Guide for Reading. Vocabulary carbonyl group aldehyde ketone carboxylic acid carboxyl group fatty acids esters dehydrogenation reaction

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1 23.3 arbonyl ompounds onnecting to Your World What is your favorite flavor, benzaldehyde or vanillin? Although these two flavors might sound like they belong in the foods of a science-fiction movie, the chances are good that you have eaten these organic molecules, called aldehydes, in ice cream or cookies. Aldehydes and other carbonyl compounds have important everyday uses even though they may have unfamiliar names. In this section, you will read about the properties that are associated with carbonyl compounds. Aldehydes and Ketones Remember that in an alcohol, an oxygen atom is bonded to a carbon group and a hydrogen atom. In an ether, an oxygen atom is bonded to two carbon groups. An oxygen atom can also be bonded to a single carbon atom by a double covalent bond. Such an arrangement is called a carbonyl group. A carbonyl group is a functional group with the general structure. The functional group is present in aldehydes and ketones. The structural formulas and space-filling models of some carbonyl-group compounds are shown in Figure An aldehyde is an organic compound in which the carbon of the carbonyl group is always joined to at least one hydrogen. The general formula for an aldehyde is R. A ketone is an organic compound in which the carbon of the carbonyl group is joined to two other carbons. The general formula for a ketone is RR. a r R Aldehyde arbonyl group b r R R Ketone arbonyl group c Guide for Reading Key oncepts What is the structure of a carbonyl group found in aldehydes and ketones? What is the general formula for a carboxylic acid? What is the general structure of an ester? ow is dehydrogenation an reaction? Vocabulary carbonyl group aldehyde ketone carboxylic acid carboxyl group fatty acids esters dehydrogenation reaction Reading Strategy utlining As you read the section, make an outline of the red and blue heads. Under each head, write a few sentences that summarize what you learned from that part of the section. Figure These low molarmass carbonyl group compounds are completely soluble in water. a Methanal is commonly known as formaldehyde. b Ethanal is commonly known as acetaldehyde. c Propanone is commonly known as acetone. Interpreting Diagrams Use the structures to explain why these compounds are soluble in water FUS bjectives Identify the structure of a carbonyl group as found in aldehydes and ketones onstruct the general formula for carboxylic acids and explain how they are named Describe an ester Explain how dehydrogenation is an reaction. Guide for Reading Build Vocabulary Graphics rganizer Use the following terms to make a concept map: carbonyl group, aldehydes, ketones, carboxylic acids, carboxyl group, and fatty acids.) Reading Strategy ompare and ontrast ave students write the general formula for an alcohol and an ether. As they read about aldehydes, ketones, and carboxylic acids, have them write those general structures and compare them with the structures of alcohols and ethers. (Students should note that all these structures contain oxygen but that the bonding is different.) 2 INSTRUT Section Resources Print Laboratory Manual, Lab 50 Small-Scale hemistry Laboratory Manual, Labs 38, 39 Guided Reading and Study Workbook, Section 23.3 ore Teaching Resources, Section 23.3 Review Transparencies, T268 T269 Section 23.3 arbonyl ompounds 737 Technology Interactive Textbook with hemasap, Assessment 23.3 Go nline, Section 23.3 ow do you think pure vanilla extract compares chemically with artificial vanilla flavoring, which is vanillin? (Vanilla extract is a mixture that contains other substances in addition to vanillin. Artificial vanilla flavoring is a pure substance vanillin.) Answers to... Figure The polar oxygen atom makes up a relatively large proportion of the molecule, so the molecule is soluble in water. Functional Groups 737

2 Section 23.3 (continued) Aldehydes and Ketones Relate ave students gently sniff the contents of labeled test tubes containing small amounts of vanillin, isoamyl acetate, and benzaldehyde. Ask: Which odor is that of vanilla ice cream? (vanillin) Bananas? (isoamyl acetate) Maraschino cherries? (benzaldehyde) Write structures for the compounds on the board. Refer to Table 23.4 for benzaldehyde and vanillin; isoamyl acetate is 2 2 ( ) 2. Explain that compounds with carbonyl groups account for the odor/taste of many foods. Use Visuals L1 Table 23.4 Write several examples of aldehydes and ketones on the board and discuss the naming of these compounds. Explain to students that the difference between an aldehyde and a ketone lies in the position of the carbonyl group. Display