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Chapter 16 Ethers, Epoxides, and Sulfides Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ethers • Formula is R—O—Rwhere R and R are alkyl or aryl. • Symmetrical or unsymmetrical © 2013 Pearson Education, Inc. Chapter 14 2 Nomenclature of Ethers, Epoxides, and Sulfides Substitutive IUPAC Names of Ethers name as alkoxy derivatives of alkanes CH3OCH2 CH3 methoxyethane CH3CH2OCH2CH2CH2Cl 3-chloro-1-ethoxypropane CH3CH2OCH2 CH3 ethoxyethane Functional Class IUPAC Names of Ethers Name the groups attached to oxygen in alphabetical order as separate words; "ether" is last word. CH3OCH2 CH3 ethyl methyl ether CH3CH2OCH2CH2CH2Cl 3-chloropropyl ethyl ether CH3CH2OCH2 CH3 diethyl ether Structure and Bonding in Ethers and Epoxides bent geometry at oxygen analogous to water and alcohols Structure and Polarity • • • • Oxygen is sp3 hybridized. Bent molecular geometry. Tetrahedral C—O—C angle is 110°. Polar C—O bonds. © 2013 Pearson Education, Inc. Chapter 14 7 Bond angles at oxygen are sensitive to steric effects O O H H 105° 108.5° O O CH3 CH3 112° H CH3 (CH3)3C C(CH3)3 132° An oxygen atom affects geometry in much the same way as a CH2 group Most stable conformation of diethyl ether resembles that of pentane. An oxygen atom affects geometry in much the same way as a CH2 group Most stable conformation of tetrahydropyran resembles that of cyclohexane. Physical Properties of Ethers Table 16.1 Ethers resemble alkanes more than alcohols with respect to boiling point boiling point 36°C 35°C O OH 117°C Intermolecular hydrogen bonding possible in alcohols; not possible in alkanes or ethers. Table 16.1 Ethers resemble alcohols more than alkanes with respect to solubility in water solubility in water (g/100 mL) very small 7.5 O OH 9 Hydrogen bonding to water possible for ethers and alcohols; not possible for alkanes. Hydrogen Bond Acceptor • Ethers cannot hydrogen bond with other ether molecules, so they have a lower boiling point than alcohols. • Ether molecules can hydrogen bond with water and alcohol molecules. • They are hydrogen bond acceptors. © 2013 Pearson Education, Inc. Chapter 14 14 Ethers as Solvents • Ethers are widely used as solvents because they can dissolve nonpolar and polar substances. they are unreactive toward strong bases. Ethers are relatively unreactive. Their low level of reactivity is one reason why ethers are often used as solvents in chemical reactions. © 2013 Pearson Education, Inc. Chapter 14 15 Ether Complexes • Grignard reagents: Complexation of an ether with a Grignard reagent stabilizes the reagent and helps keep it in solution. • Electrophiles: The ether’s nonbonding electrons stabilize the borane (BH3). © 2013 Pearson Education, Inc. Chapter 14 H _ + O B H H BH3 THF 16 Crown Ethers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Crown Ethers structure cyclic polyethers derived from repeating —OCH2CH2— units properties form stable complexes with metal ions applications synthetic reactions involving anions Crown Ether Complexes • Crown ethers can complex metal cations in the center of the ring. • The size of the ether ring will determine which cation it can solvate better. • Complexation by crown ethers often allows polar inorganic salts to dissolve in nonpolar organic solvents. © 2013 Pearson Education, Inc. Chapter 14 19 18-Crown-6 O O O K+ O O O forms stable Lewis acid/Lewis base complex with K+ Ion-Complexing and Solubility K+F– not soluble in benzene