CHAPTER 17 ALDEHYDES AND KETONES NUCLEOPHILIC ADD|TONT○ THE CARB○ NYL GROUP SOLUTIONS TO TEXT PROBLEMS 17.1 (b) The longest continuous chain in glutaraldehyde has five carbons and terminates in aldehyde functions at both ends. Pentanedial is an acceptable IUPAC name for this compound CCHCH CHCH Pentanedial (glutaraldehyde) (c) The three-carbon parent chain has a double bond between C-2 and C-3 and a phenyl sub stituent at C-3 CH CHECHCH 3-Phen (d) Vanillin can be named as a derivative of benzaldehyde. Remember to cite the remaining sub- stituents in alphabetical order. HO 4-Hydroxy-3-methoxybenzaldehyde 426 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
CHAPTER 17 ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP SOLUTIONS TO TEXT PROBLEMS 17.1 (b) The longest continuous chain in glutaraldehyde has five carbons and terminates in aldehyde functions at both ends. Pentanedial is an acceptable IUPAC name for this compound. (c) The three-carbon parent chain has a double bond between C-2 and C-3 and a phenyl substituent at C-3. (d) Vanillin can be named as a derivative of benzaldehyde. Remember to cite the remaining substituents in alphabetical order. HO CH O CH3O 4-Hydroxy-3-methoxybenzaldehyde (vanillin) 3-Phenyl-2-propenal (cinnamaldehyde) C6H5CH CHCH O 2 1 3 Pentanedial (glutaraldehyde) HCCH2CH2CH2CH O O 1 23 4 5 426 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 427 17.2 (b) First write the structure from the name given. Ethyl isopropyl ketone has an ethyl group and an isopropyl group bonded to a carbonyl group CH.CCHCH H3 Ethyl isopropyl ketone may be alternatively named 2-methyl-3-pentanone Its longest contin uous chain has five carbons. The carbonyl carbon is C-3 irrespective of the direction in which the chain is numbered, and so we choose the direction that gives the lower number to the sition that bears the methyl group (c) Methyl 2, 2-dimethylpropyl ketone has a methyl group and a 2, 2-dimethylpropyl g bonded to a carbonyl group o CH CH. CCHCCH The longest continuous chain has five carbons, and the carbonyl carbon is C-2. Thus, methyl 2, 2-dimethylpropyl ketone may also be named 4, 4-dimethyl-2-pentanone (d) The structure corresponding to allyl methyl ketone is O CH,CH=CH, Because the carbonyl group is given the lowest possible number in the chain, the substitutive name is 4-penten-2-one not 1-penten-4-o1 17.3 No Lithium aluminum hydride is the only reagent we have discussed that is capable of reducing carboxylic acids(Section 15.3) 17. 4 The target molecule, 2-butanone, contains four carbon atoms. The problem states that all of the car bons originate in acetic acid, which has two carbon atoms. This suggests the following disconnections H CHa CCH,CH3 CH; CHCH,CH CHCH CHCH The necessary aldehyde(acetaldehyde) is prepared from acetic acid by reduction followed by on CH- COH 1.LiAIH. CH, CH2OH 2.H,O CH3CH Acetic acid Ethanol Ethylmagnesium bromide may be obtained from acetic acid by the following sequence CHCHOH HBr or CH CHBI CHCHMgBr Ethanol Ethyl bromide Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 427 17.2 (b) First write the structure from the name given. Ethyl isopropyl ketone has an ethyl group and an isopropyl group bonded to a carbonyl group. Ethyl isopropyl ketone may be alternatively named 2-methyl-3-pentanone. Its longest continuous chain has five carbons. The carbonyl carbon is C-3 irrespective of the direction in which the chain is