the table using a transparency and overhead projector and point out that in ketones, as in ethers, the functional group lies between two R groups. Except for methanal, the carbonyl group in aldehydes is attached to one R group and one hydrogen atom. LASS Activity Model Building Many students may find it helpful in understanding the differences between aldehydes and ketones if they build models of compounds containing these functional groups. ave students make models of propanal, propanone, and propanoic acid. ave them examine their models and make comparisons. If model-building kits are not available, toothpicks and gumdrops or marshmallows can be used. Warn students not to eat gumdrops or marshmallows used in model building. Table 23.4 Some ommon Aldehydes and Ketones ondensed formula Structural formula IUPA name ommon name Aldehydes Methanal Formaldehyde Ethanal Acetaldehyde 6 5 Benzaldehyde Benzaldehyde phenyl-2-propenal innamaldehyde () 6 3 Ketones hapter 23 Differentiated Instruction Gifted and Talented L3 ave students study the structure of cinnamaldehyde and other carbonyl-containing compounds that contain carbon-carbon bonds conjugated with a carbon-oxygen 4-hydroxy-3- meth-oxybenzaldehyde Propanone Diphenylmethanone Vanillin Acetone (dimethyl ketone) Benzophenone (diphenyl ketone) The IUPA system may be used for naming aldehydes and ketones. For either class of compounds, first identify the longest hydrocarbon chain that contains the carbonyl group. Replace the -e ending of the hydrocarbon by -al to designate an aldehyde. In the IUPA system, the continuous-chain aldehydes are named methanal, ethanal, propanal, butanal, and so forth. Ketones are named by changing the ending of the longest continuous carbon chain that contains the carbonyl group from -e to -one. Table 23.4 illustrates the naming of some common aldehydes and ketones. If the carbonyl group of a ketone could occur at more than one place on the chain, then its position is designated by the lowest possible number. heckpoint What name ending designates a ketone? double bond. ave students consider how the location of a carbonyl group next to an alkene group affects the stability of the molecule through resonance. 738 hapter 23

3 Table 23.5 Boiling Points of Some ompounds with ne and Two arbons ompound Formula Molar mass Boiling point ( ) ne carbon Methane Properties of Aldehydes and Ketones Aldehydes and ketones can form weak hydrogen bonds between the carbonyl oxygen and the hydrogen atoms of water. The lower members of the series methanal (formaldehyde), ethanal (acetaldehyde), and propanone (acetone) are soluble in water in all proportions. As the length of the hydrocarbon chain increases, however, water solubility decreases. When the carbon chain exceeds five or six carbons, solubility of both aldehydes and ketones is very low. As might be expected, all aldehydes and ketones are soluble in nonpolar solvents. Aldehydes and ketones cannot form intermolecular hydrogen bonds because they lack hydroxyl ( ) groups. onsequently, they have boiling points that are lower than those of the corresponding alcohols. Aldehydes and ketones can attract each other, however, through polar polar interactions of their carbonyl groups. As a result, their boiling points are higher than those of the corresponding alkanes. These attractive forces account for the fact that nearly all aldehydes and ketones are either liquids or solids at room temperature. The exception is methanal, which is an irritating, pungent gas. Table 23.5 compares the boiling points of alkanes, aldehydes, and alcohols of similar molar mass. Uses of Aldehydes and Ketones A wide variety of aldehydes and ketones have been isolated from plants and animals. Many of them, particularly those with high molar masses, have fragrant or penetrating odors. They are usually known by their common names, which can indicate their natural sources or perhaps a characteristic property. Aromatic aldehydes are often used as flavoring agents. Benzaldehyde is the simplest aromatic aldehyde. Also known as oil of bitter almond, benzaldehyde is a constituent of almonds. It is a colorless liquid with a pleasant almond odor. innamaldehyde imparts the characteristic odor of oil of cinnamon. Facts and Figures Vanilla Beans Vanilla flavoring comes from the seed capsules, called beans, of several species of orchid of the genus Vanilla. The capsules are collected, boiled, dried, and cured to produce the long, thin, brown bean-like pods that are familiar to cooks everywhere. The vanilla orchid was first used as a flavoring by Primary intermolecular interactions No hydrogen-bonding or polar-polar interactions Methanal 0 21 Polar-polar interactions Methanol ydrogen bonding Two carbons Ethane No hydrogen-bonding or