Aldehydes

Type of physical science: Chemistry

Field of study: Chemical compounds

An aldehyde is an organic compound containing a carbonyl group bonded to one or two hydrogens. Aldehydes are common, naturally occurring compounds.

Overview

The oxygen atom is capable of forming two covalent bonds with other atoms. If both of these bonds are attached to a single carbon atom, then the result is a carbon-oxygen double bond (C=O). This functional group C=O is called the carbonyl group. Since carbon atoms form a total of four covalent bonds in their organic compounds, the carbon of the carbonyl group still has two bonds available for further use. When a hydrogen atom is attached to one of these bonds, the compound is classified as an aldehyde. The remaining carbon bond may be attached either to another carbon or to a hydrogen. The carbonyl group with its attached hydrogen may be referred to as the aldehyde group, which is often written as CHO or CH=O.

The carbon and oxygen atoms composing the carbonyl group have different attractions for the electrons that they share. The oxygen attracts the electrons more strongly than the carbon.

Consequently, there is an unequal distribution of these shared electrons, resulting in the oxygen becoming negatively charged and the carbon positively charged. These charges are not full units of charge, such as exist in ions, but rather fractional charges, since the electrons are still associated to some degree with both atoms. Therefore, the carbonyl group is polar.

The polarity of the carbon-oxygen double bond is responsible for many of the physical properties of aldehydes. The polarized character of the carbonyl group introduces a certain degree of attraction between aldehyde molecules. Since the oxygen atom of each aldehyde molecule carries a negative charge, it will be attracted to atoms that carry a positive charge, such as the carbon of another aldehyde molecule. This attraction affects both the melting and boiling points of aldehydes, since melting and boiling involve the separability of the molecules. The melting and boiling points of aldehydes are typically 50 degrees Celsius to 80 degrees Celsius higher than those of hydrocarbons (organic compounds containing only carbon and hydrogen atoms) which are of comparable size, shape, and molecular weight.

The polarity of the carbonyl group also affects the solubility of aldehydes. The negatively charged oxygen will also be attracted to positively charged regions of molecules that are not aldehydes. Water is a polar molecule in which the two hydrogen atoms bear partial positive charges and the oxygen atom bears a partial negative charge. The carbonyl oxygen is strongly attracted to the hydrogen of a water molecule. The force that holds the two molecules together is called a hydrogen bond. This hydrogen bonding allows low-mass aldehydes to dissolve in water more readily than nonpolar hydrocarbons, which do not hydrogen-bond with water molecules. Nevertheless, higher-mass aldehydes are insoluble in water because they have a large number of carbons, which makes the molecule resemble a hydrocarbon overcoming the effect of the hydrogen bond.

Most aldehydes have a strong aroma, the character of which varies. Low-mass aldehydes are colorless gases or liquids with characteristically pungent odors, while higher-mass aldehydes are used in perfumes.

Primary alcohols (in which the carbon bearing the -OH group is bonded only to one other carbon) decompose at 550-600 degrees Celsius in the presence of a suitable catalyst, such as copper or silver, to form hydrogen gas and an aldehyde. This reaction, called a dehydrogenation reaction, is the industrial method for producing aldehydes. Aldehydes can be prepared by the oxidation of primary alcohols, resulting in the removal of the single hydrogen that is attached to the alcohol oxygen and one of the hydrogens attached to the adjoining carbon.

Thus, the ethanol molecule, CH₃CH₂OH, becomes acetaldehyde, CH₃CHO. In the laboratory, reagents such as potassium permanganate (KMnO₄) or potassium dichromate (K₂Cr₂O₇) in acid solution can be used to prepare aldehydes from primary alcohols. Unfortunately, it is not always easy to stop the oxidation reaction at the aldehyde stage because aldehydes themselves are easily oxidized to carboxylic acids. One way to ensure that the reaction stops at the aldehyde stage is to distill the aldehyde as it is formed before it can be oxidized further. This distillation is possible because aldehydes have lower boiling points than corresponding alcohols or carboxylic acids.