numbered, and so we choose the direction that gives the lower number to the position that bears the methyl group. (c) Methyl 2,2-dimethylpropyl ketone has a methyl group and a 2,2-dimethylpropyl group bonded to a carbonyl group. The longest continuous chain has five carbons, and the carbonyl carbon is C-2. Thus, methyl 2,2-dimethylpropyl ketone may also be named 4,4-dimethyl-2-pentanone. (d) The structure corresponding to allyl methyl ketone is Because the carbonyl group is given the lowest possible number in the chain, the substitutive name is 4-penten-2-one not 1-penten-4-one. 17.3 No. Lithium aluminum hydride is the only reagent we have discussed that is capable of reducing carboxylic acids (Section 15.3). 17.4 The target molecule, 2-butanone, contains four carbon atoms. The problem states that all of the carbons originate in acetic acid, which has two carbon atoms. This suggests the following disconnections: The necessary aldehyde (acetaldehyde) is prepared from acetic acid by reduction followed by oxidation in an anhydrous medium. Ethylmagnesium bromide may be obtained from acetic acid by the following sequence: CH3CH2OH Ethanol (Prepared as previously) CH3CH2Br Ethyl bromide CH3CH2MgBr Ethylmagnesium bromide HBr or PBr3 Mg diethyl ether CH3CO2H Acetic acid CH3CH2OH Ethanol CH3CH O Acetaldehyde 1. LiAlH4 2. H2O PDC CH2Cl2 CH3CHCH2CH3 OH CH3CCH2CH3 O O CH3CH CH2CH3 2-Butanone CH3CCH2CH CH2 O CH3CCH2CCH3 CH3 O CH3 CH3CH2CCHCH3 O CH3 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
428 ALDEHY DES AND KETONES: NUCLEOPHI ON TO THE CARBONYL GROUP The preparation of 2-butanone is completed as follows I diethyl ether K, Cr,O, CH,CH+ CH, CH, MgBl CH3 CHCH,CH CHaCCH,CH Acetaldehvde 2-Butanol 2-Butanone bromide 17.5 Chloral is trichloroethanal, CCl3 CH. Chloral hydrate is the addition product of chloral and water. ClCCH Chloral hydrate 17.6 Methacrylonitrile is formed by the dehydration of acetone cyanohydrin, and thus has the structure shown H+ heat CH3CCH,.) Acetone cyanohydrin 17. The overall reaction is CBH_CH 2CH3 CH,OH Benzaldehyde Water HCI is a strong acid and, when dissolved in ethanol, transfers a proton to ethanol to give ethylox onium ion. Thus, we can represent the acid catalyst as the conjugate acid of ethanol The first three steps correspond to acid-catalyzed addition of ethanol to the carbonyl group eld a hemiacetal Step 1: CH, CH3 ChS CH+ H-O CHCH + 0: CHCH CHCH CHCH + O C6HSCH-O: Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
428 ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP The preparation of 2-butanone is completed as follows: 17.5 Chloral is trichloroethanal, . Chloral hydrate is the addition product of chloral and water. 17.6 Methacrylonitrile is formed by the dehydration of acetone cyanohydrin, and thus has the structure shown. 17.7 The overall reaction is HCl is a strong acid and, when dissolved in ethanol, transfers a proton to ethanol to give ethyloxonium ion. Thus, we can represent the acid catalyst as the conjugate acid of ethanol. The first three steps correspond to acid-catalyzed addition of ethanol to the carbonyl group to yield a hemiacetal. Step 1: Step 2: C6H5CH O C6H5CH OH CH2CH3 H O H CH2CH3 H O CH2CH3 H C6H5CH O O H C6H5CH CH2CH3 H H O O Ethanol 2CH3CH2OH Benzaldehyde diethyl acetal C6H5(OCH2CH3)2 Water H2O Benzaldehyde C6H5CH O HCl Acetone Methacrylonitrile cyanohydrin CH3CCH3 OH CN H, heat (H2O) CH3C CH2 CN Chloral hydrate