polar-polar interactions Ethanal Polar-polar interactions Ethanol ydrogen bonding Section 23.3 arbonyl ompounds 739 the Aztecs and the Mayans in Mexico and entral America for their xocolatl (chocolate) drinks. Due to advances in pollination techniques, most of the natural vanilla produced today comes from Madagascar, Reunion, and Tahiti rather than from entral America. Discuss Explain that many familiar smells and flavors come from organic compounds. For example, many ketones and aldehydes, particularly those with high molar masses, have fragrant odors. Ask, What are some functional groups other than aldehydes and ketones that can have an effect on the particular odor of an organic compound? (alcohols, esters, amines) ave students name some of their favorite scents. Ask, What are the natural sources of these perfumes? (animal oils, plant extracts) Explain that perfume makers usually combine many different compounds to create a complex scent. Although these compounds may come from animal oils and plant extracts, they can be synthesized artificially. itral, found in roses and citrus fruits, is an aldehyde. ivetone, the odor of musk, is a ketone. Use Visuals L1 Table 23.5 Ask students to think of some examples of simple organic molecules that contain carbonyl groups. ave them write the structures and build models of these molecules. Ask, What are some structural features of carbonyl-containing molecules? (arbonyl groups display trigonal planar geometry.) Point out that a carbonyl carbon represents the positive center of aldehydes and ketones and that an oxygen atom represents the negative center. Discuss the types of intermolecular forces operating between the functional groups in aldehydes and ketones. Ask, ow does the shape of a molecule affect the strength of intermolecular attractions? (If the shape causes bond polarities within the molecule to be unbalanced, the molecule is usually polar, and intermolecular forces will be stronger than if the molecule were nonpolar.) Answers to... heckpoint -one Functional Groups 739

4 Section 23.3 (continued) arboxylic Acids Discuss Write the structural formula for a carboxylic acid on the board or overhead projector. Ask, What functional groups are present and what characteristics would you expect the molecule to have? (Students should recognize the hydroxyl and carbonyl groups and may suggest that the molecule would have properties of both an alcohol and an aldehyde.) Why are carboxylic acids considered to be acids? (They contain an ionizable proton.) Explain that the presence of the double-bonded oxygen weakens the bond between the other oxygen atom and the hydrogen atom bonded to it, so that the hydrogen can ionize easily. Point out that the overall functional group is called a carboxyl group and the molecule containing it is called a carboxylic acid. Use Visuals L1 Figure ave students study the photograph. Explain that carboxylic acids are found in many natural products. Ask students to name the acids in the food products shown. Write the structural formulas for the acids on the board. Point out the carboxyl functional group in each. ave students name other examples of food products that contain acids and describe the tastes and odors associated with carboxylic acids. Figure arboxylic acids give a variety of foods spoiled as well as fresh a distinctive sour taste. Predicting What would you expect the p range of aqueous solutions containing carboxylic acids to be? 740 hapter 23 Figure Vanilla beans, the seed pods of the vanilla orchid, are the natural source of this vanilla flavoring for ice cream and other foods. Vanilla beans, which are responsible for the popular vanilla flavor, are shown in Figure The simplest aldehyde is methanal, also called formaldehyde. Methanal is very important industrially, having its greatest use in the manufacture of synthetic resins, but it is hard to handle in the gaseous state. Methanal is usually available as a 40% aqueous solution, known as formalin. Formalin can be used to preserve biological specimens. The methanal in solution combines with protein in tissues to make the tissues hard and insoluble in water, preventing the specimen from decaying. Today, formalin is used infrequently as a preservative because it causes cancer. The most common industrial ketone is propanone, also called acetone. Propanone is a colorless, volatile liquid that boils at 56. It is miscible with water in all proportions. Propanone is used as a solvent for resins, plastics, and varnishes and is often found in nail-polish removers as shown in Figure arboxylic Acids A carboxylic acid is a compound with a carboxyl group. A carboxyl group consists of a carbonyl group attached to a hydroxyl group. R r Figure Many types of nail polish remover contain acetone, a