Chapter 14 Ethers

Chapter 14 Ethers

Chapter 14 Ethers, Epoxides, and Sulfides Chem 233 Dr. Hoeger Introduction • Formula R-O-R′ where R and R′ are alkyl or aryl. • Symmetrical or unsymmetrical • Examples: CH3 O CH3 O O CH3 Ethers Ethers 2 1 Structure and Polarity • Bent molecular geometry • Oxygen is sp3 hybridized • Tetrahedral angle => Ethers Ethers 3 Boiling Points Similar to alkanes of comparable molecular weight. => Ethers Ethers 4 2 Hydrogen Bond Acceptor • Ethers cannot H-bond to each other. • In the presence of -OH or -NH (donor), the lone pair of electrons from ether forms a hydrogen bond with the -OH or -NH. => Ethers Ethers 5 Solvent Properties • Nonpolar solutes dissolve better in ether than in alcohol. • Ether has large dipole moment, so polar solutes also dissolve. • Ethers solvate cations. • Ethers do not react with strong bases. => Ethers Ethers 6 3 Ether Complexes • Grignard reagents H + _ • Electrophiles O B H H BH3 THF • Crown ethers => Ethers Ethers 7 Common Names of Ethers • Alkyl alkyl ether • Current rule: alphabetical order • Old rule: order of increasing complexity • Symmetrical: use dialkyl, or just alkyl. • Examples: CH3 CH3 O C CH3 CH3CH2 O CH2CH3 CH3 diethyl ether or t-butyl methyl ether or ethyl ether methyl t-butyl ether => Ethers Ethers 8 4 IUPAC Names • Alkoxy alkane • Examples: CH 3 O CH3 CH3 O C CH3 CH3 2-methyl-2-methoxypropane Methoxycyclohexane => Ethers Ethers 9 Cyclic Ethers • Heterocyclic: oxygen is in ring. O Epoxides (oxiranes) • H2C CH2 O • Oxetanes • Furans (Oxolanes ) O O • Pyrans (Oxanes ) O O O •Dioxanes => O Ethers Ethers 10 5 Naming Epoxides • Alkene oxide, from usual synthesis method H peroxybenzoic acid O cyclohexene oxide H • Epoxy attachment to parent compound, 1,2-epoxy-cyclohexane • Oxirane as parent, oxygen number 1 O H CH3 trans-2-ethyl-3-methyloxirane => CH3CH2 H Ethers Ethers 11 Spectroscopy of Ethers • IR: Compound contains oxygen, but O-H and C=O stretches are absent. • MS: α-cleavage to form oxonium ion, or loss of either alkyl group. • NMR: 13C-O signal between δ65-δ90, 1H-C-O signal between δ3.5-δ4. => Ethers Ethers 12 6 Williamson Synthesis • Alkoxide ion + 1° alkyl bromide (or tosylate) • Example: CH 3 CH3 CH O H _ + 3 + K CH3 O K CH 3 CH3 CH3 H CH _ 3 _ CH O 3 + CH3CH2 C Br CH3 O CH2CH2CH3 + Br CH 3 H CH3 => Ethers Ethers 13 Phenyl Ethers • Phenoxide ions are easily produced for use in the Williamson synthesis. • Phenyl halides or tosylates cannot be used in this synthesis method. _ O H O Na+ + NaOH + HOH => Ethers Ethers 14 7 Alkoxymercuration-Demercuration Use mercuric acetate with an alcohol to add RO-H to a double bond and form the Markovnikov product. => Ethers Ethers 15 Bimolecular Dehydration of Alcohols • Industrial method, not good lab synthesis. • If temperature is too high, alkene forms. H2SO4 CH3CH2 O H + H O CH2CH3 CH3CH2 O CH2CH3 140°C => Ethers Ethers 16 8 Cleavage of Ethers • Ethers are unreactive toward base, but protonated ethers can undergo substitution