Cl3CCH OH OH CCl3CH O CH3CH2MgBr Ethylmagnesium bromide Acetaldehyde CH3CH O 2-Butanone CH3CCH2CH3 O 2-Butanol CH3CHCH2CH3 OH 1. diethyl ether 2. H3O K2Cr2O7 H2SO4, H2O Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 429 Step 3: CHCH CHCH CH CH C6HSCH-O CH CH-OCH CH +H-O Hemiacetal Formation of the hemiacetal is followed by loss of water to give a carbocation Step 4: CHCH H.CH CHC-OCH. CH, t HO C6HC—9CH2CH3+:O3 H C6HSCH-OCH,CH C6HSCH-OCH, CH3+ H-O-H The next two steps describe the capture of the carbocation by ethanol to give the acetal CHCH C6HCH-ΩCH2CH3+:O C6HSCH--OCH,CH H CH3CH2 H CHCH CHCH CH CH-OCH,CH +iO C6HCH—9CH2CH3+H-O3 :OCH2CH3 CHCH H Acetal 17. 8(b) 1, 3-Propanediol forms acetals that contain a six-membered 1, 3-dioxane ring C6HSCH HOCH,CHaCH,OH CsHs H Benzaldehyde 1, 3-Propanediol 2-Phenyl-1, 3-dioxane Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Step 3: Formation of the hemiacetal is followed by loss of water to give a carbocation. Step 4: Step 5: The next two steps describe the capture of the carbocation by ethanol to give the acetal: Step 6: Step 7: 17.8 (b) 1,3-Propanediol forms acetals that contain a six-membered 1,3-dioxane ring. H C6H5 H 2-Phenyl-1,3-dioxane HOCH2CH2CH2OH 1,3-Propanediol C6H5CH O Benzaldehyde O O H O CH2CH3 C6H5CH OCH2CH3 OCH2CH3 Acetal C6H5CH OCH2CH3 O CH3CH2 H O CH2CH3 H H C6H5CH OCH2CH3 O CH3CH2 H C6H5CH OCH2CH3 O CH2CH3 H C6H5CH OCH2CH3 H H O C6H5CH H H O OCH2CH3 C6H5C HOH O H CH2CH3 OCH2CH3 H O H H CH2CH3 C6H5C HO H OCH2CH3 C6H5CH OH O CH2CH3 O H CH2CH3 H Hemiacetal C6H5CH OH H O H CH2CH3 OCH2CH3 ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 429 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
430 ALDEHY DES AND KETONES: NUCLEOPHI ON TO THE CARBONYL GROUP (c) The cyclic acetal derived from isobutyl methyl ketone and ethylene glycol bears an isobutyl group and a methyl group at C-2 of a 1, 3-dioxolane ring. (CH;),CHCH, CCH3 HOCH, CH,OH (CH3), CHCH, CH Isobutyl methyl ketone 2-1sobutyl-2-methyl-1, 3-dioxolan (d) Because the starting diol is 2, 2-dimethyl-1, 3-propanediol, the cyclic acetal is six-membered and bears two methyl substituents at C-5 in addition to isobutyl and methyl groups at C-2. HaC CH (CH),CHCH,CCH3 HOCH,CCH,OH H3 (CH),CHCHA Isobutyl methyl ketone 2, 2-Dimethyl-1,3- 2-lsobutyl-2, 5. 5-trimethyl propanediol 1.3-dioxane 17.9 The overall reaction is CHSCH(OCH,CH),+H,O= CH_CH 2CH, CH,OH Benzaldehyde diethyl acetal Water Benzaldehyde Ethanol The mechanism of acetal hydrolysis is the reverse of acetal formation. The first four steps convert the acetal to the hemiacetal CHCH CHCH-OCH.CH +HCo C6HSCH-OCH, CH3+: O OCHCH H:CH2 CHCH C6HSCH-OCH, CH3 C6HSCH-OCH, CH3+ : O CHCH H Step 3: H C6HSCH-OCH, CH3+ CHCH—QCH2CH3 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(c) The cyclic acetal derived from isobutyl methyl ketone and ethylene glycol bears an isobutyl group and a methyl group at C-2 of a 1,3-dioxolane ring. (d) Because the starting diol is 2,2-dimethyl-1,3-propanediol, the cyclic acetal is six-membered and bears two methyl substituents at C-5 in addition to isobutyl and methyl groups at C-2. 