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4OO CHAPTER 13 Aldehvdesand Ketones ffi @@ffi -"@ ffi@@=,@,,W : ru^% P,r@ffi - =iw Ardehvde Figure r5.2 Afdehydes andketonescan form hydrogenbondswith water. @^re Ketone ffik!/ W or six carbons, solubility of both aldehydesand ketones is very low. All aldehydes and ketones are soluble in nonpolar solvents. PRACTICE EXERCISE I5.4 Arrange the following substances in order of increasing solubility in water: (a) acetone (b) butanal (c) pentanol (d) benzophenone (e) benzaldehyde ll,! Redoxreoctionsof organiccompounds AIMS: To describethe processesof oxidation ond reductionin organic chemistryin termsof the lossor goin of oxygen, hydrogen,or electrons.Torelote the energy contentof o moleculeto its degreeof oxidotion or reduction. Focus Manyreactions of organic compounds involve oxidation or reduction. Now that we ale familiar with functional groups in organic chemistry we can examine oxidation-reduction reactions of organic molecules. We are interestedin redox reactionsof organic moleculesbecausethey are important in energy production in living organisms. Oxidation reactions are energy-releasing,and the more reduced a carbon compound is, the more energy the compound can release upon its complete oxidation. You may recall the principles of redox reactions from Section 6.4: Oxidation reactions involve a gain of oxygen,a loss of hydrogen, or a loss of electrons; reduction reactionsinvolve a loss of oxygen,a gain of hydrogen, or a gain of electrons. Oxidation and reduction reactionsmust be coupled; if a compound is oxidized in a reaction,some other compound in the reactionmust be reduced. We will discuss electron transfers in redox reactions in living organisms in Chapter 23. Our concern with redox reactions in this chapter will focus on those reactions that involve oxygen and hydrogen. For example,methane, a 15.5 Redox Reactionsof OrganicCompounds 401 saturated hydrocarbon, czrnbe oxidized in steps to carbon dioxide by alter nately gaining oxygensand losing hydrogens. Methane can be oxidized to methanol, then to formaldehyde, then to formic acid, and finally to carbon dioxide. H I oH ualn or | . uloarlon I U[oallon I Gain of I Lossol H-i-n -+#ii n-C-n +#ffi H-A-u #*; Loss of I H-A-on#tri o:c:o HH Methane Most energetic molecule Methanol (methyl alcohol) Many fat molecules have long hydrocarbon chains as part of their structure. As such, they are highly reduced molecules and have a relatively high energy content compared with other nutrient molecules. Least oxidized (most reduced) HH \./ H-C-C-H ./\ HH Ethane Methanal (formaldehyde) Methanoic acid (formic acid) Carbon dioxide Leastenergetic molecule In any series consisting of an alkane, alcohol, aldehyde (or ketone), carboxylic acid, and carbon dioxide, the alkane is the least oxidized (most reduced) conxpound and the carbon dioxide is the most oxidized (least reduced) conxpound.In biological systems, fats are similar in molecular structure to alkanes,and sugarsare similar to alcohols.The more reduced a compound is, the more energyit can releaseupon its complete oxidation to carbon dioxide. Therefore,the complete oxidation of a carbon atom in a fat can produce more energy than the complete oxidation of a carbon atom in a sugar.The oxidation of carbon compounds to carbon dioxide can be reversed.For example, in photoslmthesis, green plants can extract carbon dioxide from the atmosphere and produce sugars by a series of reduction reactions(seeSec.23.2). We can use the loss and gain of hydrogen to find the relative degree of oxidation of organic molecules that contain carbon-carbon double bonds and carbon-carbon triple bonds. For