Ketones

Type of physical science: Chemistry

Field of study: Chemical compounds

Organic chemical compounds consisting of molecules that contain a carbonyl group directly bonded only to carbon atoms are known as ketones. These compounds range from simple laboratory reagents to commercial chemicals produced in tonnage quantities.

Overview

Ketones are organic chemical compounds that consist of molecules containing a carbonyl group, a grouping of atoms in which two of the four available bonds of a carbon atom are linked to an oxygen atom. Chemists write the carbonyl group as

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using the lines to represent linkages, called covalent chemical bonds, and the letters C and O to represent an atom of carbon and an atom of oxygen, respectively. Ketone molecules have the two free carbon bonds of the carbonyl group attached to carbon atoms, which may in turn be linked to hydrogen, carbon, or other atoms.

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Closely related to the ketones are the aldehydes, another family of carbonyl compounds in which at least one of the two atoms linked to the carbonyl group is a hydrogen atom.

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Formaldehyde, the simplest aldehyde, is a special case in which both atoms linked to the carbonyl group are hydrogen atoms.

In the simplest ketone, acetone, the carbon atom of the carbonyl group is linked to two methyl (-CH₃) groups. Acetone is an example of a simple ketone, a term indicating that the carbon atom of the carbonyl group is linked to two identical groups. If the two groups are different, the compound is a mixed ketone. A cyclic ketone has a molecular structure in which the carbonyl group carbon atom is linked to several other carbon atoms in a ring structure; rings containing five or six carbon atoms are common, while rings of other size are found less frequently.

The nature of the carbonyl group, present in all ketones, accounts for the chemical behavior of these compounds. Consequently, two different ketones usually react similarly with a given chemical reagent, although the groups attached to the carbonyl carbon atom may affect chemical reactivity to some degree. In contrast, the physical properties of ketones depend upon the overall molecular structure. The effect of the carbonyl group is important, but the groups attached to the carbonyl carbon atom also have a direct effect on the physical properties. For example, the boiling point of acetone, a ketone, is almost 60 degrees Celsius higher than that of butane, an alkane hydrocarbon of comparable molecular weight. The difference is caused by the presence of the carbonyl group in the ketone (the alkane contains only carbon and hydrogen atoms). Among a series of ketones with different boiling points, any differences must be caused by the overall molecular structures rather than the carbonyl group, since each compound in the series contains a carbonyl group. In general, the more carbon atoms there are in the ketone molecule, the higher will be the boiling point.

A more detailed description of the nature of the carbonyl group will help in understanding the influence of this group on the chemical and physical properties of ketones. A covalent chemical bond is a bond formed by the sharing of a pair of electrons by two atoms. Thus, each line between C and O in the carbonyl group symbol represents the sharing of a pair of electrons by a carbon atom and an oxygen atom. When the covalent bond joins atoms of two different elements, as is the case in all ketone carbonyl groups, the electrons are usually shared unequally. Over time, the electrons are attracted more to the oxygen atom than to the carbon atom, and the carbonyl group develops a negative "end" at the oxygen atom and a positive "end" at the carbon atom; in other words, the carbonyl group is polar. The polarity does not represent full positive and negative electrical charges, only partial ones, represented (when shown) by the symbols δ+ and δ-:

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One of the important attractive forces between two ketone molecules is the attraction of the polar carbonyl group of one molecule for the polar carbonyl group of the other molecule. The attraction is a result of electrostatic forces, with the positive end of one carbonyl group attracting the negative end of another. These attractions, combined with other intermolecular attractive forces, are responsible for many of the observed physical properties.

The polar carbonyl group also accounts for the chemical behavior of ketones. Chemical reagents that are positive electrically, known as electrophilic ("electron-loving") reagents, react at the oxygen atom of the carbonyl group, which is the negative end of the group. Reagents that are negative electrically, known as nucleophilic ("nucleus-loving") reagents, react at the positive carbon-atom end of the carbonyl group. Even in cases in which the reaction site is not the carbonyl group itself, the electronic structure of the carbonyl group accounts for the reaction behavior. Reactions that do not occur at the carbonyl group usually take place at one of the alpha carbons, the carbon atoms immediately adjacent to the carbonyl group.

Ketones react with many different chemical reagents by reaction pathways known as reduction, nucleophilic addition, and alpha substitution.

Reduction of an organic compound involves a decrease in the oxygen content of the compound, an increase in the hydrogen content, or both. The hydrogen content increases when reduction occurs at the carbonyl group of a ketone; depending upon the chemical reagent involved in the reaction, the oxygen content either decreases at the same time or stays the same. In some cases, the carbonyl group changes into an alcohol; instead of two bonds between carbon and oxygen, as in the carbonyl group, there is only one bond between these atoms, each of which is also linked to a hydrogen atom. Alcohols produced from ketones are secondary alcohols: the -OH group is attached to a secondary carbon atom, one that is linked directly to two other carbon atoms.