17.9 The overall reaction is The mechanism of acetal hydrolysis is the reverse of acetal formation. The first four steps convert the acetal to the hemiacetal. Step 1: Step 2: Step 3: C6H5CH OCH2CH3 H H O C6H5CH OCH2CH3 O H H O H CH2CH3 C6H5CH OCH2CH3 C6H5CH OCH2CH3 O CH3CH2 H C6H5CH O H CH2CH3 OCH2CH3 OCH2CH3 H O H CH2CH3 C6H5CH OCH2CH3 O CH3CH2 H C6H5CH(OCH2CH3)2 Benzaldehyde diethyl acetal H2O Water C6H5CH O Benzaldehyde 2CH3CH2OH Ethanol HCl H (CH3)2CHCH2CCH3 O Isobutyl methyl ketone HOCH2CCH2OH CH3 CH3 2,2-Dimethyl-1,3- propanediol 2-Isobutyl-2,5,5-trimethyl- 1,3-dioxane CH3 (CH3)2CHCH2 H3C CH3 O O Ethylene glycol HOCH2CH2OH H Isobutyl methyl ketone (CH3)2CHCH2CCH3 O 2-Isobutyl-2-methyl-1,3-dioxolane (CH3)2CHCH2 O O CH3 430 ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 431 Step 4: CH,CH CHACH CdHC—9cH2CH3+:O I CH COCH, CH3+ H Hemiacetal Step 5: CH,CH3 CHCH CHCH C6HSCH--OCH, CH3 H CHCH-O C6HSCH CHCH + O: H CH, CH3 CHCH+ C6HSCH H- 17.10 The conversion requires reduction; however, the conditions necessary (LiAlH,) would also reduc the ketone carbonyl. The ketone functionality is therefore protected as the cyclic acetal HOCH, CH, OH CHC COH o-C COH Reduction of the carboxylic acid may now be carried out O 1. LiAlH, COH CHOH C Hydrolysis to remove the protecting group completes the synthesis CH, H2O,HCI CHOH Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Step 4: Step 5: Step 6: Step 7: 17.10 The conversion requires reduction; however, the conditions necessary (LiAlH4) would also reduce the ketone carbonyl. The ketone functionality is therefore protected as the cyclic acetal. Reduction of the carboxylic acid may now be carried out. Hydrolysis to remove the protecting group completes the synthesis. CH2OH CH3C CH2OH O H2O, HCl 4-Acetylbenzyl alcohol O O C H3C COH O CH2OH 1. LiAlH4 2. H2O O O C H3C O O C H3C 4-Acetylbenzoic acid COH O CH3C O HOCH2CH2OH p-toluenesulfonic acid, benzene COH O O O C H3C O H CH2CH3 O H CH2CH3 C6H5CH O C6H5CH O H H O H CH2CH3 O H CH2CH3 C6H5CH O C6H5CH OH H O H CH2CH3 H O H CH2CH3 O H CH2CH3 OCH2CH3 C6H5CH HO C6H5CH OH O H CH2CH3 H O H CH2CH3 C6H5C OCH2CH3 O H H H C6H5C OCH2CH3 H HO Hemiacetal ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 431 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
432 ALDEHY DES AND KETONES: NUCLEOPHI ON TO THE CARBONYL GROUP 17.11(b) Nucleophilic addition of butylamine to benzaldehyde gives the carbinolamine OH -CH CH..CHNH CH-NCHCH.CH Carbinolamine intermediate Dehydration of the carbinolamine produces the imine CH— NCH CHCH CH =NCH, CH, CH, CH3 (c) Cyclohexanone and tert-butylamine react according to the equation HO NC(CH, NC(CH 3)3 +( Ch3)cNH Cyclohexanone tert-Butylamine Carbinolamine N-Cyclohexylidene- intermediate NH C6H CCH3+ C6HSCCH3 -H,O C6HSCCH3 Carb 17.12 (b) Pyrrolidine, a secondary amine, adds to 3-pentanone to give a carbinolamine CHa CH,CCH, CH3 CHCH, CCH, CH3 3-Pentanone Pyrrolidine Dehydration ices the enamine CHCH,CCH,CH CH CHECCHCH hO Carbinolamine Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
17.11 (b) Nucleophilic addition of butylamine to benzaldehyde gives the carbinolamine. Dehydration of the carbinolamine produces the imine. (c) Cyclohexanone and tert-butylamine react according to the equation (d) 17.12 (b) Pyrrolidine, a secondary amine, adds to 3-pentanone to give a carbinolamine. Dehydration produces the enamine. Carbinolamine 3-Pyrrolidino-2-pentene intermediate CH3CH2CCH2CH3 OH H2O N CH3CH CCH2CH3 N 3-Pentanone CH3CH2CCH2CH3 O Carbinolamine intermediate CH3CH2CCH2CH3 OH Pyrrolidine H N N Acetophenone Carbinolamine Cyclohexylamine intermediate C6H5CCH3 OH NH NH2 C6H5CCH3 O N-(1-Phenylethylidene)- cyclohexylamine C6H5CCH3 N H2O H2O O Cyclohexanone HO NC(CH3)3 H Carbinolamine