example, ethane (an alkane) can lose hydrogen and go to ethene (an alkene) and then to ethyne (an al\me). H Loss ofhydrogen (dehydrogenation). Oxidation H H C:C Lossofhydrogen (dehydlogenation), H_C:C_H trxl0arl0n Most oxidized (leastreduced) H Ethyne Each of these lossesof hydrogen representsan oxidation of the compound that losesthe hydrogen. Ethane is the least oxidized compound, and ethlme is the most oxidized. The fewer the number of hydrogens on a carbon-carbon bond, the more oxidized is the bond. A.rcdox reaction inuoluingthe loss of hydrogen from an organic molecule is called a dehydrogenation reaction. The loss of hydrogen from ethane to give ethene and the loss of hydrogen from methanol to give methanal are examples of dehydrogenation reactions. Strongheating and a catalyst are usuallynecessaryto make dehydrogenation reactions occur in the laboratory. In living organisms, dehydrogenation reactions arc catalyzedby enzymes called dehydrogenasesand occur atverymild conditions. Like other oxidation reactions, dehydrogenation reactions can be reversed.Allcynescan be reduced to alkenes and alkenes can be reduced to alkanes by addition of hydrogen to the double bond. 4O2 CHAPTER li Aldehvdes and Ketones EXAMPTE 15.4 ldentifyingthe relativedegreeof oxidation List the compounds 2-propanol, propane, and propanone in order from most reducedto most oxidized. sotuTtoN Write the structure of each compound: o OH I cH3cHCH3 2-Propanol CH3CH2CH3 CH3CCH3 Propane Propanone Propane is most reduced; it has the maximum number of hydrogens. Propanone is the most oxidized. It has the same number of oxygens as 2-propanol but fewer hydrogens. The order is propane, 2-propanol, and propanone. PRACTICE EXERCISE I5.5 Indicate the most oxidizedcompound in eachpair. (a) l-buqme and l-butene (b) propanal and propane (c) cyclohexaneand cyclohexanol (d) 3-pentanol and 3-pentanone 15.4 Redoxreoctionsinvolvingoldehydesond ketones AIMS: To write structaresfor the oldehydeond ketoneproducts (if any) of the oxidotion of primary,secondory,ond tertiary olcohols.To write structuresfor the productsof the reductionof aldehydesond ketones. Focus Redoxreactionsareusedto interconvert aldehydes or ketones and alcohols. The transformation of an alcohol to an aldehydeor ketone can be accomplished by an oxidation reaction. Conversely,aldehydesand ketones can be reducedtoalcohols. Oxidatron of alcohols Primary alcohols can be oxidized to aldehydes,and secondary alc..,holscan be oxidized to ketones.These oxidations are represented as follows: ij]j R -H itgttL U R-,-H I'I Primary alcohol CH R-{.-R i+.- O R-: -R 1j Aldehyde Secondary alcohol Ketone 15.4RedoxReactions lnvolvingAldehydes and Ketones 405 Tertiary alcohols ire not oxidized because there is no hydrogen to remove from the carbon bearing the hydroxyl group. Oxidation of the primary alcohols methanol and ethanol by warming them at about 50'C with acidified potassium dichromate (K2Cr2O) produces formaldehyde and acetaldehyde,respectively: oHooHo rlll t-?-" --t"'# H-a-H Methanol (methyl alcoho| (bp 65 "C) Methanal (formaldehyde) (bp -21 "C). cH3-c-H ffi= cH.-t-H Ethanol (ethyl alcohol) (bp 78 "C) Ethanal (acetaldehyde) (bp 2t .C) The preparation of an aldehyde by this method is often a problem becausealdehydesare easily oxidized to carboxylic acids: n_c_H #* R_c_oH Aldehyde ./ Carboxylicacid Further oxidation is not a problem with aldehydes that have low boiling points, such as acetaldehyde,becausethe product can be distilled from the reaction mixture as it is formed. Oxidation of the secondaryalcohol2-propanol bywarmingwith acidified potassium dichromate produces acetone: OH cH.-J-cu. +*.' " ' | H25U4 cH.