Chapter 5 Aldehydes and Ketones

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Chapter 5Aldehydes and

Ketones

General, Organic, and Biological Chemistry, Fifth EditionH. Stephen Stoker

Brroks/Cole Cengage Learning. Permission required for reproduction or display.

Prepared by:GIZEL R. SANTIAGO

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Chapter 3 Topics• The Carbonyl Group• Compounds Containing a Carbonyl Group • The Aldehyde and Ketone Functional Groups • Nomenclature for Aldehydes • Nomenclature for Ketones • Isomerism for Aldehydes and Ketones • Selected Common Aldehydes and Ketones • Physical Properties of Aldehydes and Ketones • Preparation of Aldehydes and Ketones • Oxidation and Reduction of Aldehydes and Ketones • Reaction of Aldehydes and Ketones with Alcohols

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The Carbonyl GroupBoth aldehydes and ketones contain a carbonyl functional group. A carbonyl group is a carbon atom double-bonded to an oxygen atom. The structural representation for a carbonyl group is

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The Carbonyl GroupCarbon–oxygen and carbon–carbon double bonds differ in a major way. A carbon– oxygen double bond is polar, and a carbon–carbon double bond is nonpolar. The electronegativity of oxygen (3.5) is much greater than that of carbon (2.5). Hence the carbon–oxygen double bond is polarized, the oxygen atom acquiring a fractional negative charge (-) and the carbon atom acquiring a fractional positive charge (+).

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The Carbonyl Group

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The Carbonyl GroupAll carbonyl groups have a trigonal planar structure. The bond angles between the three atoms attached to the carbonyl carbon atom are 1200, as would be predicted using VSEPR theory.

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Compounds Containing A Carbonyl Group

The carbon atom of a carbonyl group must form two other bonds in addition to the carbon–oxygen double bond in order to have four bonds. The nature of these two additional bonds determines the type of carbonyl-containing compound it is.

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Compounds Containing A Carbonyl Group1. Aldehydes. In an aldehyde, one of the two additional bonds that the carbonyl carbon atom forms must be to hydrogen atom. The other may be to a hydrogen atom, an alkyl or cycloalkyl group, or an aromatic ring system.

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2. Ketones. In a ketone, both of the additional bonds of the carbonyl carbon atom must be to another carbon atom that is part of an alkyl, cycloalkyl, or aromatic group.

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3. Carboxylic acids. In a carboxylic acid, one of the two additional bonds of the carbonyl carbon atom must be to a hydroxyl group, and the other may be to a hydrogen atom, an alkyl or cycloalkyl group, or an aromatic ring system. The structural parameters for a carboxylic acid are the same as those for an aldehyde except that the mandatory hydroxyl group replaces the mandatory hydrogen atom of an aldehyde.

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Compounds Containing A Carbonyl Group4. Esters. In an ester, one of the two additional bonds of the carbonyl carbon atom must be to an oxygen atom, which in turn is bonded to an alkyl, cycloalkyl, or aromatic group. The other bond may be to a hydrogen atom, alkyl or cycloalkyl group, or an aromatic ring system. The structural parameters for an ester differ from those for a carboxylic acid only in that an —OH group has become an —O—R or —O—Ar group.

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Compounds Containing A Carbonyl Group5. Amides. The previous four types of carbonyl compounds contain the elements carbon, hydrogen, and oxygen. Amides are different from these compounds in that the element nitrogen, in addition to carbon, hydrogen, and oxygen, is present. In an amide, an amino group (—NH2) or substituted amino group replaces the —OH group of a carboxylic acid.

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Aldehydes and ketones are the first two of the five major classes of carbonyl compounds. They share the common feature of having only one oxygen atom present, the oxygen atom of the carbonyl group.

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The Aldehyde and Ketone Functional Group

An aldehyde is a carbonyl-containing organic compound in which the carbonyl carbon atom has at least one hydrogen atom directly attached to it. The remaining group attached to the carbonyl carbon atom can be hydrogen, an alkyl group (R), a cycloalkyl group, or an aryl group (Ar).

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The Aldehyde and Ketone Functional GroupLinear notations for an aldehyde functional group and for an aldehyde itself are —CHO and RCHO, respectively. Note that the ordering of the symbols H and O in these notations is HO, not OH (which denotes a hydroxyl group).