intermediate (CH3)3CNH2 tert-Butylamine NC(CH3)3 N-Cyclohexylidenetert-butylamine N-Benzylidenebutylamine CH NCH2CH2CH2CH3 OH H H2O CH NCH2CH2CH2CH3 Carbinolamine intermediate CH CH3CH2CH2CH2NH2 O Benzaldehyde Butylamine CH NCH2CH2CH2CH3 OH H 432 ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 433 C6HSCCH3+ CH-CCH H2o CHC=CH2 Piperidine Carbinolamine Here we see an example of the Wittig reaction applied to diene synthesis by use of an ylide containing a carbon-carbon double bond CHaCHCH,CH +(C6H5)3P--CHCH=CH CH, CH, CH, CH=CHCH=CH, +(CKHsaP-0 Butanal Allylidenetriphenylphosphorane 1, 3-Heptadiene(52%) Triphenylphosphine (c) Methylene transfer from methylenetriphenylphosphorane is one of the most commonly used Wittig reactions CH3+(CH)一(H2 CCH3+(C6H5)3P一 Cyclohexyl methyl Methylenetriphenyl- 2-Cyclohexylpropene Triphenylphosphine phosphorane 17.14 A second resonance structure can be written for a phosphorus ylide with a double bond between phosphorus and carbon. As a third-row element, phosphorus can have more than 8 electrons in its (CHS)3P- →(C6H5)3P=CH2 Methylenetriphenylphosphorane 17.15 (b) Two Wittig reaction routes lead to 1-pentene. One is represented retrosynthetically by the disc CH, CH, CH, CH=CH, CH,CH, CH,CH +(C H5)3P--CH 1-Pentene Methylenetriphenyl The other route is CH_ CH, CH, CH=CH? CH, CH,CH2CH--P(C6H5)3+ Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(c) 17.13 (b) Here we see an example of the Wittig reaction applied to diene synthesis by use of an ylide containing a carbon–carbon double bond. (c) Methylene transfer from methylenetriphenylphosphorane is one of the most commonly used Wittig reactions. 17.14 A second resonance structure can be written for a phosphorus ylide with a double bond between phosphorus and carbon. As a third-row element, phosphorus can have more than 8 electrons in its valence shell. 17.15 (b) Two Wittig reaction routes lead to 1-pentene. One is represented retrosynthetically by the disconnection The other route is 1-Pentene CH3CH2CH2CH CH2 Formaldehyde O HCH Butylidenetriphenylphosphorane CH3CH2CH2CH P(C6H5)3 1-Pentene CH3CH2CH2CH CH2 CH3CH2CH2CH O Butanal Methylenetriphenylphosphorane CH2 (C6H5)3P Methylenetriphenylphosphorane (C6H5) CH2 3P CH2 (C6H5)3P CCH3 O Cyclohexyl methyl ketone CCH3 CH2 2-Cyclohexylpropene (66%) Triphenylphosphine oxide O (C6H5)3P Methylenetriphenylphosphorane (C6H5)3P CH2 Allylidenetriphenylphosphorane 1,3-Heptadiene (52%) Triphenylphosphine oxide Butanal CH3CH2CH2CH CH3CH2CH2CH O (C6H5)3P CH CHCH 2 CHCH CH2 (C6H5)3P O Carbinolamine intermediate 1-Piperidino-1- phenylethene Acetophenone Piperidine C6H5CCH3 O H C6H5CCH3 OH H2O C6H5C CH2 N N N ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 433 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
434 ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 17.16 Ylides are prepared by the reaction of an alkyl halide with triphenylphosphine, followed by treat ment with strong base. 2-Bromobutane is the alkyl halide needed in this case (C6H5)3P CH3 CHCH, CH3-(CH5)3P-CHCH2CH3 Br CH3 2-Bromobutane (1-Methylpropyl)triphenyl (C HS)P-CHCH,CH, Br NaCH,SCH (C6H5)2P—CCH2CH2 Sodiomethyl 17.17 The overall reaction is CCH3+ C COOH OCCH3+ C6HS COH Cyclohexyl methyl Peroxybenzoic Cyclohexyl Benzoic In the first step, the peroxy acid adds to the carbonyl group of the ketone to form a peroxy monoester CCH CHCOOH CCH OOCCSH Peroxy monoester The intermediate then undergoes rearrangement. Alkyl group migration occurs at the same time as cleavage of the o-o bond of the peroxy ester In general, the more substituted group migrates HC H-C O+ HOCC6Hs SOCC.H 17.18 The formation of a carboxylic acid from Baeyer-Villiger oxidation of an aldehyde requires o COCC.H Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