-[-cu. H rc"3,n#il?Lo nT"?ff;3" Ketones are resistant to further oxidation. There is no need to remove them from the reaction mixture during the course of the reaction. EXAMPTE 15.5 Writingan equationfor oxidatisnof an alcohol Write an equation for the oxidation of 4-methyl-2-hexanol. SOLUTION A secondary alcohol is oxidized to a ketone. The equation is oH 9r. tl ? 9", CH3CHCI{2CHCH,CH3 curicHriHCHrCH3 +# Sincethe reactant is a secondary alcohol, oxidation to a carboxylic acid will not occur. 404 l3 Aldehydesand Ketones CHAPTER PRACTICE EXERCISE I5.6 \Mhat products are expected when the following compounds are oxidized? (a) CH.CHTCH2CH2OH (b) oH cH3cH2cHCH3 (c) oH I CH"CH,CCH" "-l CHt (d) OH T IT.7 PRACTICE EXERCISE the name and structure of the alcohol you must oxidize to make Give the following compounds. (a) CHgCHzCHO (b) o CH3CH2CCH3 (c) cH.cH,cHCHo "-l CH. Oddadon of alcohols in the Hver Ethanol is oxidized in the liver to acetaldehyde.Acetaldehyde is then oxidized to acetic acid and finallv to carbon dioxide and water: o o CH3CH2-OH Ethanol =- CHs-C-fl Acetaldehyde + CH3-C-OH._ CO, Aceticacid Carbon + HzO Water dioxide Figure15.5 can Chronic alcoholconsumption of the liver. causecirrhosis Consumption of large quantities of ethanol causesthe buildup of high concentrations of acetaldehydein the blood. This can lead to a sharp decrease in blood pressure, a more rapid heartbeat, and a generally uncomfortable feeling-a hangover. Continued overindulgence in ethanol eventually leads to yellowing and hardening of the liver, called cirrhosis (Fig. 13.3), because of the sustained high levels of acetaldehyde. Cirrhosis is an irreversible degeneration of functioning liver cells. Methanol, sometimes called wood alcohol, is extremely toxic. lVhen methanol enters the body, it is quickly absorbed into the bloodstream and passesto the liver, where it is oxidized to formaldehyde. Formaldehyde is a very reactive compound. It destroys the catalytic power of enzymes and causesliver tissue to become hard. This is whv formaldehvde solutions are 13.4 RedoxReactionsInvolvingAldehydesand Ketones 405 Fonow-upro rHECnsrrn Porxr:Fetalalcoholsyndrome Forlunately, Marla was able to follow her physician's advice, and at the end of a full-term pregnancy, she gave birlh to a healthy baby girl. The doctor's concern aboul Marla's history of alcoholism resulted from the knowledge that alcohol consumption can cause great damage to the fetus. The fetus processes ethanol more s1ow1ythan the mother, so the injurious effects of alcohol and ils oxidation product, acetaldehyde, affect the fetus for a longer time. A pregnant woman who drinks a1coholpasses these effects in a magnified form to her unborn child. The damagrng effects of alcoho] on the unbom are called fetal alcohol effects (FAE). The range of FAE can be mild to severe. Some FAE are so broad and long-lasting that they can crlpple a child's physical, social, and academic development. Fetal alcohol syndrome (FAS) is the name given to extremely severe FAE. FAS children can be underweight or below normal height and have abnormally sma11heads or other abnormal facial features, crossed eyes, underdeveloped jawbones, clef[ palates, and dysfunction of the central nervous system. FAS is the leading cause of mental retardatlon in chjldren, and the retardation may be profound. The symptoms of FAE/FAS last a tifetime, and new symptoms often appear as the child grows up. Some children who do not have visible signs of FAE/FAS at birth develop lhem over time. For example, an infanl who appears normal aL birth may display siowed development in walking or talking as a toddler. *"iliJ,illJf J,;; ffi'; "*,H""xi;:TJ:,31ti: "_ have FAE/FAS. If th ^ii - 111T.T,,. 