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A ketone is a carbonyl-containing organic compound in which the carbonyl carbon atom has two other carbon atoms directly attached to it. The groups containing these bonded carbon atoms may be alkyl, cycloalkyl, or aryl.

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The general condensed formula for a ketone is RCOR, in which the oxygen atom is understood to be double-bonded to the carbonyl carbon at the left of it in the formula.

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Cyclic aldehydes are not possible. For an aldehyde carbonyl carbon atom to be part of a ring system it would have to form two bonds to ring atoms, which would give it five bonds. Unlike aldehydes, ketones can form cyclic structures.

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Cyclic ketones are not heterocyclic ring systems as were cyclic ethers.

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Aldehydes and ketones are related to alcohols in the same manner that alkenes are related to alkanes. Removal of hydrogen atoms from each of two adjacent carbon atoms in an alkane produces an alkene. In a like manner, removal of a hydrogen atom from the —OH group of an alcohol and from the carbon atom to which the hydroxyl group is attached produces a carbonyl group.

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Nomenclature for Aldehydes

Rule 1: Select as the parent carbon chain the longest chain that includes the carbon atom of the carbonyl group. Rule 2: Name the parent chain by changing the -e ending of the corresponding alkane name to -al.

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Rule 3: Number the parent chain by assigning the number 1 to the carbonyl carbon atom of the aldehyde group. Rule 4: Determine the identity and location of any substituents, and append this information to the front of the parent chain name.

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Nomenclature for AldehydesUnlike the common names for alcohols and ethers, the common names for aldehydes are one word rather than two or three. In the IUPAC system, aromatic aldehydes—compounds in which an aldehyde group is attached to a benzene ring—are named as derivatives of benzaldehyde, the parent compound.

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The last of these compounds is named as a benzaldehyde rather than as a phenol because the aldehyde group has priority over the hydroxyl group in the IUPAC naming system.

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Nomenclature for KetonesAssigning IUPAC names to ketones is similar to naming aldehydes except that the ending -one is used instead of -al. Rule 1: S elect as the parent carbon chain the longest carbon chain that includes the carbon atom of the carbonyl group. Rule 2: Name the parent chain by changing the -e ending of the corresponding alkane name to -one. This ending, -one, is pronounced "own."

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Nomenclature for KetonesRule 3: Number the carbon chain such that the carbonyl carbon atom receives the lowest possible number. The position of the carbonyl carbon atom is noted by placing a number immediately before the name of the parent chain. Rule 4: Determine the identity and location of any substituents, and append this information to the front of the parent chain name.

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Nomenclature for Ketones

Rule 5: Cyclic ketones are named by assigning the number 1 to the carbon atom of the carbonyl group. The ring is then numbered to give the lowest number(s) to the atom(s) bearing substituents.

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Nomenclature for KetonesThe procedure for coining common names for ketones is the same as that used for ether common names. They are constructed by giving, in alphabetical order, the names of the alkyl or aryl groups attached to the carbonyl functional group and then adding the word ketone. Unlike aldehyde common names, which are one word, those for ketones are two or three words.

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Three ketones have additional common names besides those obtained with the preceding procedures. These three ketones are

Acetophenone is the simplest aromatic ketone.

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Isomerism for Aldehydes and KetonesConstitutional isomers exist for aldehydes and for ketones, and between aldehydes and ketones (functional group isomerism). The compounds butanal and 2-methylpropanal are examples of skeletal aldehyde isomers; the compounds 2-pentanone and 3-pentanone are examples of positional ketone isomers.

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Aldehydes and ketones with the same number of carbon atoms and the same degree of saturation are functional group isomers. Molecular models for the isomeric C3 compounds propanal and propanone, which both have the molecular formula C3H6O.

Isomerism for Aldehydes and Ketones

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Selected Common Aldehydes and Ketones

Formaldehyde, the simplest aldehyde, with only one carbon atom, is manufactured on a large scale by the oxidation of methanol.

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Its major use is in the manufacture of polymers. At room temperature and pressure, formaldehyde is an irritating gas. Bubbling this gas through water produces formalin, an aqueous solution containing 37% formaldehyde by mass or 40% by volume. (This represents the solubility limit of formaldehyde gas in water.) Very little free formaldehyde gas is actually present in formalin; most of it reacts with water, producing methylene glycol.