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With certain other reagents, the reduction results in the replacement of the oxygen atom of the carbonyl group by two hydrogen atoms, forming a methylene (CH₂) group.

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Nucleophilic addition reactions also occur at the carbonyl group of ketones. A typical reagent can be represented by the general formula H-Nuc, where a hydrogen atom (H) is linked to an atom or group of atoms that is electron-rich (nucleophilic). When the reagent reacts with the ketone, the bond between the hydrogen atom and the nucleophilic group breaks, the hydrogen atom combines with the carbonyl group oxygen, the nucleophilic group combines with the carbonyl group carbon atom, and there is a change from two bonds to one between the carbon and oxygen atoms.

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Frequently, the initial product that forms undergoes dehydration (loss of a molecule of water). In most of these cases, the dehydration is intramolecular (within the molecule). The -OH group is lost from the carbon atom, and a hydrogen atom is lost from the nucleophilic group; at the same time, a second bond develops between the carbon and the nucleophilic group. In some cases, the dehydration is intermolecular (between two molecules), especially when the nucleophilic reagent is an alcohol.

The carbonyl group of the ketone is the reaction site in reduction and nucleophilic addition, but in alpha substitution, the reaction takes place at an alpha carbon atom immediately adjacent to the carbonyl group carbon atom. In these reactions, a hydrogen atom bonded to the alpha carbon is replaced by another atom, frequently a chlorine or bromine atom, or a group of atoms, such as an alkyl group (a group of carbon and hydrogen atoms with the formula C_n H_{2‗I‗n‗i‗+1}, where n is an integer).

If the ketone reacts with another carbonyl compound at the alpha carbon, the reaction is called a condensation reaction; the initial condensation product, a beta-hydroxy carbonyl compound, may lose a water molecule to give an unsaturated carbonyl compound, one in which there are two bonds between the alpha carbon and the carbon atom adjacent to it (the beta-carbon).

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The previous discussion indicates that the carbonyl group is the predominant feature of ketone molecules, influencing physical properties and determining chemical behavior. The importance of this group is also evident in both the common and the systematic names of ketones. Some common names--such as acetone, acetophenone, and civetone--do not reveal much about the molecular structure of the compound, except that the "-one" suffix indicates the compound is a ketone. The name acetone comes from Latin acetum ("vinegar") and the Greek suffix "-one," which designates a female descendant, as in "anemone" ("daughter of the wind"; anemos meaning "wind"). Other common names are more informative; names such as methyl ethyl ketone and methyl isobutyl ketone not only describe each of the groups attached to the carbonyl group but also emphasize that the compound is a ketone. In methyl ethyl ketone, for example, there are a methyl group (-CH₃) and an ethyl group (-CH₂CH₃) attached to the carbonyl group.

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The systematic names of ketones use the carbonyl group as a point of reference. The longest carbon chain that includes the carbonyl group is selected for the base name, using the same stem as for the corresponding alkane. The terminal "-e" of the alkane name is replaced by "-one" (butane becomes butanone, hexane becomes hexanone, and so on). The carbonyl group is located by consecutively numbering the carbon atoms in the parent chain, starting with the end closest to the carbonyl group; the number of the carbonyl group carbon atom becomes part of the name. Substituents are designated by naming the substituent and citing the number of the carbon atom to which it is attached. For the compound shown below, the common name is methyl isobutyl ketone, and the systematic name is 4-methyl-2-pentanone.

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In this example, the five numbered carbon atoms constitute the longest chain, which includes the carbonyl group, and the base name is 2-pentanone (from the five-carbon alkane name, pentane); there is one substituent, a methyl group (-CH₃), bonded to carbon 4.

Applications

Ketones are among the most important organic compounds found in the laboratory or in industry. They are frequently used as starting materials or found as products in laboratory applications. The ketone functional group is also present, frequently along with other groups, in a number of biochemical compounds, and these compounds play a role in several important metabolic processes. Industrial applications are widespread, accounting for the production and consumption of enormous quantities each year.

One of the principal industrial applications of the simpler ketones is their use as solvents. The major use of acetone, the most important industrial ketone, is as a chemical intermediate, although it also ranks high as an industrial solvent. Each year, the amount of acetone produced exceeds about 1 billion kilograms, ranking it among the top fifty chemicals in the United States in annual production. Methyl ethyl ketone and methyl isobutyl ketone are also important industrial solvents, used in surface coatings and other applications.

