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Ethers and Epoxides 10/02/59 Ethers and Epoxides เอกสารประกอบการสอน รายวิชา 01403224 ดร.นงพงา จรัสโสภณ Brevetoxin Structure and nomenclature of ethers Ethers are compounds of formula R -0-R', where R and R' may be alkyl groups or aryl (benzene ring) groups. 1 10/02/59 3 Ethers and Their Relatives • Diethyl ether is used industrially as a solvent • Tetrahydrofuran (THF) is a solvent that is a cyclic ether • Thiols (R–S–H) and sulfides (R–S–R) are sulfur (for oxygen) analogs of alcohols and ethers 4 Nomenclature Common names (Alkyl Alkyl Ether Names) Common names of ethers are formed by naming the two alkyl groups on oxygen and adding the word ether. Under the current system, the alkyl groups should be named in alphabetical order. 2 10/02/59 IUPAC Nomenclature • Ethers are named as “Alkoxy alkanes”, where the longer chain alkyl group is the alkane and thus is written at the end. CH3-O-CH3 CH3-O-C2H5 Methoxyethane Methoxymethane The common alkoxy substituents are given names derived from their alkyl component Alkyl Name Alkoxy Group Name group CH3– Methyl CH3O– Methoxy CH3CH2– Ethyl CH3CH2O– Ethoxy (CH3)2CH– Isopropyl (CH3)2CHO– Isopropoxy (CH3)3C– tert-Butyl (CH3)3CO– tert-Butoxy C6H5– Phenyl C6H5O– Phenoxy 3 10/02/59 Examples H3C-O-CH3 H3C-CH2-O-CH2-CH3 H2C=CH-O-CH=CH2 Common Dimethyl ether Diethyl ether Divinyl ether IUPAC Methoxy methane Ethoxy ethane Vinyloxyethene H3C-O-C6H13 H3C-O-CH2-CH3 H3C-O-CH=CH2 Common Hexyl methyl ether Ethyl methyl ether Methyl vinyl ether IUPAC Methoxy hexane mehoxy ethane Methoxy ethene Examples Common: Diphenyl ether Common: Methyl Phenyl ether IUPAC : Phenoxy benzene (anisole) IUPAC : Methoxy benzene 4 10/02/59 9 Nomenclature of Cyclic Ether 1. Epoxide • Epoxides can be named in three different ways—As epoxyalkanes, oxiranes, or alkene oxides. • To name an epoxide as an epoxyalkane, first name the alkane chain or ring to which the O atom is attached, and use the prefix “epoxy” to name the epoxide as a substituent. Use two numbers to designate the location of the atoms to which the O’s are bonded. 10 • Epoxides bonded to a chain of carbon atoms can also be named as derivatives of oxirane, the simplest epoxide having two carbons and one oxygen atom in a ring. • The oxirane ring is numbered to put the O atom at position one, and the first substituent at position two. • No number is used for a substituent in a monosubstituted oxirane. 5 10/02/59 11 • Epoxides are also named as alkene oxides, since they are often prepared by adding an O atom to an alkene. To name an epoxide in this way: Mentally replace the epoxide oxygen with a double bond. Name the alkene. Add the word oxide. 2. Oxetanes 4. Pyrans, Oxanes 3. Furans, Oxolanes 3. Dioxanes 6 10/02/59 13 Structure, Properties, and Sources of Ethers • R–O–R ~ tetrahedral bond angle (112° in dimethyl ether) • Oxygen is sp3-hybridized • Oxygen atom gives ethers a slight dipole moment • Diethyl ether prepared industrially by sulfuric acid– catalyzed dehydration of ethanol – also with other primary alcohols 14 Physical Properties • Alcohols, ethers and epoxides exhibit dipole-dipole interactions because they have a bent structure with two polar bonds. • Alcohols are capable of intermolecular hydrogen bonding. Thus, alcohols are more polar than ethers and epoxides. • Steric factors affect hydrogen bonding. 7 10/02/59 15 Synthesis of Ethers and Epoxides 8 10/02/59 17 1. The Williamson Ether Synthesis • Reaction of metal alkoxides and primary alkyl halides and tosylates • Best method for the preparation of ethers • Alkoxides prepared by reaction of an alcohol with a strong base such as sodium hydride, NaH 18 2. Silver Oxide-Catalyzed Ether Formation • Reaction of alcohols with Ag2O directly with alkyl halide forms ether in one step • Glucose reacts with excess iodomethane in the presence of Ag2O to generate a pentaether in 85% yield 9 10/02/59 19 3. Alkoxymercuration of Alkenes • React alkene with an alcohol and mercuric acetate or trifluoroacetate • Demercuration with NaBH4 yields an ether • Overall Markovnikov addition of alcohol to alkene Reaction of Ethers and Epoxides 10 10/02/59 21 Reactions of Ethers: Acidic Cleavage • Ethers are generally unreactive • Strong acid will cleave an ether at elevated temperature • HI, HBr produce an alkyl halide from less hindered component by SN2 (tertiary ethers undergo SN1) 22 Reactions of Ethers: Claisen Rearrangement • Specific to allyl aryl ethers, ArOCH2CH=CH2 • Heating to 200–250°C leads to an o-allylphenol • Result is alkylation