common ketone. arbonyl group ydroxyl group arboxyl group (also written 2 or ) The general formula for a carboxylic acid is R. arboxylic acids are weak acids because they ionize slightly in solution to give a carboxylate ion and a hydrogen ion. R m R arboxylic arboxylate ydrogen ion acid ion (proton) In the IUPA system, carboxylic acids are named by replacing the -e ending of the parent alkane with the ending -oic acid. Remember, the parent alkane of a carboxylic acid is the hydrocarbon with the longest continuous carbon chain containing the carboxyl group. Differentiated Instruction Less Proficient Readers L1 Write the general structures of an aldehyde, a ketone, a carboxylic acid, and an ester on the board. ave students copy the structures in their notebooks. ave them label the carbonyl groups, hydroxyl groups, aliphatic, and aryl groups. ave them indicate whether each structure is an aldehyde, ketone, ester, or carboxylic acid. 740 hapter 23

5 Table 23.6 Saturated Aliphatic arboxylic Acids Formula arbon atoms IUPA name ommon name Melting point ( ) 1 Methanoic acid Formic acid 8 2 Ethanoic acid Acetic acid 17 2 Propanoic acid Propionic acid 22 ( 2 ) 2 4 Butanoic acid Butyric acid 6 ( 2 ) 4 6 exanoic acid aproic acid 3 ( 2 ) 6 8 ctanoic acid aprylic acid 16 ( 2 ) 8 10 Decanoic acid apric acid 31 ( 2 ) Dodecanoic acid Lauric acid 44 ( 2 ) Tetradecanoic acid Myristic acid 58 ( 2 ) exadecanoic acid Palmitic acid 63 ( 2 ) ctadecanoic acid Stearic acid 70 arboxylic acids are abundant and widely distributed in nature. Many have common names derived from a Greek or Latin word that describes their natural sources. For example, the common name for ethanoic acid is acetic acid, which comes from the Latin word acetum, meaning vinegar. ommon household vinegar contains about 5% (v/v) acetic acid. The formation of acetic acid in wine causes the wine to turn sour, become vinegar, and develop a pungent aroma. ne type of vinegar made from wine is shown in Figure Many continuous-chain carboxylic acids were first isolated from fats and are called fatty acids. Propionic acid, the threecarbon acid, literally means first fatty acid. ommon names are used more often than IUPA names for carboxylic acids. Table 23.6 lists the names and formulas of some common aliphatic carboxylic acids. The low-molar-mass members of the aliphatic carboxylic acid series are colorless, volatile liquids. They have sharp, unpleasant odors. The higher members of the series are nonvolatile, waxy, odorless solids with low melting points. Stearic acid, an 18-carbon acid obtained from beef fat, is used to make inexpensive wax candles. Like alcohols, carboxylic acids form intermolecular hydrogen bonds. Thus, carboxylic acids have higher boiling and melting points than other compounds of similar molar mass. All aromatic carboxylic acids are crystalline solids at room temperature. The carboxyl group in carboxylic acids is polar and readily forms hydrogen bonds with water molecules. Figure shows a model of ethanoic acid. Methanoic, ethanoic, propanoic, and butanoic acids are completely miscible with water. The solubility of carboxylic acids of higher molar mass drops sharply, however. Most carboxylic acids dissolve in organic solvents such as ethanol or propanone. Figure Ethanoic acid, shown in this formula and space-filling model, is a colorless, volatile liquid. Applying oncepts Is ethanoic acid soluble in water? Why? Discuss Ask students why carboxylic acids are considered weak acids. Write the chemical equation for the ionization of dilute aqueous ethanoic acid on the board. Point out that the acid is only partly ionized at equilibrium. hallenge students to write the equilibrium constant expression for the reaction shown. ave them write the chemical equation for the ionization of propanoic acid. Use Visuals L1 Table 23.6 Use an overhead projector to display a transparency of Table Discuss the IUPA rules for naming carboxylic acids. Remind students that the systematic names are derived from the parent alkanes. Discuss the basis for the observed trends in physical properties for the homologous series of aliphatic carboxylic acids. Ask, What is the name of an aliphatic carboxylic acid that is a solid at room temperature? (Answers will vary, but should be restricted to those acids with melting points higher than 25.) What is the name of an aliphatic carboxylic acid that is a liquid at room temperature? (Answers will vary, but should be restricted to those acids with melting points lower than 25.) What is the common name of an 18-carbon aliphatic carboxylic acid? What is a source of this substance? (stearic acid; found in beef fat) heckpoint What is a fatty acid? Section 23.3 arbonyl ompounds 741 Answers to... Figure below p 7 Figure Yes. The carboxyl group is polar and can form hydrogen bonds with water molecules. The combined van der Waals forces between the single methyl groups of ethanoic acid are not great enough to prevent water miscibility. heckpoint a continuouschain carboxylic acid first isolated from fats Functional Groups 741