Several methods for the industrial production of acetone exist. One commercial preparation involves the dehydrogenation (removal of two hydrogen atoms) of isopropyl alcohol at high temperatures (about 300 degrees Celsius) over a copper oxide catalyst. Another important process, the air oxidation of cumene (isopropylbenzene) to phenol, produces acetone as a by-product. Cumene is a component of petroleum, but it is produced commercially by the reaction of benzene with propylene in the presence of a catalyst. The preparation of acetone by oxidation of cumene is interesting since the process is also one of the major pathways to phenol. These two products, phenol and acetone, are the starting materials for the preparation of bisphenol A, a key intermediate in the synthesis of polycarbonate polymers and epoxy resins. The polycarbonates are extremely tough and shatter-resistant, and the epoxy resins are widely used as adhesives and in paints and other surface coatings. Acetone is a starting material not only for bisphenol A but also for methyl isobutyl ketone and methyl methacrylate, the monomer used to prepare the polymer poly(methyl methacrylate). Poly(methyl methacrylate) is the colorless, optically transparent material known as Lucite or Plexiglas.

Cyclohexanone, a six-carbon cyclic ketone, is another important chemical intermediate. It plays a key role in the industrial synthesis of the six-carbon monomer starting materials used in the preparation of such polymers as nylon 6 and nylon 66. The preparation of cyclohexanone involves the oxidation of the alcohol cyclohexanol, which is a product of the hydrogenation (reduction) of phenol.

Many ketones have characteristic, pleasant fragrances that lead to their use in cosmetics and foods. For example, biacetyl finds use as a flavoring agent in margarine. Several naturally occurring ketones have a musk odor, leading to their use in the perfume industry. Muskone, obtained from the female musk deer, is a large-ring ketone containing fifteen carbon atoms in the ring; civetone, obtained from the civet cat, is also a cyclic ketone, with seventeen carbon atoms in the ring. Pure civetone has an extremely obnoxious odor, but when highly diluted, as is the case in perfume formulations, the odor is pleasant. The ketone carvone exists in two molecular forms, the only difference being that the two forms are mirror images; one form of carvone is responsible for the odor of caraway, and the other form is responsible for the odor of spearmint.

The ketones that are important in biochemical processes usually are not simple ketones but are compounds that contain several functional groups, including the ketone carbonyl group. Examples of such compounds include pyruvic acid, an important compound in amino acid metabolism, and dihydroxyacetone phosphate, fructose 6-phosphate, and fructose 1,6-diphosphate, compounds involved in glycolysis, one of the anaerobic fermentations by which various organisms extract chemical energy from organic fuels in the absence of oxygen. The sugars D-ribulose 5-phosphate and D-xylulose 5-phosphate, both of which are keto sugars, or ketoses, are intermediates in the pentose phosphate pathway, an alternate pathway of glucose degradation.

The steroid hormones are another group of biochemical compounds that are of considerable interest, and many of these compounds contain the ketone functional group, along with other functional groups. These important compounds include the estrogen estrone, the androgens testosterone and dihydrotestosterone, the progestational hormone progesterone, and the steroid hormones of the adrenal cortex, cortisone, cortisol, aldosterone, and corticosterone. Closely related are the synthetic progestin components of oral contraceptives, such as norethynodrel and norethindrone.

The versatility of ketones as laboratory reagents accounts for the extensive use of these compounds as starting materials, intermediates, and products in organic research. Numerous applications described in the primary chemical literature illustrate their importance to organic synthesis.

Context

From the days of alchemy, ketones have played a major role in the development of organic chemistry from both a theoretical and a practical point of view. Continued interest in ketones as laboratory-scale reagents and as useful products is a reflection of their versatility and usefulness.

Long before the development of organic structural theory permitted a detailed description of ketones, investigators such as Johann Rudolf Glauber, an early seventeenth century German alchemist, had isolated and described the properties of acetone. Nearly three centuries later, essentially the same method of preparation--decomposition of a metal salt of acetic acid--was in use for the commercial production of acetone.

Nevertheless, during World War I, the British needed more acetone than could be produced by the destructive distillation of calcium acetate, the method in use at the time. Large quantities of acetone were needed for the manufacture of cordite, a military explosive, and as a solvent for lacquers used to coat the fabric coverings of military aircraft. Cordite, a so-called smokeless powder, is a mixture of about 60 percent gun cotton (cellulose nitrate) and about 40 percent nitroglycerine, thickened with petrolatum and gelatinized by dissolving in acetone.

During the war years, Chaim Azriel Weizmann, a Soviet-born chemist, was working on methods for making synthetic rubber while serving as a professor at the University of Manchester. In his search for appropriate starting materials, he examined a fermentation process that converted starch or sugar to n-butyl alcohol. Weizmann found that anaerobic fermentation of corn mash or black-strap molasses inoculated with various soil bacteria, including Clostridium acetobutylicum, produced both n-butyl alcohol and acetone in a ratio of 2:1 or 3:1. Acetone was the product of interest at the time, but in later years, n-butyl alcohol became the main product and acetone the by-product.