Many tests for the detection of aldehydes are based on their ease of oxidation. Treating an aldehyde with potassium dichromate in acid solution results in the formation of the corresponding carboxylic acid. During this process, the red Cr⁶⁺ ion is converted into the green Cr³⁺ ion. Thus, a change in solution color from red to green is an indicator that a reaction has occurred. This test, which is called the chromic acid test, is not specific for aldehydes since primary and secondary alcohols also give identical results. Aldehydes are so easily oxidized that they react with very mild oxidizing agents such as the Ag+ ion, which does not oxidize other species such as alcohols. This is the basis for the Tollen's reagent. When an aldehyde is treated with Ag+ in an aqueous alkaline ammonia solution, the aldehyde is oxidized to the salt of a carboxylic acid and the Ag+ ion is reduced to silver metal. The silver metal forms a black precipitate, or if the reaction vessel is very clean, the silver plates the glass to form a silver mirror. Aldehydes can also be oxidized by Cu²⁺ ions.

In the Benedict's test, an aldehyde is treated with a basic solution of Cu²⁺ ions. The Cu²⁺ solution is blue in color. As the aldehyde is oxidized to the salt of the carboxylic acid, the Cu²⁺ ion is converted to Cu₂O, which is a red precipitate. Aldehydes are so easily oxidized that even oxygen in the atmosphere can bring about a gradual change. Thus, opened containers of aldehydes often become contaminated over time with their corresponding carboxylic acid and must be purified before their use.

The carbonyl group has a great capacity for adding a variety of reagents, especially if the reagent contains a hydrogen atom that can be added to the carbonyl oxygen forming an -OH group. For example, aldehydes can be reduced to alcohols by the addition of hydrogen gas in the presence of a catalyst such as nickel metal. Metal hydrides such as sodium borohydride, NaBH₄, also cause this reduction. In the laboratory, chemists frequently carry out these reduction reactions utilizing metal hydrides because the experimental procedures are more convenient.

Alcohols can be added to the carbonyl group of an aldehyde to form a compound called a hemiacetal. For example, if ethanol, CH₃CH₂OH, is added to acetaldehyde,CH₃CHO, then the resulting hemiacetal has the CH₃CH₂O of the ethanol attached to the carbon of the carbonyl group (now an ether) and the H of the ethanol attached to the oxygen of the carbonyl group. Thus, a hemiacetal is a compound containing one -OR (R is a carbon chain) group, one -OH group, and an -H attached to the same carbon. This reaction is extremely important in the chemistry of carbohydrates. Simple sugars may contain both alcohol and aldehyde groups. In these molecules, an alcohol group at one end of the molecule reacts with the aldehyde group at the other end of the molecule to form a hemiacetal. The sugar molecule is no longer a chain but rather a ring structure. The OH group of a hemiacetal can react with another alcohol molecule to form an acetal, which has two ether groups attached to the carbon which was originally the carbonyl carbon. An acetal is a compound containing two -OR groups and an -H on the same carbon. For example, the -OH of one sugar hemiacetal can react with the -OH of another sugar hemiacetal to form disaccharides, which are the sweeteners often found in foods.

Many of these simple sugar molecules can join together via this reaction to yield polysaccharides, such as starch and cellulose.

The common names for aldehydes are determined by the names of the acids formed when they are oxidized. This system results from the fact that the carboxylic acids were well known before aldehydes were discovered. Thus, the common name for an aldehyde is derived by taking the name for the corresponding carboxylic acid and changing the "-ic" acid portion of the name to "-aldehyde." For example, when CH₃CHO is oxidized, it gives acetic acid,CH₃CO₂H. The name of the aldehyde is acetaldehyde. To name the aldehyde systematically, the name of the alkane containing the same number of carbons is used changing the "-ane" ending to "-anal." Thus, acetaldehyde could also be named ethanal since the two-carbon alkane is ethane.

Applications

The simplest aldehyde is formaldehyde, HCHO. It is a flammable, colorless, toxic gas with a characteristic pungent odor that is commercially prepared by the oxidation of methyl alcohol. When inhaled, formaldehyde irritates the membranes of the eyes, nose, and throat. It is an example of a lachrymator (from the Latin for tears). Formaldehyde is an irritant because it combines with the proteins in tissues, hardening and killing them. In the process, it also kills any microorganisms that might be present in the tissue. Formaldehyde is widely utilized in preserving biological specimens such as tissues, organs, and organisms. It serves to prevent decay and to harden the tissue to increase the ease with which it may be handled. Since formaldehyde boils at -21 degrees Celsius, it is typically used as a 37 percent aqueous solution called formalin. Formalin is sometimes a component in embalming fluids. Formaldehyde is also used as an antiseptic, disinfectant, and fungicide since it inactivates certain enzymes. Industrially, formaldehyde is the most important aldehyde, as it is used to produce paper, plywood, home insulation, leather, drugs, and cosmetics.