reactions with strong acids. • Alcohol leaving group is replaced by a halide. • Reactivity: HI > HBr >> HCl => Ethers Ethers 17 Mechanism for Cleavage • Ether is protonated. H + _ _ CH3 O CH3 H Br CH3 O CH3 + Br • Alcohol leaves as halide attacks. H _ + Br CH3 O CH3 Br CH3 + H O CH3 • Alcohol is protonated, halide attacks, and another molecule of alkyl bromide is formed. => Ethers Ethers 18 9 Phenyl Ether Cleavage • Phenol cannot react further to become halide. • Example: O CH2CH3 OH HBr + CH3CH2 Br => Ethers Ethers 19 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. => Ethers Ethers 20 10 Sulfides (Thioethers) • R-S-R′, analog of ether. • Name sulfides like ethers, replacing “sulfide” for “ether” in common name, or “alkylthio” for “alkoxy” in IUPAC system. • Example: S CH3 methyl phenyl sulfide or methylthiobenzene => Ethers Ethers 21 Thiols and Thiolates • R-SH about same acidity as phenols. _ + CH3CH2 SH + NaOH CH3CH2 S Na + HOH • Thiolates are better nucleophiles, weaker bases, than alkoxides. Br _ SCH3 CH3S CH3 C CH3 CH3 C CH3 CH3OH H H 2° halide Substitution product => Ethers Ethers 22 11 Sulfide Reactions • Sulfides are easily oxidized to sulfoxides and sulfones. O O H2O2 H2O2 CH3 S CH3 CH3 S CH3 CH3 S CH3 CH3COOH CH3COOH O • Sulfides react with unhindered alkyl halides to give sulfonium salts. + _ CH3 S CH3 + CH3 I CH3 S CH3 I CH3 => Ethers Ethers 23 Synthesis of Epoxides • Peroxyacid epoxidation • Cyclization of Halohydrin _ _ HO H OH O H O H2O, Cl2 H H H H Cl H Cl H => Ethers Ethers 24 12 Ring Opening in Acid • Trans diol formed in water solvent. + O H , H2O HO H H H H OH • Alkoxy alcohol formed in alcohol solvent. + O H , CH3OH HO H H H H OCH3 • 1,2-Dihalide formed with HI or HBr. Ethers Ethers 25 Biosynthesis of Steroids => Ethers Ethers 26 13 Ring Opening in Base Epoxide’s high ring strain makes it susceptible to nucleophilic attack. => Ethers Ethers 27 Epoxide Opening in Base • With aqueous hydroxide, a trans 1,2-diol is formed. • With alkoxide in alcohol, a trans 1,2-alkoxy alcohol is formed. • These are the same products that were formed in acid. • Different products are formed in acid and base if epoxide is unsymmetrical. => Ethers Ethers 28 14 Orientation of Epoxide Opening • Base attacks the least hindered carbon. _ O O OH CH3CH2OH HC CH2 HC CH2 HC CH2 CH CH OCH2CH3 CH OCH2CH3 3 OCH2CH3 3 3 • In acid, the nucleophile attacks the protonated epoxide at the most substituted carbon. H OH H3C OH H3C + O HC CH2 HC CH2 HC CH 2 OCH2CH3 OCH2CH3 H + CH3 => CH3CH2OH Ethers Ethers 29 Reaction with Grignard and R-Li • Strong base opens the epoxide ring by attacking the less hindered carbon. • Example: OH MgBr CH CHCH O 1) ether 2 3 H2C CHCH3 + + 2) H3O => Ethers Ethers 30 15 Epoxy Resins Polymer of bisphenol A and epichlorohydrin CH3 O HO C OH H2C CHCH2Cl epichlorohydrin CH3 bisphenol A => Ethers Ethers 31 End of Chapter 14 Homework: 33, 35, 37, 39, 40, 42, 45-47 Ethers Ethers 32 16.

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