Ion-Complexing and Solubility O O O K+F– O O benzene O add 18-crown-6 Ion-Complexing and Solubility O O O O F– O O K+ O O benzene O O 18-crown-6 complex of K+ dissolves in benzene O O Ion-Complexing and Solubility O O O O O O K+ O O benzene O F– carried into benzene to preserve electroneutrality O O O + F– Application to organic synthesis Complexation of K+ by 18-crown-6 solubilizes potassium salts in benzene. Anion of salt is in a relatively unsolvated state in benzene (sometimes referred to as a "naked anion"). Unsolvated anion is very reactive. Only catalytic quantities of 18-crown-6 are needed. Example KF CH3(CH2)6CH2Br 18-crown-6 benzene CH3(CH2)6CH2F (92%) Preparation of Ethers Acid-Catalyzed Condensation of Alcohols* 2 CH3CH2CH2CH2OH H2SO4, 130°C CH3CH2CH2CH2OCH2CH2CH2CH3 (60%) Method is good for primary alcohols. Diethyl ether is made on industrial scale using this method. Ethylene will form at higher temperatures. Secondary and tertiary alcohols give alkenes as main product. Addition of Alcohols to Alkenes (CH3)2C=CH2 + CH3OH H+ (CH3)3COCH3 tert-Butyl methyl ether tert-Butyl methyl ether (MTBE) was produced on a scale exceeding 15 billion pounds per year in the U.S. during the 1990s. It is an effective octane booster in gasoline, but contaminates ground water if allowed to leak from storage tanks. Further use of MTBE is unlikely. The Williamson Ether Synthesis Williamson Ether Synthesis • This method involves an SN2 attack of the alkoxide on an unhindered primary halide or tosylate. • The alkoxide is commonly made by adding Na, K, or NaH to the alcohol © 2013 Pearson Education, Inc. Chapter 14 31 Example CH3CH2CH2CH2ONa + CH3CH2I CH3CH2CH2CH2OCH2CH3 + NaI (71%) Another Example Alkyl halide must be primary or methyl Alkoxide ion can be derived from methyl, primary, secondary, or tertiary alcohol. CH2Cl + CH3CHCH3 ONa CH2OCHCH3 CH3 (84%) Origin of Reactants CH3CHCH3 CH2OH OH HCl CH2Cl Na + CH3CHCH3 ONa CH2OCHCH3 CH3 (84%) What happens if the alkyl halide is not primary? CH2ONa + CH3CHCH3 Br CH2OH + H2C CHCH3 Elimination by the E2 mechanism becomes the major reaction pathway. Reactions of Ethers: Acid-Catalyzed Cleavage of Ethers Ethers can be cleaved by heating with concentrated HBr and HI. Reactivity: HI > HBr Example CH3CHCH2CH3 HBr OCH3 heat CH3CHCH2CH3 Br (81%) + CH3Br Mechanism of Ether Cleavage • Step 1: Protonation of the oxygen. • Step 2: The halide will attack the carbon and displace the alcohol (SN2). © 2013 Pearson Education, Inc. Chapter 14 39 Mechanism of Ether Cleavage Step 3: The alcohol reacts further with the acid to produce another mole of alkyl halide. • This does not occur with aromatic alcohols (phenols). © 2013 Pearson Education, Inc. Chapter 14 40 Cleavage of Cyclic Ethers O HI 150°C ICH2CH2CH2CH2I (65%) Mechanism •• ICH2CH2CH2CH2I O •• HI •• – •• I • • •• HI •• O+ H •• •• I •• •• •• O H Autoxidation of Ethers • In the presence of atmospheric oxygen, ethers slowly oxidize to hydroperoxides and dialkyl peroxides. • Both are highly explosive. • Precautions: Do not distill to dryness. Store in full bottles with tight caps. © 2013 Pearson Education, Inc. Chapter 14 43 Autoxidation of Ethers © 2013 Pearson Education, Inc. Chapter 14 44 Preparation of Epoxides: A Review and a Preview © 2013 Pearson Education, Inc. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 14 45 Preparation of Epoxides Epoxides are prepared by two major methods. Both begin with alkenes. Reaction of alkenes with peroxy acids (Section 6.19) Conversion of alkenes to vicinal halohydrins, followed by treatment with base (Section 16.10, this chapter) © 2013 Pearson Education, Inc. Chapter 14 46 Synthesis of Epoxides • Peroxyacids are used to convert alkenes to epoxides. • Most commonly used peroxyacid is metachloroperoxybenzoic acid (MCPBA). • The reaction is carried out in an aprotic acid to prevent the opening of the epoxide. © 2013 Pearson Education, Inc. Chapter 14 47 Halohydrin Cyclization • If an alkoxide and a halogen are located in the same molecule, the alkoxide may displace a halide ion and form a ring. • Treatment of a halohydrin with a base leads to an epoxide through this internal SN2 attack. © 2013 Pearson Education, Inc. Chapter 14 48 Another look H H OH NaOH O H2 O H H Br •• – •• O •• via: (81%) H H © 2013 Pearson Education, Inc. •• Br •• •• Chapter 14 49 Epoxidation via Vicinal Halohydrins Br Br2 NaOH H2O O OH anti addition inversion Corresponds to overall syn addition of oxygen to the double bond. © 2013 Pearson Education, Inc. Chapter 14 50 Reactions of Epoxides: A Review and a Preview In General... Reactions of epoxides involve attack by a nucleophile and proceed with ring-opening. For ethylene oxide: Nu—H + H2C CH2 O Nu—CH2CH2O—H Example CH2 H2C O NaOCH2CH3 CH3CH2OH CH3CH2O CH2CH2OH (50%) CH3CH2 Mechanism •• – O •• •• CH2 H2C O •• •• •• CH3CH2 CH3CH2 •• O •• •• O •• CH2CH2 CH2CH2 •• O •• – O •• •• H •• O •• CH2CH3 H – •• •• O •• CH2CH3 Example CH2 H2C O KSCH2CH2CH2CH3 ethanol-water, 0°C CH3CH2CH2CH2S CH2CH2OH (99%) In General... For epoxides where the two carbons of the ring are differently substituted: Nucleophiles attack here when the reaction is catalyzed by acids: Anionic nucleophiles attack here: R CH2 C H O Anionic Nucleophile Attacks Less-crowded Carbon MgBr + CHCH3 H2C O 1. diethyl ether 2. H3O+ CH2CHCH3 OH (60%) Lithium Aluminum Hydride Reduces Epoxides CH(CH2)7CH3 H2C O Hydride attacks less-crowded carbon. H3C 1. LiAlH4, diethyl ether 2. H2O CH(CH2)7CH3 OH (90%) Acid-Catalyzed Ring-Opening Reactions of Epoxides Example CH2 H2C O CH3CH2OH CH3CH2OCH2CH2OH H2SO4, 25°C (87-92%) CH3CH2OCH2CH2OCH2CH3 formed only on heating and/or longer reaction times. Example CH2 H2C O HBr 10°C BrCH2CH2OH (87-92%) BrCH2CH2Br formed only on heating and/or longer reaction times. Mechanism CH2 H2C H2C O •• •• •• •• Br •• •• – •• Br • • •• H CH2 + O •• H •• • Br • • • CH2CH2 •• O •• H Acid-Catalyzed Hydrolysis of Ethylene Oxide Step 1 CH2 H2C O •• H •• •• O + H H H2C H •• O •• H CH2 + O •• H Acid-Catalyzed Hydrolysis of Ethylene Oxide H Step 2 O •• H •• H2C H + O •• H + • H O• CH2CH2 CH2 •• O •• H Acid-Catalyzed Hydrolysis of Ethylene Oxide H + H O •• Step 3 H H H H O •• •• •O• • • H CH2CH2 + • H O• CH2CH2 •• O •• H •• O •• H Acid-Catalyzed Ring Opening of Epoxides Characteristics: Nucleophile attacks more substituted carbon of protonated epoxide. Inversion of configuration at site of nucleophilic attack. Nucleophile Attacks More-substituted Carbon H3C CH3 C C H O CH3OH H2SO4 CH3 OCH3 CH3CH OH CCH3 CH3 (76%) Stereochemistry H H OH O HBr H H Br (73%) Inversion of configuration at carbon being attacked by nucleophile Stereochemistry H3C H R H3C H CH3 R O CH3OH H2SO4 R CH3O H H S OH CH3 (57%) Inversion of configuration at carbon being attacked by nucleophile Stereochemistry H3C H R CH3 R R CH3OH O CH3O H2SO4 H3C H H H S CH3 + CH3O H H3C H + H3C H + O H OH anti-Hydroxylation of Alkenes H O H CH3COOH O H H H2O H OH HClO4 H (80%) OH + enantiomer