One of the most important uses of formaldehyde is in the production of condensation polymers with phenols. In this process, the formaldehyde reacts with the phenol to produce water molecules and a polymer, which is glassy in appearance. These polymers can be softened and molded into any desired shape. Upon cooling, these polymers, called plastics, become hard and keep their shapes. Formica, used to laminate surfaces, and Melmac, used in dinnerware, are polymers made from formaldehyde and the amine melamine.

Acetaldehyde is the two-carbon aldehyde. It has a boiling point of 21 degrees Celsius and is not easy to handle as either a liquid or a gas. Like formaldehyde, it has an irritating, pungent odor. If acetaldehyde is treated with strong acid, three molecules combine into a ring called paraldehyde. Paraldehyde is a liquid with a boiling point of 122 degrees Celsius, so it is more easily handled than acetaldehyde. When treated with weak acid, paraldehyde is broken down into its constituent acetaldehyde molecules. When dissolved in water, paraldehyde has a calming effect on the nervous sytem which allows a patient to fall asleep within ten to fifteen minutes. Consequently, this aldehyde has been prescribed as a sedative-hypnotic.

Chloral (CCl₃CHO) is a more effective sedative.

Chloral is an oily liquid that in the presence of water yields a white crystalline hydrate called chloral hydrate. Chloral was first tested pharmaceutically in the mistaken belief that it would be converted to the anesthetic chloroform in the body. Actually, the body reduces chloral to its corresponding alcohol, trichloroethanol (Cl₃CCH₂OH), which is responsible for its depressant properties. Chloral hydrate has a penetrating, slightly acrid odor and a bitter caustic taste. It is very soluble in both water and alcohol, and dissolving chloral hydrate in alcohol helps to mask its disagreeable taste. In addition, alcohol synergistically increases the depressant effect of chloral hydrate to produce a very potent sedative. Alcohol mixtures of chloral hydrate are commonly referred to as "knockout drops" or "Mickey Finns."

Sedatives of this type are addictive, and overdoses can cause death.

Acetaldehyde is present in the body as a metabolic intermediate (a compound which, when formed in one reaction, is immediately consumed in another reaction). Therefore, the compound is never present in very large quantities. When ethanol is ingested, it is oxidized to acetaldehyde in the liver. This acetaldehyde is then further oxidized to acetic acid, which is ultimately converted to carbon dioxide and water. Other alcohols are not suitable for consumption because their oxidation products are more toxic than acetaldehyde. For example, methanol is converted in the liver into formaldehyde. For persons whose metabolism is slow to convert acetaldehyde to acetic acid, ingesting alcohol can be very unpleasant. Acetaldehyde build-up causes nausea and many of the other symptoms associated with hangovers. Large doses of acetaldehyde can cause death by respiratory paralysis, which is why drinking large quantities of alcohol over a very short time span can have serious consequences. Disulfiram is a drug that is administered in the treatment of alcoholism. This drug produces a sensitivity to alcohol that results in a highly unpleasant reaction when the patient under treatment ingests even small amounts of alcohol. It works by blocking the oxidation of alcohol at the acetaldehyde stage, allowing it to accumulate.

Many higher-mass aldehydes have pleasant fragrances or flavors. For example, citral is a ten-carbon aldehyde that is responsible for the characteristic odor of lemons and is used in lemon flavorings. Aromatic aldehydes are compounds in which the aldehyde group is attached to a benzene ring. These compounds all have notable fragrances. Three examples of naturally occurring aromatic aldehydes are benzaldehyde (flavor and odor of almonds), vanillin (vanilla flavor), and cinnamaldehyde (flavor and odor of cinnamon). These compounds are not very soluble in water but do dissolve in ethanol. For this reason, perfumes and flavoring extracts containing these compounds also contain alcohol.

Context

Compounds containing the aldehyde group have been utilized for dietary purposes since very early times because of the sweetness of these compounds and their characteristic flavors. The first documented laboratory synthesis of an aldehyde was accomplished by the German chemist Justus von Liebig in 1835. He dehydrogenated ethanol to generate a compound (acetaldehyde) to which he gave the Latin name alcohol dehydrogenatus. The term aldehyde arose as an abbreviation for this name.

