous membrane irritation problems. Aspirin is one of the few pain relievers that is not physically addicting. It is generally taken orally in tablet form. A typical 5-grain tablet contains 0.32 gram of acetylsalicylic acid pressed together with a starch binder. Tablets of buffered aspirin and combination pain relievers contain other ingredients in addition to the aspirin. After the tablet is ingested, the ester is hydrolyzed to generate salicylic acid, which is the therapeutic form of the drug.

Since salicylic acid contains both the hydroxy group and the carboxylic acid group, there are two sites in the molecule where an ester can be formed. In attempts to modify salicylic acid to reduce its irritational properties, chemists synthesized esters of both of these groups. One ester that was prepared as a possible therapeutic agent was methyl salicylate, an ester which results from the esterification of the carboxylic acid group with methyl alcohol. This compound has a strong and pleasant wintergreen odor and, in fact, can be isolated from the wintergreen plant. Although it exhibits the same biological properties as aspirin, methyl salicylate is generally too irritating to be taken internally in quantities that are of therapeutic value. When applied topically, methyl salicylate is absorbed through the skin, where it is hydrolyzed into salicylic acid. The salicylic acid causes a mild, local irritation, which warms the area and relieves muscle pain. Thus, methyl salicylate is the active ingredient in many liniments and skin rubs. This ester is also used in small amounts in flavorings.

The macromolecule cellulose is formed by joining approximately five thousand molecules of the sugar glucose to one another. Each of these glucose units possesses three hydroxy groups, which can be esterified. When two or three of the hydroxy groups in each of the glucose units react with acetic acid, cellulose acetate is formed. This cellulose acetate can be dissolved in acetone and squirted through a hole to produce a thin stream of liquid. A thin fiber of cellulose acetate is produced as the acetone evaporates. This fiber is called acetate rayon, which was used as an early silk substitute. If the acetone solution is passed through a slit, then a flexible film that can be used in film cameras is produced upon the evaporation of the acetone.

Esterification reactions are classified as condensation reactions--reactions in which two molecules are joined together to form a larger molecule, thus eliminating a molecule of water.

Large molecules called polymers can be formed by condensation reactions. If a molecule containing two carboxylic acid groups reacts with a molecule containing two hydroxy groups, then a molecule is formed in which the two original molecules are joined together as an ester.

This molecule still contains one carboxylic acid group that can be esterified and one hydroxy group that can be esterified. The polyester polymer is built up by continually esterifying these free hydroxy and carboxylic acid groups. Many different polyesters are utilized in the textile industry. Dacron, probably the best-known polyester fiber, is a polymer of terephthalic acid and ethylene glycol. This fiber is used extensively in wash-and-wear clothing. Since Dacron is a physiologically inert substance, it also finds use in medical applications, such as in arterial grafts and artificial heart valves. When this same polymer is formed as a film instead of a fiber, it is called Mylar. Mylar films can be coated magnetically and used in audiotapes and videotapes.

Methyl methacrylate is a simple four-carbon ester that contains a carbon-carbon double bond. Molecules containing a carbon-carbon double bond have two adjacent carbons that have only three atoms or groups attached to each of them instead of four. The double bonds in these molecules enable neighboring molecules to join together (polymerize) and form long-chain addition polymers containing many ester units. These ester units branch off of the long polymer chain. When methyl methacrylate polymerizes, a product called Plexiglas is obtained. Plexiglas is a transparent substance that is glasslike in appearance and can be substituted for glass in some applications. Plexiglas is lighter than glass and shatterproof. It is, however, softer than glass and scratches relatively easily.

Esters are also formed from alcohols and inorganic acids such as sulfuric, phosphoric, and nitric acids. Nitrate esters--of which nitroglycerin is an example--are explosive. Phosphate esters are biologically important. Some examples include nucleic acids and lipids, which compose membranes. Phosphate esters are widely used in industry as solvents, plasticizers, flame retardants, gasoline additives, and insecticides. The esters of sulfuric and sulfurous acids are used in the manufacturing of dyes and pharmaceuticals.

Context

Esters are widely distributed in nature and have been utilized in many applications. In primitive times, people flavored their foods with natural spices and herbs. They developed more sophisticated flavoring agents by developing plant extracts and essential oils. Over time, it became recognized that these characteristic flavors often were caused by naturally occurring esters. Since about 1950, food manufacturers have replaced or augmented many natural flavors with synthetic esters.

