CH 18: Ethers and Epoxides Renee Y. Becker Valencia Community College CHM 2211 1 Ethers and Their Relatives • An ether has two organic groups (alkyl, aryl, or vinyl) bonded to the same oxygen atom, R–O–R • Diethyl ether is used industrially as a solvent • Tetrahydrofuran (THF) is a solvent that is a cyclic ether • Epoxides contain a C-O-C unit which make-up a three membered ring • Thiols (R–S–H) and sulfides (R–S–R) are sulfur (for oxygen) analogs of alcohols and ethers 2 3 Naming Ethers • Ethers are named two ways: – As alkoxy derivatives of alkanes – Derived by listing the two alkyl groups in the general structure of ROR’ in alphabetical order as separate words and adding the word ether • When both alkyl groups are the same, the prefix diprecedes the name of the alkyl group • (Ethers can be described as symmetrical or unsymmetrical) 4 Naming Ethers CH3CH2 O CH2CH3 Diethyl ether Ethoxyethane CH3CH2 O CH3 Ethyl methyl ether Methoxyethane CH3CH2 O CH2CH2CH2Cl 3-Chloropropyl ethyl ether 1-Chloro-3-ethoxypropane 5 Naming Ethers • Epoxides (oxiranes) – “epoxy” always preceeds the name of the alkane O O 1,2-Epoxycyclohexane 2-Methyl-2,3-epoxybutane 6 Example 1: Name OCH3 1 O 6 O 2 O 7 3 O Br 4 O Cl 8 O Br 5 Br OCH3 7 Example 2: Draw 1. Isopropyl methyl ether 2. 4-t-butoxy-1-cyclohexene 3. Phenyl propyl ether 4. O- nitro anisole 8 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 1.2 D) 9 Structure, Properties, and Sources of Ethers (comparative boiling points) CH3CH2OCH2CH3 Diethyl ether bp =35oC solubility in water: 7.5 g/100mL CH3CH2CH2CH2CH3 Pentane bp =36oC solubility in water: insoluble CH3CH2CH2CH2OH 1-Butanol bp =117oC solubility in water: 9 g/100mL 10 Preparation of Ethers • Acid catalyzed synthesis of ethers: 2 OH butanol H2SO4 O 130oC Dibutyl ether + H2O Limited to symmetrical ethers. WHY? 11 Mechanism 1: Acid catalyzed synthesis of ethers HCl 2 OH O + 130 C H OH H O H3 O+ + Cl- H O H O+ H---Cl H + H3O+ + H O+ - Cl O 12 Preparation of Ethers • Williamson ether synthesis •Metal alkoxides react with primary alkyl halides by an SN2 pathway to yield ethers. •Secondary and tertiary substrates react following an E2 mechanism 13 Mechanism 2: Williamson Ether Synthesis OH O – Na+ NaH O THF + CH3CH2----I + H2 + NaI THF O H O– Na+ THF + Na+ H- CH3CH2----I + H2 O NaI + 14 Example 3: Williamson ether synthesis O CH3CH2O + CH3CH2Br + Br SN2 E2 ? ? 15 Example 4 How would you prepare the following compounds using a Williamson synthesis? • Methyl propyl ether • Anisole (methyl phenyl ether) 16 Example 5: Predict the product: Br O + CH3CH2CHCH3 17 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 18 19 20 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) 21 22 Mechanism 3: Acidic Cleavage Note that the halide attacks the protonated ether at the less highly substituted site. 23 Example 6 24 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 25 Mechanism 4: Claisen Rearrangement • Concerted pericyclic 6-electron, 6-membered ring transition state • Mechanism consistent with 14C labeling 26 Example 7 27 Cyclic Ethers: Epoxides • Cyclic ethers behave like acyclic ethers, except if ring is 3-membered • Dioxane and tetrahydrofuran are used as solvents 28 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 29 30 Preparation of Epoxides Using a Peroxyacid • Treat an alkene with a peroxyacid 31 Mechanism 5: Preparation of Epoxides Using a Peroxyacid peroxyacetic acid H C6H5 H O O + H5C6 H (E)-1,2-diphenylethene O H CH3 C6H5 O H5C6 OH + O H trans-1,2-diphenyloxirane CH3 acetic acid 32 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 33 34 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 (anti-addition) 35 Mechanism 6: Acid catalyzed ring-openings 36 Ethylene Glycol • 1,2-ethanediol from acid catalyzed hydration of ethylene • Widely used as automobile antifreeze (lowers freezing point of water solutions) 37 Halohydrins from Epoxides • Anhydrous HF, HBr, HCl, or HI combines with an epoxide • Gives trans product 38 39 40 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 – Complete the reaction O CH2 OH- H2O, 100oC Methylenecyclohexane oxide 41 Mechanism 7: Base-Catalyzed Epoxide Opening O - CH2 OH O– CH 2OH H O H OH CH 2OH + - OH 100 C 42 Mechanism 7: Base-Catalyzed Epoxide Opening 43 Addition of Grignards to Ethylene Oxide • Adds –CH2CH2OH to the Grignard reagent’s hydrocarbon chain • Acyclic and other larger ring ethers do not react 44 Thiols • Thiols (RSH), are sulfur analogs of alcohols – Named with the suffix -thiol – SH group is called “mercapto group” 45 Example 8: Name 1 SH 2 SH Br 3 SH 46 Example 9: Draw 1. Cyclopentanethiol 2. 3-methyl-4-heptanethiol 3. 4-ethyl-4-isopropyl-2-methyl-3-hexanethiol 47 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 48 Example 10: Name or Draw 1 2 3 4 S S ethyl phenyl sulfide sec-butyl isopropyl sulfide 49 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 50 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 51