of the phenol in an ortho position 11 10/02/59 23 Claisen Rearrangement Mechanism • Concerted pericyclic 6-electron, 6-membered ring transition state • Mechanism consistent with 14C labelling 24 Cyclic Ethers: Epoxides • Cyclic ethers behave like acyclic ethers, except if ring is 3-membered • Dioxane and tetrahydrofuran are used as solvents 12 10/02/59 25 Epoxides (Oxiranes) • Three membered ring ether is called an oxirane (root “ir” from “tri” for 3-membered; prefix “ox” for oxygen; “ane” for saturated) • Also called epoxides • Ethylene oxide (oxirane; 1,2-epoxyethane) is industrially important as an intermediate • Prepared by reaction of ethylene with oxygen at 300 °C and silver oxide catalyst 26 Preparation of Epoxides Using a Peroxyacid • Treat an alkene with a peroxyacid 13 10/02/59 27 Epoxides from Halohydrins • Addition of HO-X to an alkene gives a halohydrin • Treatment of a halohydrin with base gives an epoxide • Intramolecular Williamson ether synthesis 28 Ring-Opening Reactions of Epoxides • Water adds to epoxides with dilute acid at room temperature • Product is a 1,2-diol (on adjacent C’s: vicinal) • Mechanism: acid protonates oxygen and water adds to opposite side (trans addition) 14 10/02/59 29 Ethylene Glycol • 1,2-ethanediol from acid catalyzed hydration of ethylene • Widely used as automobile antifreeze (lowers freezing point of water solutions) 30 Halohydrins from Epoxides • Anhydrous HF, HBr, HCl, or HI combines with an epoxide • Gives trans product 15 10/02/59 31 Regiochemistry of Acid-Catalyzed Opening of Epoxides • Nucleophile preferably adds to less hindered site if primary and secondary C’s • Also at tertiary because of carbocation character 32 Base-Catalyzed Epoxide Opening • Strain of the three-membered ring is relieved on ring- opening • Hydroxide cleaves epoxides at elevated temperatures to give trans 1,2-diols 16 10/02/59 33 Addition of Grignards to Ethylene Oxide • Adds –CH2CH2OH to the Grignard reagent’s hydrocarbon chain • Acyclic and other larger ring ethers do not react 34 Crown Ethers • Large rings consisting repeating (-OCH2CH2-) or similar units • Named as x-crown-y ▫ x is the total number of atoms in the ring ▫ y is the number of oxygen atoms ▫ 18-crown-6 ether: 18-membered ring containing 6 oxygens atoms • Central cavity is electronegative and attracts cations 17 10/02/59 35 Uses of Crown Ethers • Complexes between crown ethers and ionic salts are soluble in nonpolar organic solvents • Creates reagents that are free of water that have useful properties • Inorganic salts dissolve in organic solvents leaving the anion unassociated, enhancing reactivity 36 Thiols and Sulfides • Thiols (RSH), are sulfur analogs of alcohols ▫ Named with the suffix -thiol ▫ SH group is called “mercapto group” (“capturer of mercury”) 18 10/02/59 37 Sulfides • Sulfides (RSR), are sulfur analogs of ethers ▫ Named by rules used for ethers, with sulfide in place of ether for simple compounds and alkylthio in place of alkoxy 38 Thiols: Formation and Reaction • From alkyl halides by displacement with a sulfur nucleophile such as SH ▫ The alkylthiol product can undergo further reaction with the alkyl halide to give a symmetrical sulfide, giving a poorer yield of the thiol 19 10/02/59 39 Using Thiourea to Form Alkylthiols • Thiols can undergo further reaction with the alkyl halide to give dialkyl sulfides • For a pure alkylthiol use thiourea (NH2(C=S)NH2) as the nucleophile • This gives an intermediate alkylisothiourea salt, which is hydrolyzed cleanly to the alkyl thiourea 20 10/02/59 41 Oxidation of Thiols to Disulfides • Reaction of an alkyl thiol (RSH) with bromine or iodine gives a disulfide (RSSR) • The thiol is oxidized in the process and the halogen is reduced 42 Sulfides • Thiolates (RS) are formed by the reaction of a thiol with a base • Thiolates react with primary or secondary alkyl halide to give sulfides (RSR’) • Thiolates are excellent nucleophiles and react with many electrophiles 21 10/02/59 43 Sulfides as Nucleophiles • Sulfur compounds are more nucleophilic than their oxygen-compound analogs ▫ 3p electrons valence electrons (on S) are less tightly held than 2p electrons (on O) • Sulfides react with primary alkyl halides (SN2) to give + trialkylsulfonium salts (R3S ) 44 Oxidation of Sulfides • Sulfides are easily oxidized with H2O2 to the sulfoxide (R2SO) • Oxidation of a sulfoxide with a peroxyacid yields a sulfone (R2SO2) • Dimethyl sulfoxide (DMSO) is often used as a polar aprotic solvent 22 10/02/59 23.
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