The curative powers of willow bark have been known for centuries. Hippocrates recommended chewing it for pain relief around 400 B.C. In 1763, Edward Stone reported to the Royal Society of London that extracts of the bark of the willow could be used as a therapeutic agent for malaria. His extract of willow bark reduced the fever that accompanies the disease. A century later, willow extract was found to alleviate the symptoms of rheumatism. In 1860, the active ingredient in these extracts was isolated and identified as salicylic acid. In 1893, Felix Hoffman, who was employed as a chemist by a German Bayer Company, modified salicylic acid to acetyl-salicylic acid. This product, marketed as "aspirin," is one of the world's most widely used drugs. In general, esters are the most important carboxylic acid derivatives that are used in pharmacy and medicine. Numerous examples of ester-type compounds are utilized as therapeutic agents. Many local anesthetics are esters, as are such drugs as atropine and cocaine.

In 1847, Ascanio Sobrero, an Italian chemist, synthesized the explosive nitrate ester nitroglycerin. Since there were few chemical explosives known at that time, it rapidly became the most widely used explosive for both wartime and peacetime uses. In 1866, Swedish chemist Alfred Bernhard Nobel found that, when mixed with diatomaceous earth, the liquid nitroglycerin was absorbed and stabilized, although the mixture could be ignited with a fuse. This material, which became known as dynamite, has remained one of the most widely used explosives.

Nitroglycerin is also used to treat the medical condition angina pectoris.

Work at Du Pont provided the basis for polyester fibers when investigations of macromolecules showed that acids containing two carboxylic acid groups and polyhydroxy alcohols could be polymerized. These polyesters were first produced on an industrial scale in Great Britain in 1948. Polyester production surpassed nylon production for the first time in 1970 to rank first in production among noncellulose, man-made fibers. Polyesters will continue to play a significant role in the industrial arena.

Principal terms

ALCOHOL: a compound that is composed of a hydroxy (-OH) functional group attached to a carbon atom

CARBOXYLIC ACID: a compound that contains the carboxyl functional group -CO₂H

ESTERIFICATION: the reaction of an alcohol with an acid to form an ester

FUNCTIONAL GROUP: an atom or group of atoms in an organic compound that gives the compound some of its characteristic properties

HYDROLYSIS: a chemical reaction in which a molecule is cleaved by water

SAPONIFICATION: a chemical reaction of an ester with sodium or potassium hydroxide to yield an alcohol and a carboxylate ion

Bibliography

Asimov, Isaac. THE WORLD OF CARBON. New York: Abelard-Schuman, 1958. This is a well-written overview of the different classes of carbon compounds. Presented in a clear, nontechnical fashion. The chapter entitled "Sour Substances" discusses carboxylic acids and their ester derivatives in the context of their natural occurrences and the usage of these compounds.

Barton, T. J., and J. A. Moore. ORGANIC CHEMISTRY: AN OVERVIEW. Philadelphia: Saunders College Publishing, 1978. Contains a concise treatment of the preparation and reactions of esters. Included is a good discussion of the metabolism of esters in living systems.

Lee, Jessie C., and Frederick A. Bettelheim. INTRODUCTION TO GENERAL, ORGANIC, AND BIOCHEMISTRY. Philadelphia: Saunders College Publishing, 1984. A text designed for college freshmen with emphasis on those aspects of organic chemistry that relate to living systems. The chapter entitled "Carboxylic Acids and Esters" discusses several practical applications of esters, as well as the preparation, properties, and reactions of these compounds.

Saunders, K. J. ORGANIC POLYMER CHEMISTRY. London: Chapman and Hall, 1973. This book contains a chapter on cellulose polymers and another on polyesters. These chapters provide a thorough treatment of topics such as the historical applications, synthetic aspects, and properties of these classes of polymers. The discussion of basic principles adequately prepares the reader to understand the technical details presented.

Stoker, H. S., and E. B. Walker. FUNDAMENTALS OF CHEMISTRY. Boston: Allyn & Bacon, 1988. Presents chemical principles along with numerous applications designed to interest students preparing for health or biologically related careers. The chapter on esters gives many interesting examples of their utility and discusses esters of inorganic acids. Cross-referencing provides access to related topics within the text.

Carbon and Carbon Group Compounds