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Formalin is used for preserving biological specimens, anyone who has experience in a biology laboratory is familiar with the pungent odor of formalin. Formalin is also the most widely used preservative chemical in embalming fl uids used by morticians. Its mode of action involves reaction with protein molecules in a manner that links the protein molecules together; the result is a “hardening” of the protein.

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Selected Common Aldehydes and KetonesAcetone, a colorless, volatile liquid with a pleasant, mildly “sweet” odor, is the simplest ketone and is also the ketone used in largest volume in industry. Acetone is an excellent solvent because it is miscible with both water and nonpolar solvents. Acetone is the main ingredient in gasoline treatments that are designed to solubilize water in the gas tank and allow it to pass through the engine in miscible form. Acetone can also be used to remove water from glassware in the laboratory. And it is a major component of some nail polish removers.

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Selected Common Aldehydes and KetonesSmall amounts of acetone are produced in the human body in reactions related to obtaining energy from fats. Normally, such acetone is degraded to CO2 and H2O. Diabetic people produce larger amounts of acetone, not all of which can be degraded. The presence of acetone in urine is a sign of diabetes. In severe diabetes, the odor of acetone can be detected on the person’s breath.

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Naturally Occurring Aldehydes and Ketones Aldehydes and ketones occur widely in nature. Naturally occurring compounds of these types, with higher molecular masses, usually have pleasant odors and flavors and are often used for these properties in consumer products (perfumes, air fresheners, and the like). The unmistakable odor of melted butter is largely due to the four-carbon diketone butanedione.

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Naturally Occurring Aldehydes and Ketones Many important steroid hormones are ketones, including testosterone, the hormone that controls the development of male sex characteristics; progesterone, the hormone secreted at the time of ovulation in females; and cortisone, a hormone from the adrenal glands that is used medicinally to relieve inflammation.

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Physical Properties of Aldehydes and KetonesThe C1 and C2 aldehydes are gases at room temperature. The C3 through C11 straight-chain saturated aldehydes are liquids, and the higher aldehydes are solids. The presence of alkyl groups tends to lower both boiling points and melting points, as does the presence of unsaturation in the carbon chain. Lower-molecular-mass ketones are colorless liquids at room temperature.

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Physical Properties of Aldehydes and Ketones

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Physical Properties of Aldehydes and KetonesThe boiling points of aldehydes and ketones are intermediate between those of alcohols and alkanes of similar molecular mass. Aldehydes and ketones have higher boiling points than alkanes because of dipole–dipole attractions between molecules. Carbonyl group polarity makes these dipole–dipole interactions possible.

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Physical Properties of Aldehydes and KetonesThe boiling points of aldehydes and ketones are intermediate between those of alcohols and alkanes of similar molecular mass. Aldehydes and ketones have higher boiling points than alkanes because of dipole–dipole attractions between molecules. Carbonyl group polarity makes these dipole–dipole interactions possible.

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Physical Properties of Aldehydes and KetonesAldehydes and ketones have lower boiling

points than the corresponding alcohols because no hydrogen bonding occurs as it does with alcohols. Dipole–dipole attractions are weaker forces than hydrogen bonds.

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Physical Properties of Aldehydes and KetonesWater molecules can hydrogen-bond with aldehyde and ketone molecules. This hydrogen bonding causes low-molecular-mass aldehydes and ketones to be water soluble. As the hydrocarbon portions get larger, the water solubility of aldehydes and ketones decreases.

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Physical Properties of Aldehydes and KetonesAlthough low-molecular-mass aldehydes have pungent, penetrating, unpleasant odors, higher-molecular-mass aldehydes (above C8) are more fragrant, especially benzaldehyde derivatives. Ketones generally have pleasant odors, and several are used in perfumes and air fresheners.

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Preparation of Aldehydes and KetonesAldehydes and ketones can be produced by the oxidation of primary and secondary alcohols, respectively, using mild oxidizing agents such as KMnO4 or K2Cr2O7.

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Preparation of Aldehydes and Ketones

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Oxidation and Reduction Aldehydes and KetonesAldehydes readily undergo oxidation to carboxylic acids, and ketones are resistant to oxidation.