11I chances ofFAE /FAs'"-^1"^r:^* 'l"l; inLensined FAE/FAS lllll?lL,Tl":n#ff women abstain from aicohol consumption and smoking during pregnancy. Fathers are not relieved of responslbitity in preventing FAE/FAS. One study shows the birth weight of babies born to fathers who drink regularly averages 181 g less lhan children born to fathers who drink only occasronally. used to preservebiological specimens.When methanol is ingested,temporary or permanent blindnessmay occur. The consumption of even moderate amounts of ethanol by pregnant women can have devastating effects on the unborn fetus. In the Case in Point for this chapter, you may recall that Marla, a pregnant woman with a history of alcoholism,was strongly advisedby her doctor to avoid alcohol during her pregnancy.In the Follow-up to the Casein Point, above,we will learn more about the consequencesof using alcohol during pregnancy. Reduction of aldehydes and ketones Aldehydesand ketonescan be reducedto alcoholsby the addition ofhydrogen, H-H, to the -C:O double bond. Reductionsof an aldehydeand a ketone can be representedas follows: o A tt-\--rl OH Reduction. +rH'------ I R-C-H I ooH t . ,D. , ---;n-Kle O U C T I O n - D A R-c-R n-f - * n H Aidehyde Primary alcohol H Ketone Secondary alcohol As we can see from these reactions,the reduction of aldehydesproduces primary alcohols,and the reduction of ketonesproduces secondaryalcohols.A variety of reagentsare availablefor the reduction of -C:O groups 406 l3 Aldehydesand Ketones CHAPTER An important bioiogical example of reduction of an aldehydeoccurs in fermentation. At the last step in the metabolism of glucose, yeastand other organismsreduce acetaldehvdeto ethanol. groups.For example,hydrogen gascan be used with a platto -CH-OH inum or palladium catalyst. o a-> tl Pt, + H-H >T tl \-,.- \-.- Cyclohexanol Cyclohexanone The hydrogenation of C:O bonds is similar to the hydrogenation of C-C bonds discussed previously. Other reducing reagents for C:O bonds include compounds called hydrides. Lithium aluminum hydride (LiAlH4) and sodium tetrahydroborate (NaBHa) are often used. o OH II cH.cH2-c-H " T |\ITJ, ' I CH3CH2-C-H I H l-Propanol (propyl alcohol) Propanal o ilo ;\-i-7\ ll I \2 ll \.2 N^or /-\J I'I\i.'J",ll Diphenylmethanone henzophenone) \'2 || H Diohenvlmethanol Reductions of aldehydes and ketones in living organisms are catalyzed by dehydrogenase enzyrnes. Enzymes work reversibly, and dehydrogenases catalyzeboth the oxidation of alcohols and the reduction of aldehydesand ketones. Il.5 Aldehydedetection or o o Benedict's, AIM: To describethe resultsof o Tollens', Fehling'stest on on aldehyde,o ketone,ond on olpho' hydroxy ketone. Focus Aldehydes can be distinguished from ketones byusing mild oxidizing agents. Aldehydes and ketones react with a wide variety of compounds. In general, however, aldehydes are more reactive than ketones. Chemists have taken advantage of the easewith which an aldehyde can be oxidized to develop severalvisual tests for their detection. The most widely used tests for aldehyde detection are Tollens',Benedict's,and Fehling's. Tbllens'test The mild oxidizing agent used in this test, Tollens'reagent, is an alkaline solution of siluer nitrate.It is clear and colorless.To prevent the silver ions from precipitating as silver oxide (Ag2O)at the high pH, a few drops of an 13.5 Aldehyde Detection 407 ammonia solution are added.The ammonia forms a water-soluble complex with silver ions: llg* (aq) + 2NH3@4) - + [Ag(NH3)2] (dq) When an aldehyde is oxidized with Tollens' reagent,the corresponding carboxylic acid is formed, and simultaneously, silver ions are reduced to metallic