● CHAPTER 16 ETHERS, EPOXIDES, AND SULFIDES SOLUTIONS TO TEXT PROBLEMS 16.1 (b) Oxirane is the IUPAC name for ethylene oxide. A chloromethyl group(CICH,)is attached to position 2 of the ring in 2-(chloromethyl)oxirane CH,HC— CHCHO Oxirane 2-( Chloromethyl)oxiran This compound is more commonly known as epichlorohydrin (c) Epoxides may be named by adding the prefix epoxy to the IuPAC name of a parent compound, specifying by number both atoms to which the oxygen is attached CHa CHLCH=CH, H,C--CHCH=CH 1-Butene 3, 4-Epoxy-l-butene 16.2 1, 2-Epoxybutane and tetrahydrofuran both have the molecular formula C4HO--that is, they are constitutional isomers-and so it is appropriate to compare their heats of combustion directly. Angle strain from the three-membered ring of 1, 2-epoxybutane causes it to have more internal energy than tetrahydrofuran, and its combustion is more exothermic H,C一 CHCHCH 1, 2-Epoxybutane Tetrahydrofuran eat of combustion 2546 kJ/mol heat of combustion 2499 kJ/mol (609.1 kcal/mol (597.8 kcal/ 401 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
401 CHAPTER 16 ETHERS, EPOXIDES, AND SULFIDES SOLUTIONS TO TEXT PROBLEMS 16.1 (b) Oxirane is the IUPAC name for ethylene oxide. A chloromethyl group (ClCH2@) is attached to position 2 of the ring in 2-(chloromethyl)oxirane. This compound is more commonly known as epichlorohydrin. (c) Epoxides may be named by adding the prefix epoxy to the IUPAC name of a parent compound, specifying by number both atoms to which the oxygen is attached. 16.2 1,2-Epoxybutane and tetrahydrofuran both have the molecular formula C4H8O—that is, they are constitutional isomers—and so it is appropriate to compare their heats of combustion directly. Angle strain from the three-membered ring of 1,2-epoxybutane causes it to have more internal energy than tetrahydrofuran, and its combustion is more exothermic. 1,2-Epoxybutane; heat of combustion 2546 kJ/mol (609.1 kcal/mol) H2C CHCH2CH3 Tetrahydrofuran; heat of combustion 2499 kJ/mol (597.8 kcal/mol) O O 1-Butene CH3CH2CH CH2 3,4-Epoxy-1-butene H2C CHCH CH2 O H2C CH2 Oxirane H2C CHCH2Cl 2-(Chloromethyl)oxirane O O 3 2 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
402 ETHERS, EPOXIDES, AND SULFIDES 16.3 An ether can function only as a proton acceptor in a hydrogen bond, but an alcohol can be either roton acceptor or a donor. The only hydrogen bond possible between an ether and an alcohol therefore the one shown Ether Alcohol 16.4 The compound is 1, 4-dioxane; it has a six-membered ring and two oxygens separated by CHy--Ch 1.4-dioxane (“6- crown-2”) 16.5 Protonation of the carbon-carbon double bond leads to the more stable carbocation (CH,),C=CH,+ H (CH3)2C—CH 2-Methylpropene tert-Butyl cation Methanol acts as a nucleophile to capture tert-butyl cation CH3 (CH3)2C-CH,+ (CH3)C Deprotonation of the alkyloxonium ion leads to formation of tert-butyl methyl ether. (CH3)3COCH3 H,OCH tert-Butyl methyl ether 16.6 Both alkyl groups in benzyl ethyl ether are primary, thus either may come from the alkyl halide in a williamson ether synthesis. The two routes to benzyl ethyl ether are CBH CH,ONa CH_ CH, Br C6HSCH2OCH, CH3 NaBr Sodium benzyloxide Bromoethane Benzyl ethyl ether Sodium C6H CH Br CHCH,ONa- CHS CH,OCH,CH3 NaBr Benzyl bromide Sodium ethoxide Benzyl ethyl ether 16.7(b) A primary carbon and a secondary carbon are attached to the ether oxygen. The secondary car- bon can only be derived from the alkoxide, because secondary alkyl halides cannot be used in the preparation of ethers by the williamson method. The only effective method uses an allyl (CH3)2CHONa t H,C=CHCH,Br- H,C=CHCH,OCH(CH3)2 NaBi Sodium isopropoxide Allyl bromide Allyl isopropyl ether Elimination will be the major reaction of an isopropyl halide with an alkoxide base Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