The discovery of the "silver on glass" mirror is attributed to von Liebig, who in 1835 observed that, when heating an aldehyde with a solution containing silver ions in a glass vessel, a brilliant deposit of metallic silver was formed on the surface of the glass (a practical use of the Tollen's test). The industrial process known as silvering was introduced in 1840 and has been used extensively to produce household mirrors. A modification of this silvering process, called the cold process, takes advantage of the power of sugar to reduce the silver ion. This method has been generally adopted for the silvering of mirrors used in astronomical telescopes.

Aldehydes were among the first organic compounds used as hypnotics, with chloral being introduced in 1869 and paraldehyde in 1882. To circumvent the disadvantages of these two compounds, such as objectionable tastes and stomach irritation, milder and less harmful analogs have been developed, which are commonly prescribed sedatives. Many other compounds of medicinal value, such as natural and synthetic steroids, also possess the aldehyde group. Often, medicinal agents possess either objectionable taste or odor characteristics. To make these preparations more palatable and acceptable to the patient, flavoring agents and perfumes are added. Benzaldehyde, vanillin, and ethyl vanillin are aldehydes that are commonly used for this purpose. Aldehydes will continue to play important pharmaceutical roles in the future.

A number of aldehydes have important industrial uses and are manufactured on a large scale. They are used as solvents, polymer components, flavoring agents, perfume ingredients, and chemical intermediates for synthetic processes. Bakelite, invented by Leo Hendrik Baekeland in 1909, was one of the earliest widely used polymers. It dominated the world of plastics into the 1930's and remains an important plastic of the thermosetting type (meaning it cannot be melted after it sets). Formaldehyde polymers are used not only as plastics but also, even more significantly, as adhesives and coatings.

A number of compounds with important physiological roles such as retinene and pyridoxal phosphate contain the aldehyde group. Retinene, metabolically obtained by the oxidation of vitamin A1, forms a pigment called rhodopsin, which is essential to human vision upon combination with a protein. Pyridoxyl phosphate is a coenzyme that, when combined with a variety of enzymes, catalyzes reactions essential to life. Sugars are also another important class of naturally occurring compounds in which the aldehyde group is often present.

Principal terms

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

CARBONYL GROUP: the functional group in organic chemistry that consists of a carbon atom that is double bonded with an oxygen atom (C=O)

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

OXIDATION REACTION: the loss of one or more electrons from an atom, molecule, or ion; in organic reactions, the loss of hydrogen or the gain of oxygen

OXIDIZING AGENT: a substance that can cause oxidation

POLARITY: the tendency of a molecule to have positive and negative poles because of the unequal sharing of a pair of electrons

REDUCTION REACTION: the gain of one or more electrons to an atom, molecule, or ion; in organic reactions, the gain of hydrogen or the loss of oxygen

Bibliography

Barton, T. J., and J. A. Moore. ORGANIC CHEMISTRY: AN OVERVIEW. Philadelphia: Saunders College Publishing, 1978. The chapter entitled "Aldehydes and Ketones" provides a concise treatment of the properties and reactions of aldehydes. Included is an informative discussion on the chemistry of formaldehyde.

Fessenden, J. S., and R. J. Fessenden. FUNDAMENTALS OF ORGANIC CHEMISTRY. New York: Harper & Row, 1990. A brief text intended for students in science-related fields. Discussions of the applications of organic chemistry in consumer products and in nature are an important feature of this text. The section concerning aldehydes includes a treatment of the chemistry of vision.

Lee, Jessie C., and Frederick A. Bettelheim. INTRODUCTION TO GENERAL, ORGANIC, AND BIOCHEMISTRY. Philadelphia: Saunders College Publishing, 1984. Geared for college undergraduates with an emphasis on organic chemistry. The chapter concerned with aldehydes contains a very good discussion of the formation of hemiacetal and acetals.

Saunders, K. J. ORGANIC POLYMER CHEMISTRY. London: Chapman and Hall, 1973. Contains an extensive treatment of the class of phenol-formaldehyde polymers. Treatment of the topic includes historical perspective, synthetic aspects, and polymer properties. Includes a discussion of basic principles.

Solomons, T. W. G. ORGANIC CHEMISTRY. New York: John Wiley & Sons; 1988. A popular college-level organic chemistry text that devotes two chapters to aldehydes and the related carbonyl compounds ketones. The topics of preparation and reactions of aldehydes are covered in a thorough manner. This text assumes knowledge of basic chemical principles.

Carbon and Carbon Group Compounds