6 Section 23.3 (continued) Esters TEAER Demo Making an Ester Purpose Students will observe the synthesis of an ester from an acid and an alcohol. Materials water bath, hot plate, test tubes, several carboxylic acids, several corresponding alcohols, concentrated sulfuric acid, filter paper, watch glass Safety and Disposal Sulfuric acid is caustic and can burn skin and clothing. Neutralize the combined reaction mixtures and flush them down the drain. Procedure Explain that esters are derivatives of carboxylic acids in which the group of the carboxyl group has been replaced by an R group from an alcohol. The synthesis of an ester from an acid and an alcohol is called esterification. Synthesize a variety of fragrant esters in class and give students a chance to smell the products. Set up a water bath on a hot plate to maintain the temperature at about 80. To synthesize the ester, mix 1 2 ml (or g if the acid is a solid) of each carboxylic acid with 3 5 ml of the corresponding alcohol in a test tube. Add 1-2 drops of concentrated sulfuric acid and heat for 15 minutes in the water bath. Dip a piece of filter paper into the test tube, and place it on a clean watch glass. ave students record the esters you prepare and their associated odors. Ask them to write the chemical equation for each reaction. Point out that the only difference in these reactions is in the R group attached to the carboxyl and alcohol functional groups. Expected utcome Acid Alcohol dor of Ester acetic ethyl apple acetic isoamyl banana acetic amyl apricot acetic octyl orange butyric ethyl pineapple salicylic methyl wintergreen Figure Esters impart the characteristic aromas and flavors of many flowers and fruits. Marigolds, raspberries, and bananas all contain esters. 742 hapter 23 Facts and Figures Scientists Growing Flavors Scientists are learning how to produce flavor molecules and other naturally occurring compounds in plant cell cultures and genetically altered yeast cells. For example, Japanese scientists are using a tissue culture technique called root culture to produce ginseng for teas and medicines and for the red pigment used in cosmetics. Esters Esters are probably the most pleasant and delicious organic compounds one can study. Many esters have pleasant, fruity odors. Esters give blueberries, pineapples, apples, pears, bananas, and many other fruits their characteristic aromas. They give many perfumes their fragrances. Esters are derivatives of carboxylic acids in which the of the carboxyl group has been replaced by an R from an alcohol. Esters contain a carbonyl group and an ether link to the carbonyl carbon. The general formula for an ester is RR. The R groups can be short-chain or long-chain aliphatic (alkyl) or aromatic (aryl) groups, saturated or unsaturated. arbonyl group (from the acid) R r Figure shows some of the flowers and fruits that contain esters. Simple esters are neutral substances. Although the molecules are polar, they cannot form hydrogen bonds with one another because they do not contain hydrogen attached to oxygen or another electronegative atom. As a result, only weak attractions hold ester molecules to one another. As you might expect, esters have much lower boiling points than the strongly hydrogen-bonded carboxylic acids from which they are derived. The lowformula-mass esters are somewhat soluble in water, but esters containing more than four or five carbons have very limited solubility. Esters may be prepared from a carboxylic acid and an alcohol. The process is called esterification. The reactants, usually a carboxylic acid and a primary or secondary alcohol, are heated with a mineral acid as a catalyst. The reaction is reversible. R + R m + R + R arboxylic Alcohol arboxylate Water acid ester R Alkyl or aryl group (from the alcohol) In the United States, scientists are obtaining vanilla flavor from cells surgically removed from a vanilla orchid and cultured in a glass bioreactor. Prior to any commercial production, a patent application is usually filed, and manufacturers must seek approval from the Food and Drug Administration to use the prepared flavor molecules in food products. 742 hapter 23