402 ETHERS, EPOXIDES, AND SULFIDES 16.3 An ether can function only as a proton acceptor in a hydrogen bond, but an alcohol can be either a proton acceptor or a donor. The only hydrogen bond possible between an ether and an alcohol is therefore the one shown: 16.4 The compound is 1,4-dioxane; it has a six-membered ring and two oxygens separated by CH2—CH2 units. 16.5 Protonation of the carbon–carbon double bond leads to the more stable carbocation. Methanol acts as a nucleophile to capture tert-butyl cation. Deprotonation of the alkyloxonium ion leads to formation of tert-butyl methyl ether. 16.6 Both alkyl groups in benzyl ethyl ether are primary, thus either may come from the alkyl halide in a Williamson ether synthesis. The two routes to benzyl ethyl ether are 16.7 (b) A primary carbon and a secondary carbon are attached to the ether oxygen. The secondary carbon can only be derived from the alkoxide, because secondary alkyl halides cannot be used in the preparation of ethers by the Williamson method. The only effective method uses an allyl halide and sodium isopropoxide. Elimination will be the major reaction of an isopropyl halide with an alkoxide base. (CH3)2CHONa Sodium isopropoxide Allyl bromide H2C CHCH2Br CHCH2OCH(CH3)2 NaBr Allyl isopropyl ether Sodium bromide H2C C6H5CH2Br CH 3CH2ONa Benzyl bromide Sodium ethoxide C6H5CH2OCH2CH3 NaBr Benzyl ethyl ether Sodium bromide C6H5CH2ONa CH 3CH2Br Sodium benzyloxide Bromoethane C6H5CH2OCH2CH3 NaBr Benzyl ethyl ether Sodium bromide (CH3)3C OCH3 H O H CH3 H2OCH3 (CH3)3COCH3 tert-Butyl methyl ether (CH3)2C CH3 (CH3)3C OCH3 H O H CH3 H (CH3)2C CH2 2-Methylpropene (CH3)2C CH3 tert-Butyl cation O O 1,4-dioxane (‘‘6-crown-2”) O R R Ether H O R Alcohol Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ETHERS, EPOXIDES, AND SULFIDES 403 (c) Here the ether is a mixed primary-tertiary one. The best combination is the one that uses the primary alkyl halide CH3)COK CH CH, Br -(CH3),COCH, C6Hs t KBi Benzyl bromide Benzyl tert-butyl ether Potassium rt-butoxide The reaction between( CH3)3CBr and C6HS CH,O is elimination, not substitution. 16.8 CH,,OCH,CH3 60, Diethyl ether Oxygen Carbon Water 16.9(b) If benzyl bromide is the only organic product from reaction of a dialkyl ether with hydrogen bromide, then both alkyl groups attached to oxygen must be benzyl C6HSCH,OCH, C6H 2C6H5 CH,Br H,o Benzyl bromide Water (c) Since I mole of a dihalide, rather than 2 moles of a monohalide, is produced per mole of ether, the ether must be cyclic. 2HB, BrCH,CH, CH,CH, CH, Br H,O Tetrahydropyran 16.10 As outlined in text Figure 16.4, the first step is protonation of the ether oxygen to give a dialkylox onium ion Tetrahydrofuran Hydrogen Dialkyloxonium lodide In the second step, nucleophilic attack of the halide ion on carbon of the oxonium ion gives +C3-H Dialkyloxonium 4-lodo-l-butanol The remaining two steps of the mechanism correspond to those in which an alcohol is converted to an alkyl halide, as discussed in Chapter 4 OH, H+ +h,o: 4.Iodobutane Water Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(c) Here the ether is a mixed primary–tertiary one. The best combination is the one that uses the primary alkyl halide. The reaction between (CH3)3CBr and C6H5CH2O is elimination, not substitution. 16.8 16.9 (b) If benzyl bromide is the only organic product from reaction of a dialkyl ether with hydrogen bromide, then both alkyl groups attached to oxygen must be benzyl. (c) Since 1 mole of a dihalide, rather than 2 moles of a monohalide, is produced per mole of ether, the ether must be cyclic. 