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Oxidation and Reduction Aldehydes and KetonesIn aldehyde oxidation, the aldehyde gains an oxygen

atom (supplied by the oxidizing agent). An increase in the number of C—O bonds is one of the operational definitions for the process of oxidation. Oxidation of an aldehyde involves breaking a carbon–hydrogen bond, and oxidation of a ketone involves breaking a carbon–carbon bond.

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Oxidation and Reduction Aldehydes and KetonesSeveral tests, based on the ease with which

aldehydes are oxidized, have been developed for distinguishing between aldehydes and ketones, for detecting the presence of aldehyde groups in sugars (carbohydrates), and for measuring the amounts of sugars present in a solution. The most widely used of these tests are the Tollens test and Benedict’s test.

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Oxidation and Reduction Aldehydes and KetonesThe Tollens test, also called the silver mirror test, involves a solution that contains silver nitrate (AgNO3) and ammonia (NH3) in water. When Tollens solution is added to an aldehyde, Ag+ ion (the oxidizing agent) is reduced to silver metal, which deposits on the inside of the test tube, forming a silver mirror. The appearance of this silver mirror is a positive test for the presence of the aldehyde group.

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Oxidation and Reduction Aldehydes and Ketones

The Ag+ ion will not oxidize ketones.

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Oxidation and Reduction Aldehydes and KetonesBenedict’s test is similar to the Tollens test in that a

metal ion is the oxidizing agent. With this test, Cu2+ ion is reduced to Cu+ ion, which precipitates from solution as Cu2O . Benedict’s solution is made by dissolving copper sulfate, sodium citrate, and sodium carbonate in water.

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Reduction of Aldehydes and Ketones Aldehydes and ketones are easily reduced by hydrogen gas (H2), in the presence of a catalyst (Ni, Pt, or Cu), to form alcohols. The reduction of aldehydes produces primary alcohols, and the reduction of ketones yields secondary alcohols.

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Reduction of Aldehydes and Ketones

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Reduction of Aldehydes and Ketones It is the addition of hydrogen atoms to the carbon–oxygen double bond that produces the alcohol in each of these reactions.

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Reduction of Aldehydes and Ketones This hydrogen addition process is very similar to the addition of hydrogen to the carbon– carbon double bond of an alkene to produce an alkane.

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Reduction of Aldehydes and Ketones Aldehyde reduction and ketone reduction to produce alcohols are the “opposite” of the oxidation of alcohols to produce aldehydes and ketones.

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Reaction of Aldehydes and Ketones with Alcohols Aldehydes and ketones react with alcohols to form hemiacetals and acetals. Reaction with one molecule of alcohol produces a hemiacetal, which is then converted to an acetal by reaction with a second alcohol molecule.

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Reaction of Aldehydes and Ketones with Alcohols

The Greek prefix hemi- means “half.” When one alcohol molecule has reacted with the aldehyde or ketone, the compound is halfway to the final acetal.

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Hemiacetal Formation Hemiacetal formation is an addition reaction in which a molecule of alcohol adds to the carbonyl group of an aldehyde or ketone. The H portion of the alcohol adds to the carbonyl oxygen atom, and the R—O portion of the alcohol adds to the carbonyl carbon atom.

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Hemiacetal Formation

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Hemiacetal Formation Formally defined, a hemiacetal is an organic compound in which a carbon atom is bonded to both a hydroxyl group (OOH) and an alkoxy group (OOR). The functional group for a hemiacetal is thus

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Hemiacetal Formation The carbon atom of the hemiacetal functional group is often referred to as the hemiacetal carbon atom; it was the carbonyl carbon atom of the aldehyde or ketone that reacted. A reaction mixture containing a hemiacetal is always in equilibrium with the alcohol and carbonyl compound from which it was made, and the equilibrium lies to the carbonyl compound side of the reaction.

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Acetal Formation If a small amount of acid catalyst is added to a hemiacetal reaction mixture, the hemiacetal reacts with a second alcohol molecule, in a condensation reaction, to form an acetal.

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Acetal Formation An acetal is an organic compound in which a carbon atom is bonded to two alkoxy groups (—OR). The functional group for an acetal is thus

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Acetal Formation A specific example of acetal formation from a hemiacetal is

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Acetal Formation

Note that acetal formation does not involve addition to a carbon–oxygen double bond as hemiacetal formation does; no double bond is present in either of the reactants involved in acetal formation. Acetal formation involves a substitution reaction; the —OR group of the alcohol replaces the —OH group on the hemiacetal.