7 The formation of the ester ethyl ethanoate from ethanoic acid and ethanol is an example of esterification. The product of this reaction, ethyl ethanoate, is shown in Figure Ethanoic acid 32 3 Ethanol m + If an ester is heated with water for several hours, usually very little happens. In strong acid or base solutions, however, the ester breaks down. An ester is hydrolyzed by the addition of water to produce a carboxylic acid and an alcohol. The reaction is rapid in acidic solution. ydroxide ions also promote this reaction. The usual agent for ester hydrolysis is an aqueous solution of sodium hydroxide or potassium hydroxide. Because many esters do not dissolve in water, a solvent such as ethanol is added to make the solution homogeneous. The reaction mixture is usually heated. All of the ester is converted to products. The carboxylic acid product is in solution as its sodium or potassium salt. If the reaction mixture is acidified, the carboxylic acid forms. xidation-reduction Reactions All the classes of organic compounds you have just studied are related by and reduction reactions. Recall from hapter 20 that is the gain of oxygen, loss of hydrogen, or loss of electrons and reduction is the loss of oxygen, gain of hydrogen, or gain of electrons. Also remember that one does not occur without the other. In organic chemistry, the number of oxygen atoms and hydrogen atoms attached to carbon indicates the degree of of a compound. The fewer hydrogens on a carbon carbon bond, the more oxidized the bond. Thus, a triple bond (an alkyne) is more oxidized than a double bond (an alkene), which is more oxidized than a single bond (an alkane). In other words, an alkane can be oxidized to an alkene and then to an alkyne; ethane (an alkane) can be oxidized to ethene (an alkene), then to ethyne (an alkyne). Differentiated Instruction 23 2 Ethyl Water ethanoate 2 + m Ethyl ethanoate Ethanoic acid Ethanol 2 + Na - Na Ethyl ethanoate Sodium ethanoate Ethanol - Na + + l + Nal Sodium ethanoate Ethanoic acid Section 23.3 arbonyl ompounds 743 English Learners/Less Proficient Readers L1 Students may find it difficult to understand of -reduction reactions on this and that and reduction can be defined the next few pages of the text, noting the in terms of the loss or gain of hydrogen. Pair number of hydrogen and oxygen atoms up students who are having trouble with this attached to carbon at each step. concept. ave students write out each series 2 Ethyl ethanoate Figure Ethyl ethanoate, shown with its structural formula and space-filling model, is a lowmolar-mass ester. Relate Explain that the formation of an ester from a carboxylic acid and an alcohol is an excellent example of dynamic equilibrium. Under the proper conditions, the equilibrium can be shifted greatly in favor of the products. ther conditions favor the decomposition of the ester into the carboxylic acid and alcohol. ave students review Le htelier s principle and use it to predict conditions that would favor the reactants and conditions that would favor the products. Ask, ow can a chemist improve the yield of an ester from an esterification reaction? (Use an excess of a reactant, or remove the water or ester by distillation as it is formed.) Discuss Discuss the base-promoted hydrolysis of an ester. Write a number of examples of reactions on the chalkboard or overhead projector. Explain which part of the ester yields the alcohol and which part yields carboxylic acid products. Ask students about the difference in volatility of the products and how a chemist might separate them once they are formed. Write the molecular structure for propyl ethanoate on the board. Ask, What products would be formed from the base-promoted hydrolysis of propyl ethanoate? (propanol (propyl alcohol) and ethanoic (acetic) acid) xidation Reduction Reactions Discuss Go through the sequence of s that occurs for alkanes. Explain that carbon dioxide is the most oxidized form of carbon, and an alkane is the most reduced form of carbon. Discuss each step as being an example of an or reduction. Remind students that the loss of hydrogen is, and the gain of hydrogen is reduction. In any series of s or reductions involving hydrocarbons, an alkane is the least oxidized and an alkyne is most oxidized. Functional Groups 743

8 Section 23.3 (continued) TEAER Demo xidation of Alcohols Purpose Students observe the of primary and secondary alcohols to carboxylic acids and ketones respectively by chromic acid. Materials r 3 ; concentrated sulfuric acid; water; 3 test tubes, acetone, ethanol, isopropanol, and t-butyl alcohol Procedure Explain how the loss of a molecule of hydrogen by an alcohol can lead to the formation of a carbonyl group. Prepare an acidic solution of r 3 by dissolving 0.25 g of r 3 in 0.75 ml of water and adding 0.25 ml of concentrated sulfuric acid. A clear orange solution should be obtained. Set up three test tubes each containing 1 ml of acetone. Dissolve about 5 drops of ethanol, isopropanol, and t- butyl alcohol in separate tubes. Add 1 2 drops of chromic acid reagent to each tube and gently tap the tubes to mix. Note any