16.10 As outlined in text Figure 16.4, the first step is protonation of the ether oxygen to give a dialkyloxonium ion. In the second step, nucleophilic attack of the halide ion on carbon of the oxonium ion gives 4-iodo-1-butanol. The remaining two steps of the mechanism correspond to those in which an alcohol is converted to an alkyl halide, as discussed in Chapter 4. Water I H2O I OH2 1,4-Diiodobutane I I I Hydrogen iodide H I 4-Iodo-1-butanol I OH OH2 I I Iodide ion Dialkyloxonium ion O H 4-Iodo-1-butanol I OH O Tetrahydrofuran I Iodide ion H I Hydrogen iodide Dialkyloxonium ion O H 2HBr heat O Tetrahydropyran BrCH2CH2CH2CH2CH2Br 1,5-Dibromopentane H2O Water Dibenzyl ether C6H5CH2OCH2C6H5 2C6H5CH2Br Benzyl bromide Water HBr heat H2O Diethyl ether CH3CH2OCH2CH3 Oxygen 6O2 Carbon dioxide 4CO2 Water 5H2O (CH KBr 3)3COCH2C6H5 Benzyl tert-butyl ether Potassium bromide (CH3)3COK Potassium tert-butoxide C6H5CH2Br Benzyl bromide ETHERS, EPOXIDES, AND SULFIDES 403 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
404 ETHERS, EPOXIDES, AND SULFIDES 16.11 The cis epoxide is achiral. It is a meso form containing a plane of symmetry. The trans isomer is hiral; its two mirror-image representations are not superposable Plane of symmetry H3C f影 cis-2,3-Epoxybutane Plane of symmetry passes Irans.2.3)_nc)forms ofage Neither the cis nor the trans epoxide is optically active when formed from the alkene. The cis epox ide is achiral; it cannot be optically active. The trans epoxide is capable of optical activity but is formed as a racemic mixture because achiral starting materials are used 16.12 (b) Azide ion [: N=N=N:] is a good nucleophile, reacting readily with ethylene oxide to yield 2-azidoethanol HC—CH NaN N, CHCH.OH Ethylene oxide (c) Ethylene oxide is hydrolyzed to ethylene glycol in the presence of aqueous base HC—CH HOCH,CH,OH (a) Phenyllithium reacts with ethylene oxide in a manner similar to that of a Grignard reagent. H,CCH2 1.CH, Li, diethylether C6H-CH, CH,OH Ethylene oxide 2-Phenylethanol (e) The nucleophilic species here is the acetylenic anion CH,CH,C=C:, which attacks a carbon atom of ethylene oxide to give 3-hexyn-l-ol HC—CH2 CH2CH2C≡CCH2CH2OH 16.13 Nucleophilic attack at C-2 of the starting epoxide will be faster than attack at C-1, because C-1 is ding to attack at c-1. is likely pound B Compound B not only arises by methoxide ion attack at C-2 but also satisfies the stereo- chemical requirement that epoxide ring opening take place with inversion of configuration at the site of substitution Compound B is correct. Compound C, although it is formed by methoxide substitution at the less crowded carbon of the epoxide, is wrong stereochemically. It requires Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
16.11 The cis epoxide is achiral. It is a meso form containing a plane of symmetry. The trans isomer is chiral; its two mirror-image representations are not superposable. Neither the cis nor the trans epoxide is optically active when formed from the alkene. The cis epoxide is achiral; it cannot be optically active. The trans epoxide is capable of optical activity but is formed as a racemic mixture because achiral starting materials are used. 16.12 (b) Azide ion is a good nucleophile, reacting readily with ethylene oxide to yield 2-azidoethanol. (c) Ethylene oxide is hydrolyzed to ethylene glycol in the presence of aqueous base. (d) Phenyllithium reacts with ethylene oxide in a manner similar to that of a Grignard reagent. (e) The nucleophilic species here is the acetylenic anion CH3CH2C>C: , which attacks a carbon atom of ethylene oxide to give 3-hexyn-1-ol. 16.13 Nucleophilic attack at C-2 of the starting epoxide will be faster than attack at C-1, because C-1 is more sterically hindered. Compound A, corresponding to attack at C-1, is not as likely as compound B. Compound B not only arises by methoxide ion attack at C-2 but also satisfies the stereochemical requirement that epoxide ring opening take place with inversion of configuration at the site of substitution. Compound B is correct. Compound C, although it is formed by methoxide substitution at the less crowded carbon of the epoxide, is wrong stereochemically. It requires Ethylene oxide H2C CH2 NH3 NaC CCH2CH3 3-Hexyn-1-ol (48%) CH3CH2C CCH2CH2OH O C6H5CH2CH2OH 2-Phenylethanol 