silver. For example, acetaldehyde goes to acetic acid. The silver is usually deposited as a mirror on the inside surface of the reaction vessel. The appearance of a silver mirror is a positive test for an aldehyde. If acetaldehydeis treated with Tollens' reagent' the reaction is o + 2tAg(NHg)21++ 2o}l- ------ cHs-c-o-NH4+ CH3-C-H Acetaldehyde + 2Ag(s) + 3NH3 + H2o Silver (mirror) Aceticacid (asammoniumsalt) Tollens' reagent The aldehyde,acetaldehyde,is oxidized to a carboxylic acid, acetic acid; it is a reducing agent.The silver ions are reduced to metallic silver; they are oxidizing agents.Mirrors (Fig. f 3.4) are often silvered byusingTollens' reagent. The commercial process uses glucose or formaldehyde as the reducing agent. PRACTICE EXERCISE T5.8 . Figute15.4 The reflectivecoatingon a dentist's mirror is producedon the back of a sheet of glasswhen formaldehydereducessilverions in solutionto silvermetal.The formaldehydeis oxidizedto formic acid in the process. You have two unlabeled test tubes, one containing pentanal and the other containing 2-pentanone. \Mhat simple test could you do to find .:, out which tube contains pentanal and which tube contains 2-pen" tanone? '" Benedict's and Fehling's tests Benedict's andFeliingb reagents are deepblue alkaline solutions of copper sulfate of slightly differing compositions. lVhen an aldehyde is oxidized with Benedict's or Fehling's reagents, a brick red precipitate of copper(I) oxide (CuzO)is obtained. The reaction with acetaldehydeis o CH3-C-H Acetaldehyde + o tl 2ctt*2 + 5OH- ------ CHa-C-O Copper(Il) ion complex (blue solution) Acetic acid (asacetate ion) + CuzO(s) + 3H2O Copper(I) oxide (red precipitate) The acetaldehydeis oxidized to acetic acid; copper(Il) ions (Cu2*) are reducedto copper(I) ions (Cu+). Alpha-hydrory ketones Ketones are not usually oxidized by mild oxidizing agents such as Tollens' and Benedict's solutions. However, ketones that contain a carbonyl group attached to a carbon that bears a hydroxyl group give positive testswith Tol- 408 13 Aldehydesand Ketones CHAPTER Personsin good health do not have sugar in their urine. Clinitest tablets are used to screenfor sugar in urine. The reaction ofthe tablet with the urine givescoloredproducts corresponding to differing levelsof sugar.The reaction chemistry is that of a Benedict'stest. lens',Benedict's,and Fehling'sreagents.Thesecompoundsarecalledalphahydrory ketones,whichhauethisgeneralformula: oHo til R-C-C-R I H Alpha-hydroxy ketone PRACTICEEXERCISEI5.9 Determine which of the following substancesgive a positive test (red precipitate) with Benedict'sreagent. (a) (d) o (\'r- CH3CCH3 \/ (b) CH3CH2CHO (c) (e) OH \r' HOO ltl I CH3CHCHO CH3CHCCH2CH3 Il.6 Additionsto the corhonylgrouP AIM: To illustrotewith equotionsthe formotion of o hydrote,o hemiocetaland on ocetol,ond o hemiketolond a ketol' Focus Reactions of aldehydes and ketones are reactions ofthe carbonyl group. The most characteristic reactions of both aldehydes and ketones are additionreactions. A wide variety of compounds add to the carbon-oxygen double bond of the carbonyl group. We have already seen additions to carboncarbon double bonds in SectionsL2.2 and I2.4, and additions to carbonoxygen double bonds are quite similar. Our main concern here is with the addition of water and alcohols to the carbonyl group. Addition of water Water will add to most aldehydes and ketones to form hydrates-compounds that haue two hydroxyl groups on the same carbon.The reaction is reversible.with few exceptions, the equilibrium lies to the left, in favor of the starting materials. ooH ,,1 R-C-H(R) + H OH : R-C-H(R) OH Carbonyl group ofaldehyde or ketone Water Hydrate