changes that occur. A positive test for a primary or secondary alcohol is indicated by a change in color of the solution from orange to blue-green. No change should occur in the case of t-butyl alcohol. ave students record their observations and write the chemical equations for any reactions they observed. Ask, Why was no reaction observed for the test tube containing t-butyl alcohol? (There is no hydrogen on the carbon bearing the hydroxyl group.) Figure xidation reactions occur in many daily activities. a These basketball players are energized by reactions taking place within the cells of their bodies. b Much of the world relies on hydrocarbon combustion as a source of energy for vehicles. a b Methane (most energetic molecule) gain of oxygen The loss of hydrogen is a dehydrogenation reaction. Strong heating and a catalyst are usually necessary to make dehydrogenation reactions occur. Dehydrogenation is an reaction because the loss of each molecule of hydrogen involves the loss of two electrons from the organic molecule. The remaining carbon electrons pair to make a second or third bond. Least oxidized (most reduced) loss of hydrogen (dehydrogenation) These reactions are reversible. Alkynes can be reduced to alkenes, and alkenes can be reduced to alkanes by addition of hydrogen to a double bond. xidation in organic chemistry also involves the number and degree of of oxygen atoms attached to carbon. For example, methane, a saturated hydrocarbon, can be oxidized in steps to carbon dioxide. This occurs if it alternately gains oxygen atoms and loses hydrogen atoms. Methane is oxidized to methanol, then to methanal, then to methanoic acid, and finally to carbon dioxide. The same sequence of s occurs for other alkanes. Each series consists of an alkane, alcohol, aldehyde (or ketone), carboxylic acid, and carbon dioxide. The carbon dioxide is most oxidized or least reduced, and the alkane is least oxidized or most reduced. loss of hydrogen (dehydrogenation) Most oxidized (least reduced) (hydrogenation) (hydrogenation) reduction reduction Most oxidized (least reduced) Methanol (methyl alcohol) loss of hydrogen gain of hydrogen Methanal (formaldehyde) gain of oxygen gain of hydrogen Methanoic acid (formic acid) loss of hydrogen Most reduced (least oxidized) arbon dioxide (least energetic molecule) The more reduced a carbon compound is, the more energy it can release upon its complete to carbon dioxide. The of organic compounds is exothermic. The energy-releasing properties of reactions are extremely important for the production of energy in living systems. They also explain why the combustion of hydrocarbons such as methane is a good source of energy. Figure shows how energy production by reactions is used in both living and nonliving systems. Primary alcohols can be oxidized to aldehydes, and secondary alcohols can be oxidized to ketones, as shown below. R R -2 R R R R -2 Primary alcohol Aldehyde Secondary alcohol Ketone 744 hapter 23 Differentiated Instruction Gifted and Talented L3 ave students assign numbers to carbon for each step in the sequence of reactions shown in the text in which methane is converted to carbon dioxide. Ask, ow many molecules of hydrogen are given up by methane? (2) ow many electrons are lost? (4) ow many molecules of 2 would ethane yield? (3) 744 hapter 23

9 Tertiary alcohols, however, cannot be oxidized because there is no hydrogen atom present on the carbon bearing the hydroxyl group. A comparison of the structure of primary, secondary, and tertiary alcohols follows. Primary alcohol Secondary alcohol Tertiary alcohol The primary alcohols methanol and ethanol can be oxidized to aldehydes by warming them at about 50 with acidified potassium dichromate (K 2 r 2 7 ). In these reactions, methanol produces formaldehyde, and ethanol produces acetaldehyde. K r S 4 Preparing an aldehyde by this method is often a problem because aldehydes are easily oxidized further 05_793_ to carboxylic 6_0535acids. K r S 3 4 Methanol Methanal Ethanol Ethanal (methyl alcohol) (formaldehyde) (ethyl alcohol) (acetaldehyde) (bp 65 ) (bp 21 ) (bp 78 ) (bp 21 ) R K r R 2 S 4 For: Links on xidation and Reduction Visit: Web ode: cdn Download a worksheet on xidation and Reduction for students to complete, and find additional teacher support from NSTA SciLinks. ASSESS Evaluate Understanding Write the molecular structures of vanillin, 3-heptanone, ethyl butanoate, and pentanoic acid on the board. ave students classify each compound as an aldehyde, ketone, ester, or carboxylic acid. Aldehyde arboxylic acid xidation of the secondary alcohol 2-propanol by warming it with acidified potassium dichromate produces acetone. K r S Propanol (isopropyl alcohol) Propanone (acetone) Unlike aldehydes, ketones are resistant to further, so