1. C6H5 Li, diethyl ether 2. H3O Ethylene oxide H2C CH2 O HOCH2CH2OH Ethylene glycol NaOH H2O Ethylene oxide H2C CH2 O Ethylene oxide N3CH2CH2OH 2-Azidoethanol NaN3 ethanol–water H2C CH2 O N N ] [ N H3CH HCH3 O cis-2,3-Epoxybutane (Plane of symmetry passes through oxygen and midpoint of carbon–carbon bond.) CH3 H H3C H O H H 3CH CH3 O Nonsuperposable mirror-image (enantiomeric) forms of trans-2,3-epoxybutane Plane of symmetry 404 ETHERS, EPOXIDES, AND SULFIDES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ETHERS, EPOXIDES, AND SULFIDES 405 substitution with retention of configuration, which is not the normal mode of epoxide ring 16.14 Acid-catalyzed nucleophilic ring opening proceeds by attack of methanol at the more substituted carbon of the protonated epoxide. Inversion of configuration is observed at the site of attack. The correct product is compound A. H CHO 0 HO: CH Protonated Compound A form of 1-methyl-1. 2. epoxycyclopentane The nucleophilic ring openings in both this problem and Problem 16 13 occur by inversion of configuration. Attack under basic conditions by methoxide ion, however, occurs at the less hindered carbon of the epoxide ring, whereas attack by methanol under acid-catalyzed conditions occurs at the more substituted carbon 16.15 Begin by drawing meso-2, 3-butanediol, recalling that a meso form is achiral. The eclipsed confor- mation has a plane of symmetry Ho meso-23-Butanediol Epoxidation followed by acid-catalyzed hydrolysis results in anti addition of hydroxyl groups to the double bond. trans-2-Butene is the required starting material CH Ho H3C CH, COOH H,O+ H3C H > CH3 trans-2-Butene trans-2,3-Epoxybutane meso-23-Butanediol Osmium tetraoxide hydroxylation is a method of achieving syn hydroxylation. The necessary start ing material is cis-2-butene HO COOH. OsO, CH,COH. HO cis-2-Butene meso-23-Butanediol 16.16 Reaction of (R)-2-octanol with p-toluenesulfonyl chloride yields a p-toluenesulfonate ester (tosylate) having the same configuration; the stereogenic center is not involved in this step Reaction Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
substitution with retention of configuration, which is not the normal mode of epoxide ring opening. 16.14 Acid-catalyzed nucleophilic ring opening proceeds by attack of methanol at the more substituted carbon of the protonated epoxide. Inversion of configuration is observed at the site of attack. The correct product is compound A. The nucleophilic ring openings in both this problem and Problem 16.13 occur by inversion of configuration. Attack under basic conditions by methoxide ion, however, occurs at the less hindered carbon of the epoxide ring, whereas attack by methanol under acid-catalyzed conditions occurs at the more substituted carbon. 16.15 Begin by drawing meso-2,3-butanediol, recalling that a meso form is achiral. The eclipsed conformation has a plane of symmetry. Epoxidation followed by acid-catalyzed hydrolysis results in anti addition of hydroxyl groups to the double bond. trans-2-Butene is the required starting material. Osmium tetraoxide hydroxylation is a method of achieving syn hydroxylation. The necessary starting material is cis-2-butene. 16.16 Reaction of (R)-2-octanol with p-toluenesulfonyl chloride yields a p-toluenesulfonate ester (tosylate) having the same configuration; the stereogenic center is not involved in this step. Reaction O O C H3C CH3 H H C cis-2-Butene via meso-2,3-Butanediol (CH3)3COOH, OsO4(cat) (CH3)3COH, HO C H HO OH CH3 H CH3 H CH3 C H CH3 O C C O Os trans-2-Butene C C H3C H H CH3 trans-2,3-Epoxybutane O C C meso-2,3-Butanediol H3O C H CH3 CH3 H C H3C H OH H HO CH3 CH3COOH O meso-2,3-Butanediol C H HO OH CH3 H CH3 C CH3OH H O H O HO CH3 CH3 H H H Protonated form of 1-methyl-1,2- epoxycyclopentane Compound A H OCH3 CH3 HO CH3 ETHERS, EPOXIDES, AND SULFIDES 405 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website