Large-scale methods for producing acetone and other commercially important ketones depend heavily upon petroleum feedstock supplies, although microbiological fermentation, such as the Weizmann process, provides independence from petroleum reserves by relying on renewable resources. Economic and environmental factors, however, may determine the practicability of these commercial processes. A trend toward more extensive use in automobiles of engineering plastics such as nylon, polycarbonate, and poly(methyl methacrylate), and new applications in the construction industry, may increase the demand for ketone starting materials.

Principal terms:

ALCOHOLS: compounds that have a hydroxyl group (-OH) bonded to a saturated carbon atom; a carbon atom that is bonded to four atoms

CARBONYL GROUP: a portion of an organic molecule consisting of a carbon atom and an oxygen atom joined by two covalent bonds

CONDENSATION REACTION: a reaction between two carbonyl components that involves a combination of nucleophilic addition and alpha substitution

COVALENT CHEMICAL BOND: an attractive force resulting from the sharing of two electrons that holds two atoms together in a molecule

ELECTROPHILIC REAGENT: a substance that accepts an electron pair from a nucleophile in a reaction that forms a polar bond

NUCLEOPHILIC REAGENT: a substance that donates an electron pair to an electrophile in a reaction that forms a polar bond

ORGANIC OXIDATION: a reaction that decreases the hydrogen content or increases the oxygen content of a molecule

ORGANIC REDUCTION: a reaction that increases the hydrogen content or decreases the oxygen content of a molecule

POLARITY: the unsymmetrical distribution of electrons in molecules that results when one atom attracts electrons more strongly than another

Bibliography

Amend, John R., Bradford P. Mundy, and Melvin T. Arnold. GENERAL, ORGANIC, AND BIOLOGICAL CHEMISTRY. Philadelphia: Saunders College Publishing, 1990. An introductory textbook intended for students in a first course in allied health chemistry. Chapter 19 presents material on aldehydes and ketones, including examples and illustrations that emphasize the role of chemistry in everyday life. The reader may need to refer to earlier chapters in the book for full understanding of the chapter 19 material.

Asimov, Isaac. THE WORLD OF CARBON. London: Abelard-Schuman, 1958. Although written for the young reader, this book is an excellent introductory discussion of organic chemistry for all those who have an interest in the world around them. Factual information is presented in short, crisp sentences.

Bettelheim, Frederick A., and Jerry March. INTRODUCTION TO GENERAL, ORGANIC, AND BIOCHEMISTRY. 3d ed. Philadelphia: Saunders College Publishing, 1990. This introductory textbook is written for students who have little or no chemistry background. Chapter 13 presents material on aldehydes and ketones and includes colorful illustrations and relevant examples. The reader may need to refer to earlier chapters for full understanding of the chapter 13 material.

Budavari, Susan, ed. THE MERCK INDEX. 11th ed. Rahway, N.J.: Merck, 1989. The best single-source reference for finding information on the molecular structure and the properties of individual ketones and other organic compounds.

Chenier, Philip J. SURVEY OF INDUSTRIAL CHEMISTRY, New York: John Wiley, 1986. A basic text of introductory material, covering all important areas of the chemical industry yet limited in scope. Includes material on acetone and other important ketones and describes the relationship of these compounds to other important industrial chemicals.

Cooley, Lloyd C. "Acetone." INDUSTRIAL AND ENGINEERING CHEMISTRY, Industrial Edition 29 (December, 1937): 1399-1407. This review article on acetone includes interesting historical information on this important ketone. The article discusses several commercial processes for the production of acetone. A number of technical details are given, but the technical information is presented in language that does not require a strong technical background for understanding.

Read, John. A DIRECT ENTRY TO ORGANIC CHEMISTRY. New York: Harper and Brothers, 1960. This book starts from first principles and assumes no previous knowledge on the part of the reader. Attempts to outline in a limited number of words a scientific picture of the nature and scope of organic chemistry.

Waddams, A. L. CHEMICALS FROM PETROLEUM. 4th ed. Houston: Gulf, 1980. An introduction to the petrochemicals industry. Whether scientifically qualified or not, new entrants to the field have found this book to be useful. Provides information on the preparation of such important ketones as acetone, methyl ethyl ketone, and cyclohexanone, as well as their relationship to the petrochemicals industry.

Weiss, John M. "Acetone: Tonnage By-Product." CHEMICAL AND ENGINEERING NEWS 36 (June 9, 1958): 79-82. This review article describes some of the history of the commercial production of acetone and traces the factors responsible for the fluctuation of acetone from by-product to main product to by-product.