there is no need to remove them from the reaction mixture during the reaction. Tests for aldehydes make use of the ease with which these compounds are oxidized. Benedict s and Fehling s reagents are deep-blue alkaline solutions of copper(ii) sulfate. Figure illustrates Fehling s test for an aldehyde. When an aldehyde is oxidized with Benedict s or Fehling s reagent, a red precipitate of copper(i) oxide (u 2 ) is formed. The aldehyde is oxidized to its acid, and copper(ii) ions (u 2 ) are reduced to copper(i) ions (u ). Figure When an aldehyde is mixed with Fehling s reagent (left test tube) and heated, the blue copper(ii) ions in Fehling s reagent are reduced to form u 2, a red precipitate (right test tube). Inferring What is the state of copper in the product? Section 23.3 arbonyl ompounds 745 Answers to... Figure Functional Groups 745

10 Section 23.3 (continued) Quick LAB Quick LAB Testing for an Aldehyde bjective After completing this activity, students will be able to describe a positive test for an aldehyde. Teaching Tips It may be necessary to warm the test tube in a bath of warm water. lass Time 30 minutes Safety Do not store Tollens reagent for extended periods of time. It decomposes on standing and yields an explosive mixture. Avoid skin contact with these chemicals. Expected utcome The deposition of a silver mirror on the inner walls of the test tube is a positive indicator for the presence of an aldehyde functional group. (The silver mirror can be removed from the inside of the test tube with nitric acid.) Analyze and onclude 1. A layer of reflective, silver metal is deposited on the inner walls of test tube 1. No reaction is observed for test tubes 2 and In an alkaline solution of silver nitrate, the aldehyde is oxidized to a carboxylic acid according to the following reaction: R(aq) + 2Ag(N 3 ) 2 (aq) 2Ag(s) + R (aq) + N + 4 (aq) + 2 (l) + 3N 3 (aq) 3. The test is useful for distinguishing aldehydes from alcohols and ketones. onnecting oncepts Students comparisons should include descriptions of how the bonding in carboxylic acids causes them to be weak acids. If your class subscribes to the Interactive Textbook, use it to review key concepts in Section with hemasap Quick LAB Testing for an Aldehyde Purpose To distinguish an aldehyde from an alcohol or a ketone using Tollens s reagent. Materials 1M sodium hydroxide 5% silver nitrate 6M aqueous ammonia 4 small test tubes test tube rack plastic droppers glucose solution propanone ethanol 23.3 Section Assessment 746 hapter 23 Procedure 1. Add 1 drop of 1M sodium hydroxide to 2 ml of 5% silver nitrate in a test tube. Add 6M aqueous ammonia drop by drop, gently agitating the tube after each addition until the brownish precipitate dissolves. This will be your Tollens s reagent. 2. Place 10 drops of Tollens s reagent in each of three clean, labeled test tubes. 3. To test tube 1, add 2 drops of glucose solution. To test tube 2, add 2 drops of propanone. To test tube 3, add 2 drops of ethanol. Gently agitate each test tube to mix the contents. 4. bserve the test tubes, leaving them undisturbed for at least 5 minutes. 13. Key oncept Describe the structure of the carbonyl groups that are characteristic of aldehydes and ketones. 14. Key oncept What is the general formula for a carboxylic acid? 15. Key oncept What is the general structure of an ester? 16. Key oncept Explain why dehydrogenation is an reaction. 17. What products are expected when the following compounds are oxidized? a. 2 b A carbonyl group is a carbon atom double bonded to an oxygen atom. Aldehyde: carbon of the carbonyl group is always joined to at least one hydrogen (R); ketone: carbon is joined to two other carbons (RR). arboxylic acid: carbon is attached to a hydroxyl group (R). Ester: a derivative of a carboxylic acid in which the of the Section 23.3 Assessment Analyze and onclude 1. What evidence of a chemical reaction did you observe in test tube 1? In test tube 2? In test tube 3? 2. Write the equation for any chemical reaction you observed. 3. If you observed a chemical reaction in one or more of the test tubes, what practical uses might the reaction have? 18. Give the IUPA name for the aldehyde and the ketone. a. 2 b Acids Review the acids and bases in hapter 19. Write a paragraph comparing the properties of inorganic acids with those of carboxylic acids. Explain whether carboxylic acids are weak or strong acids. Assessment 23.3 heck your understanding of the important ideas and concepts in Section withhemasap carboxyl group has been replaced by an R from an alcohol (RR) 14. R or R a compound with the molecular formula RR or R 2 R 16. The loss of hydrogen ( 2 ) also results in the loss of two electrons. 17. a. no reaction b. 2-butanone 18. a. propanal b. 3-hexanone 746 hapter 23