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And Epoxidesepoxides Ethers,Ethers, SulfidesSulfides (omit), andand EpoxidesEpoxides Chapter 11 1 StructureStructure • The functional group of an ether is an oxygen atom bonded to two carbon atoms. – In dialkyl ethers, oxygen is sp3 hybridized with bond angles of approximately 109.5°. – In dimethyl ether, the C-O-C bond angle is 110.3°. H H •• HOC CH •• H H 2 StructureStructure – In other ethers, the ether oxygen is bonded to an sp2 hybridized carbon. – In ethyl vinyl ether, for example, the ether oxygen is bonded to one sp3 hybridized carbon and one sp2 hybridized carbon. Ethoxyethene CH3 CH2 -O- CH= CH2 (Ethyl vinyl ether) 3 Nomenclature •IUPAC: the longest carbon chain is the parent. Name the OR group as an alkoxy substituent. • Common names: name the groups bonded to oxygen in alphabetical order followed by the word ether. OH CH3 CH OCCH CH3 CH2 OCH2 CH3 3 3 OCH2 CH3 CH3 Ethoxyethane trans- 2-Ethoxy- 2-Methoxy-2- (Diethyl ether) cyclohexanol methylpropane (tert- Butyl methyl ether) 4 Nomenclature • Although cyclic ethers have IUPAC names, their common names are more widely used. – IUPAC: prefix ox- shows oxygen in the ring. – The suffixes -irane,irane -etane,etane -olane,olane and -ane show three, four, five, and six atoms in a saturated ring. 23 O O 1O O O O Oxirane Oxetan e Oxolane Oxane 1,4-Dioxane (Ethylene oxide) (Tetrahydrofuran) (Tetrahydropyran) 5 Physical Properties • Although ethers are polar compounds, only weak dipole-dipole attractive forces exist between their molecules in the pure liquid state. Note bene: there is NO intramolecular H-bonding 6 Physical Properties • Boiling points of ethers are – lower than alcohols of comparable MW. – close to those of hydrocarbons of comparable MW. • Ethers are hydrogen bond acceptors. – They are NOT soluble in H2O, but – They are more soluble in H2O than are hydrocarbons.. Note bene: there IS intermolecular H-bonding 7 Preparation of Ethers • Williamson ether synthesis: Ether synthesis by the SN2 displacement of halide, tosylate, or mesylate by alkoxide ion. CH3 CH3 - + SN 2 + - CH3 CHO Na + CH3 I CH3 CHOCH3 + Na I Sodium Iodomethane 2-Methoxypropane isopropoxide (Methyl iodide) (Isopropyl methyl ether) OH OR' 1) Nao R R(H) 2) R'X R R(H) R(H) 1o or 2o R(H) ROH Ether 8 Preparation of Ethers –Yields are highest with methyl and 1° halides, –lower with 2° halides (competing β- elimination) CH3 CH3 - + SN2 + - CH3 CO K + CH3 Br CH3 COCH3 + K Br CH3 CH3 Potassium Bromomethane 2-Methoxy-2-methylpropane tert-butoxide (Methyl bromide) (tert-Butyl methyl ether) 9 Preparation of Ethers reaction fails with 3° halides (β-elimination only). CH3 CH 3 - + E2 + - CH3 CBr+ CH3 O Na CH 3 C= CH2 + CH3 OH + Na Br CH3 2-Bromo-2- Sodium 2-Methylpropene methylpropane methoxide 10 Preparation of Ethers • Acid-catalyzed dehydration of alcohols – Diethyl ether and several other ethers are made this way on an industrial scale. – A specific example of an SN2 reaction in which a poor leaving group (OH-) is converted to a better one (H2O). H SO 2CH CH OH 2 4 CH CH OCH CH + H O 3 2 140°C 3 2 2 3 2 Ethanol Diethyl ether 11 Preparation of Ethers – Step 1: Proton transfer gives an oxonium ion. fast and O + O reversib le - CH3 CH2 -O-H + H-O-S-O-H CH3 CH2 -O-H + O- S- O-H O H O An oxonium ion – Step 2: Nucleophilic displacement of H2O by the OH group of the alcohol gives a new oxonium ion. + + SN2 CH3 CH2 -O-H+ CH3 CH2 -O-H CH3 CH2 -O-CH2 CH3 + O-H H H H A new oxonium ion 12 Preparation of Ethers Step 3: Proton transfer to solvent completes the reaction. proton + transfer + CH 3 CH 2 -O-CH2 CH3 + O-H CH3 CH 2 -O-CH2 CH3 + HO-H H H H 13 Preparation of Ethers • Acid-catalyzed addition of alcohols to alkenes – Yields are highest using an alkene that can form a stable carbocation and – using methanol or a 1° alcohol that is not prone to undergo acid-catalyzed dehydration. 2 4 2 2 14 Preparation of Ethers 1. Use an alkene that can form a stable carbocation (2o or 3o) 2. Use a 1° alcohol (that is not prone to undergo acid-catalyzed dehydration). CH acid CH 3 catalyst 3 CH3 C= CH2 + CH3 OH CH3 COCH3 CH3 2-Methoxy-2-methyl propane 15 Preparation of Ethers – Step 1: Protonation of the alkene gives a carbocation. CH 3 + CH3 CH C= CH + HOCH CH CCH + O CH 3 2 3 3 + 3 3 H H – Step 2: Reaction of the carbocation (an electrophile) with the alcohol (a nucleophile) gives an oxonium ion. CH 3 CH 3 CH CCH + HOCH CH CCH 3 + 3 3 3 + 3 O H CH3 16 Preparation of Ethers Step 3: Proton transfer to solvent completes the reaction. CH3 + CH 3 CH3 OH+ CH3 CCH3 CH3 O H + CH3 CCH3 + O H O CH3 HCH3 17 Epoxides • Epoxide: A cyclic ether in which oxygen is one atom of a three-membered ring. – Simple epoxides are named as derivatives of oxirane. – Where the epoxide is part of another ring system, it is shown by the prefix epoxy-.