17.16 Ylides are prepared by the reaction of an alkyl halide with triphenylphosphine, followed by treatment with strong base. 2-Bromobutane is the alkyl halide needed in this case. 17.17 The overall reaction is In the first step, the peroxy acid adds to the carbonyl group of the ketone to form a peroxy monoester of a gem-diol. The intermediate then undergoes rearrangement. Alkyl group migration occurs at the same time as cleavage of the O@O bond of the peroxy ester. In general, the more substituted group migrates. 17.18 The formation of a carboxylic acid from Baeyer–Villiger oxidation of an aldehyde requires hydrogen migration. C6H5COOH O O CH O2N m-Nitrobenzaldehyde O COH O2N m-Nitrobenzoic acid OH C H O OCC6H5 O O2N H3C O C O O HOCC6H5 C OCC6H5 O OH O H3C O C6H5COOH O CCH3 Peroxy monoester OH OOCC6H5 O CCH3 Peroxybenzoic acid O C6H5COOH Cyclohexyl methyl ketone O CCH3 Benzoic acid O C6H5COH Cyclohexyl acetate O OCCH3 Br CH3 CHCH2CH3 (C6H5)3P (1-Methylpropyl)triphenylphosphonium bromide Sodiomethyl methyl sulfoxide O NaCH2SCH3 Ylide CH3 CCH2CH3 (C6H5)3P Br CH3 CHCH2CH3 (C6H5)3P (1-Methylpropyl)triphenylphosphonium bromide CH3CHCH2CH3 Br 2-Bromobutane (C6H5)3P Triphenylphosphine 434 ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 435 17.19 (a) First consider all the isomeric aldehydes of molecular formula CsHioo. Pentanal 3-Methylbutanal H (S)-2-Methylbutanal (R)-2-Methylbutanal 2, 2-Dimethylpropanal There are three isomeric ketones. 2-Pentanone 3-Pentanone 3.Methyl-2-butanone (b) Reduction of an aldehyde to a primary alcohol does not introduce a stereogenic center into the molecule. The only aldehydes that yield chiral alcohols on reduction are therefore those that already contain a stereogenic center. H OH CH,OH (S)-2-Methylbutanal (S)-2-Methyl-1-butanol (R)-2-Methylbutanal (R)-2-Methyl-l-butanol Among the ketones, 2-pentanone and 3-methyl-butanone are reduced to chiral alcohols 2-Pentanone 2-Pentanol (chiral but racemic OH 3-Pentanone 3-Pentanol 3-Methyl-2-butanone Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
17.19 (a) First consider all the isomeric aldehydes of molecular formula C5H10O. There are three isomeric ketones: (b) Reduction of an aldehyde to a primary alcohol does not introduce a stereogenic center into the molecule. The only aldehydes that yield chiral alcohols on reduction are therefore those that already contain a stereogenic center. Among the ketones, 2-pentanone and 3-methyl-butanone are reduced to chiral alcohols. NaBH4 CH3OH O 3-Methyl-2-butanone OH 3-Methyl-2-butanol (chiral but racemic) NaBH4 CH3OH O 3-Pentanone OH 3-Pentanol (achiral) NaBH4 CH3OH O 2-Pentanone OH 2-Pentanol (chiral but racemic) (R)-2-Methylbutanal (R)-2-Methyl-1-butanol NaBH4 CH3OH H O H OH H H (S)-2-Methylbutanal (S)-2-Methyl-1-butanol NaBH4 CH3OH O OH H H 2-Pentanone 3-Pentanone 3-Methyl-2-butanone O O O H H (S)-2-Methylbutanal O (R)-2-Methylbutanal O 2,2-Dimethylpropanal H H H O O H Pentanal O H 3-Methylbutanal ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 435 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