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Acetal Hydrolysis

A hydrolysis reaction is the reaction of a compound with H2O, in which the compound splits into two or more fragments as the elements of water (H— and —OH) are added to the compound. The products of acetal hydrolysis are the aldehyde or ketone and alcohols that originally reacted to form the acetal.

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Acetal Hydrolysis

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Acetal Hydrolysis

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Acetal HydrolysisThe carbonyl hydrolysis product is an aldehyde if the acetal carbon atom has a hydrogen atom attached directly to it, and it is a ketone if no hydrogen attachment is present.

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Nomenclature for Hemiacetals and Acetals A “descriptive” type of common nomenclature that includes the terms hemiacetal and acetal as well as the name of the carbonyl compound (aldehyde or ketone) produced in the hydrolysis of the hemiacetal or acetal is commonly used in describing such compounds. Two examples of such nomenclature are

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Formaldehyde-Based PolymerFormaldehyde, the simplest aldehyde, is a prolific “polymer former.” As representative of its polymer reactions, let us consider the reaction between formaldehyde and phenol, under acidic conditions, to form a phenol–formaldehyde network polymer. A network polymer is a polymer in which monomers are connected in a three-dimensional cross-linked network.

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Formaldehyde-Based PolymerWhen excess formaldehyde is present, the polymerization proceeds via mono-, di-, and trisubstituted phenols that are formed as intermediates in the reaction between phenol and formaldehyde.

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Formaldehyde-Based PolymerThe substituted phenols then interact with each other by splitting out water molecules. The fi nal product is a complex, large, three-dimensional network polymer in which monomer units are linked via methylene (—CH2—) bridges.

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Formaldehyde-Based Polymer

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Formaldehyde-Based PolymerThe first synthetic plastic, Bakelite, produced in 1907, was a phenol–formaldehyde polymer. Early uses of Bakelite were in the manufacture of billiard balls and “plastic” jewelry. Modern phenol–formaldehyde polymers, called phenolics, are adhesives used in the production of plywood and particle board.

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Sulfur-Containing Carbonyl GroupThe introduction of sulfur into a carbonyl group

produces two different classes of compounds depending on whether the sulfur atom replaces the carbonyl oxygen atom or the carbonyl carbon atom. Replacement of the carbonyl oxygen atom with sulfur produces thiocarbonyl compounds— thioaldehydes (thials) and thioketones (thiones)—the simplest of which are

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Sulfur-Containing Carbonyl Group

Thiocarbonyl compounds such as these are unstable and readily decompose.

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Sulfur-Containing Carbonyl GroupReplacement of the carbonyl carbon atom with

sulfur produces sulfoxides, compounds that are much more stable than thiocarbonyl compounds. The oxidation of a thioether (sulfide) constitutes the most common route to a sulfoxide.

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A highly interesting sulfoxide is DMSO (dimethyl sulfoxide), a sulfur analog of acetone, the simplest ketone.

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DMSO is an odorless liquid with unusual properties. Because of the presence of the polar sulfur–oxide bond, DMSO is miscible with water and also quite soluble in less polar organic solvents. When rubbed on the skin, DMSO has remarkable penetrating power and is quickly absorbed into the body, where it relieves pain and infl ammation.

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Sulfur-Containing Carbonyl GroupFor many years it has been heralded as a “miracle drug” for

arthritis, sprains, burns, herpes, infections, and high blood pressure. However, the FDA has steadfastly refused to approve it for general medical use. For example, the FDA says that DMSO’s powerful penetrating action could cause an insecticide on a gardener’s skin to be carried accidentally into his or her bloodstream. Another complication is that DMSO is reduced in the body to dimethyl sulfide, a compound with a strong garlic-like odor that soon appears on the breath.

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Sulfur-Containing Carbonyl GroupThe FDA has approved DMSO for use in certain

bladder conditions and as a veterinary drug for topical use in nonbreeding dogs and horses. For example, DMSO is used as an anti-inflammatory rub for race horses.

End of Chapter 5Aldehydes and

Ketones

General, Organic, and Biological Chemistry, Fifth EditionH. Stephen Stoker

Brroks/Cole Cengage Learning. Permission required for reproduction or display.

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Chapter 5 Aldehydes and Ketones

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