- H 1 23 O H2 C CH2 1 O 2 H Oxirane 1,2-Epoxycyclohexane (Ethylene oxide) (Cyclohexene oxide) 18 Epoxides • Epoxide: A cyclic ether in which oxygen is one atom of a three-membered ring. – Common names are derived from the name of the alkene from which the epoxide is formally derived. H H 1 23 H CH H3 C 3 H2 C CH2 C C O 1 O O 2 H Oxirane cis- 2,3-Dimethyloxirane 1,2-Epoxycyclohexane (Ethylene oxide) (cis- 2-Butene oxide) (Cyclohexene oxide) 19 Synthesis of Epoxides • Ethylene oxide, one of the few epoxides manufactured on an industrial scale, is prepared by air oxidation of ethylene. Ag 2CH2 =CH2 + O2 2 H2 C CH2 O Oxirane (Ethylene oxide) procaine 20 Synthesis of Epoxides • The most common laboratory method is oxidation of an alkene using a peroxycarboxylic acid (a peracid). O COOH O CH3 COOH Cl Peroxyacetic acid (Peracetic acid) meta- Chloroperoxy- benzoic acid (MCPBA) 21 Synthesis of Epoxides • Epoxidation of cyclohexene H O O + RCOOH O + RCOH CH 2 Cl2 H Cyclohexene A peroxy- 1,2-Epoxycyclohexane A carboxylic carboxylic acid (Cyclohexene oxide) acid 22 Synthesis of Epoxides • Epoxidation is stereospecific: – Epoxidation of cis-2-butene gives only cis-2,3- dimethyloxirane. – Epoxidation of trans-2-butene gives only trans-2,3-dimethyloxirane. O HCH H CH 3 3 H C H CC RCO H 3 + CC 3 CC H CH H 3 H3 C H O H3 C trans-2-Butene trans-2,3-Dimethyloxirane (a racemic mixture) 23 Synthesis of Epoxides • A mechanism for alkene epoxidation. R R O C 3 OC O H 2 H O O 4 1 O CC CC 24 Synthesis of Epoxides • Epoxides are also synthesized via halohydrins. OH O Cl2 , H2 O NaOH, H2 O CH 3 CH= CH2 CH CH- CH CH CH CH 3 2 S 2 3 2 Cl N PropeneA chlorohydrin Methyloxirane (racemic) (racemic) 2 2 25 Synthesis of Epoxides The second step is an internal SN2 reaction. O O internal S N 2 CC+ Cl CC Cl An epoxide 26 Synthesis of Epoxides – Halohydrin formation is both regioselective and stereoselective; for alkenes that show cis,trans isomerism, it is also stereospecific. – Conversion of a halohydrin to an epoxide is stereoselective: H H CH H H 1. Cl , H O H3 C 3 CC 2 2 CC H C CH 3 3 2. NaOH, H2 O O cis- 2-Butene cis- 2,3-Dimethyloxirane 27 Synthesis of Epoxides • Problem: Account for the fact that conversion of cis-2-butene to an epoxide by the halohydrin method gives only cis-2,3-dimethyloxirane. H H CH H H 1. Cl , H O H3 C 3 CC 2 2 CC H C CH 3 3 2. NaOH, H2 O O cis- 2-Butene cis- 2,3-Dimethyloxirane 28 Reactions of Epoxides • Ethers are not normally susceptible to attack by nucleophiles. • Because of the strain associated with the three-membered ring, epoxides readily undergo a variety of ring-opening reactions. Nu CC + HNu: C C O HO 29 Reactions of Epoxides • Acid-catalyzed ring opening – In the presence of an acid catalyst, such as sulfuric acid, epoxides are hydrolyzed to glycols. H+ OH O + H2 O HO Oxirane 1,2-Ethanediol (Ethylene oxide) (Ethylene glycol) 30 Reactions of Epoxides Step 1: Proton transfer to oxygen gives a bridged oxonium ion intermediate. Step 2: Backside attack by water (a nucleophile) on the oxonium ion (an electrophile) opens the ring. Step 3: Proton transfer to solvent completes the reaction. H H O HH O 3 2 H H +O 3 OH + H2 CCH2 H2 CCH2 CH2 CH2 CH2 CH2 + H3 O O O 2 OH + + OH 1 HHO H (1) H 31 Reactions of Epoxides Attack of the nucleophile on the protonated epoxide shows anti stereoselectivity. – Hydrolysis of an epoxycycloalkane gives a trans- 1,2-diol. OH OH H+ O + H2 O + OH OH 1,2-Epoxycyclopentane trans- 1,2-Cyclopentanediol (Cyclopentene oxide) (a racemic mixture) (achiral) 32 O OH + H3O CCH 2 CCH R 2 R R R OH 33 Reactions of Epoxides The value of epoxides is the variety of nucleophiles that will open the ring and the combinations of functional groups that can be prepared from them. CH 3 CH 3 CH 3 HOCH 2 CHOH A glycol HC CCH CHOH HSCH2 CHOH 2 A β-mercaptoalcohol A β-alkynylalcohol + H2 O/ H3 O + - 1.
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