Ether Notes

ETHER Like water, ethers have a bent structure, with an sp3 hybrid oxygen atom giving a nearly tetrahedral bond angle. •• H O C 110° C H H H H H dimethyl ether •• •• H •• •• •• O O (CH3)3C H Water 105° 132° C(CH3)3 Di-ter.butyl ether Bonding in ethers is readily understood by comparing ethers with water and alcohol. Van der Waals strain involving alkyl groups causes the bond angle at oxygen to the larger in ethers than in alcohol, and larger in alcohols than in water. An extreme example is di-tert-butyl ether, where steric hindrance between the tert-butyl groups is responsible for a dramatic increase in the C – O – C bond angle. R—O—R (Dialkyl ether), alkoxy alkane. It's General formula is CnH2n + 2O. CH3—O—CH2CH3 (Methoxy ethane) or ethyl methyl ether or 2–oxa butane Ether is monoalkyl derivative of R–OH and dialkyl derivative of H2O H R — OH R—O—R +R 2H +2R H—O—H Classification : They may be classified as : (a) (b) Simple or symmetrical ether. e.g. R–O–R Mixed or unsymmetrical ether e.g. R–O–R' Structure : The molecule of ether is bent due to lone pair of electron on oxygen atom- bond electron repulsion. The bond angle is 1100. It is greater than that of water 1050 due to the repulsion between bulky alkyl groups. Due to bent structure, it posses dipole moment and hence are polar molecules. GENERAL METHODS OF PREPARATION (A) From alkyl halides : (i) Ex. By Williamson's synthesis : R — X + Na — O — R  R — O — R + NaX [ SN2 Reaction] CH3 — I + C2H5 O– Na+  CH3 — CH2O — CH3 + NaI MECHANISM : [ SN2 Reaction] C2H5O C— I H H H H H Slow C2H5O ------ C ----- I H Fast C2H5—O—CH3+ I ALCOHOL, PHENOL & ETHER Ex. CH3 CH3 Ex. CH3 C ONa + CH3 CH2  Cl CH3 CH2 O CH2 CH3 CH3 CH3 Ex. C CH—Cl + CH3CH2 — ONa  No reaction [Stable by Resonance] (ii) Reaction with Dry Ag2O : 2RX + Ag2O  Ex. 2CH3—CH2—Cl + Ag2O (B) From R–OH: R—O—R + 2AgX  CH3CH2OCH2CH3 + 2AgCl conc. H2SO4  (i) By dehydration : (ii) Reaction with CH2N2 (diazomethane) : R — OH R — OH + CH2 — N2   BF  3 ? R — O — CH2 — H + N2 PHYSICAL PROPERTIES (i) CH3OCH3 , CH3OCH2CH3 are gases and higher are volatile liquids. (ii) Ether are less polar [=1.18D]. (iii) Ethers are less soluble in H2O. (iv) Ethers have less BP then corresponding alcohol. Ex. Ethers are less soluble in H2O . Why ? Sol. Reason : Due to less polar, it forms weaker H–Bonding with H2O. Ex. Ethers have less BP then corresponding alcohol. Why ? Sol. Reason : No H–Bonding in ether molecules. CHEMICAL PROPERTIES Ethers are less polar so less reactive and do not react with active metals [Na,K], cold dil. acid, oxidising and reducing agent. Reason : They do not have any active functional group. 1. Basic nature : Due to presence of .p on oxygen atom ether behave as lewis base Ethers react with cold conc. acid and form oxonium ion Ex. (diethyl oxonium chloride) Ex. (diethyl oxonium hydrogen sulphate) Ether form dative bond with Lewis acids like BF3, AlCl3, RMgX etc. R Ex. R .. O: R B F F F .. O .. R Mg X [Ether is used as solvent] for Grignard reagent. .. O .. R R R 2. Halogenation : 3. Formation of peroxides : Ether add up atmospheric oxygen or ozonised oxygen. It is explained by Free radical mechanism as intermediates is free radical. C2H5—O—C2H5 (Non polar) CH3CH2 — O — CH2 — Ph O2 (nonpolar) Long contact CH3CH2 CHCH3 O sunlight or UV O2 Long contact O  CH3 — CH2 — O — CH — Ph stable by resonance O2 O H CH3 CH2 O CH O Ph O H ALCOHOL, PHENOL & ETHER Peroxides are unstable and explosives. Test for peroxides FeSO4 /KCNS  Red colour ether (peroxide) CNS ether (Peroxides) + Fe +2  Fe +3   Fe(CNS) 3 (Red) 4. Reaction with hot dil. H2SO4 : R—O—R hot dil. H2SO4 5. Reaction with hot conc. H2SO4 : R—O—R hot conc. H2SO4 2R—OH cold dil. CH3 CH2 O CH2 CH3 6. Reaction with PCl5 : ROR + PCl5 7. Reaction with BCl3 : 3ROR + BCl3 8. Reaction with RCOCl : ROR + RCOCl 9. Reaction with CO : ROR + CO 10. Reaction with C2H5Na : 11. Dehydration : 12. Reduction : 13. Oxidation : Combustion : Oxonium salt hot dil. Ethyl alcohol hot conc. Ethyl hydrogen sulphate 2RCl + POCl3 heat  AlCl3 ZnCl2 heat 3RCl + (RO)3B RCOOR +RCl BF3/HgO 500 atm 150° C RCOOR  CH3CH2OH + CH2 Red P + HI heat CH3CH2OCH2CH3 CH3CH2 — O — CH2CH3 14. No Reaction cold conc. H2SO4 2RHSO4 H /K2Cr 2O7 + C2H5OC2H5 + 6O2 2CH3CH2OH  [O] CH2+C2H6 2CH3CH3 2CH3CHO 4CO2 + 5H2O (explosive mixture) HI > HBr > HCl 15. Reaction with HX : Reactivity of HX (A) REACTION WITH COLD CONC. HX : Ethers forms oxonium salt with cold and conc. HCl (less reactive) Cold conc. HI and HBr (more reactive) break C–O bond. [O] 2CH3COOH CH3 Ex. CH3 C O CH2 CH3 cold & conc. HI ? CH3 Sol. Mechanism  If oxonium ion gives more stable carbocation [ Ph CH 2 , CH2    CH— C H2, (CH3)3 C ] then SN1 reaction occurs.   If oxonium ion gives less stable carbocation [P h , CH2 = C H, CH3 C H2] then SN2 reaction occurs, and X attacks at less hindered carbon. Ex. Sol. CH3CH2—O—CH2Ph Mechanism : Cold conc. HI CH3CH2—OH + PhCH2—I, write mechanism of given reaction. CH3CH2OH + Ph CH2 Ex. CH3CH2 — O — CH3 anhy. HI I PhCH2I + CH3CH2–OH ? Sol. Ex. Oxonium ion gives less stable carbocation SN2 reaction I attacks at less hinderd carbon. CH3 — CH2 — O — Ph aq. HBr ? Sol. Mechanism : If excess of HI is used then two moles of alkyl hallides are formed. CH3CH2 — O — CH2Ph HI CH3CH2OH + PhCH2I HI CH 3CH 2 – I + PhCH 2 – I ALCOHOL, PHENOL & ETHER (B) REACTION WITH HOT AND CONC. HX : hot & conc. HI CH3CH2 — O — CH3 hot & conc. HBr Ex. C2H5 — O — C2H5 Sol. C2H5 — Br + C2H5 — Br CH3CH2 — I + CH3 — I ?+? Uses of ether : (i) General anaesthetics agent. (ii) Refrigerant a mixture of ether and dry ice gives temperature as low 110°C. (iii) Solvent for oil, fats, resins, Grignard reagent. (iv) For providing inert & moist free medium to organic reaction example : Wurtz reactions. (v) In perfumery. (vi) Di-isopropyl ether Petrol as an antiknock comp. (vii) Mixture of alcohol and ether is used as a substitute of petrol. Trade name "Natalite" (viii) Halothane (CF3CHClBr) used as an anaesthetic because it produces unconsciousness without affecting lung and heat. PREPARATION OF EPOXIDES : (i) Epoxidation of alkenes by reaction with peroxy acids (ii) Base-promoted ring closure of vicinal halohydrins (iii) Epoxidation of alkenes by reaction with peroxy acids Epoxidation of alkenes by reaction with peroxy acids : O C=C + R–C–OOH Peroxy acid Ex. : O + R–C–OH C—C O Epoxide (a) O O (b) (c) O + CH3–C–OOH + CH3–C–OH Epoxidation is a stereospecific syn addition : C6H5 H C=C H C6H5 O + (E) -1,2-diphenyl ethene CH3–C–OOH H C6H5 H O C6H5 O + CH3–C–OH trans -2,3-diphenyl oxirane MECHANISM : Base-promoted ring closure of vicinal halohydrins : x2 R2C = CR2  H 2o R R O .. R O R H H OH NaOH Ex. (i) OH Br trans-2-bromocyclohexanol H3C (ii) (iii) O H 1, 2-epoxycyclohexane Br H Br2/H2 O Anti addition H3C H cis-2-butane H H3C R+ X – C––C .. .. .. .. .. –O R R R R C––C R + HO—H .. –O R X R C––C .. .. Step II H R R X .. .. ..– HO .. R C––C .. Step I R X .. .. Mechanism : H3C H3C H3C H H OH H inversion of configuration H3C H O cis-2, 3-epoxybutane Br CH3 H trans-2-butane Br2/H2O Anti addition H H3C H CH3 H OH inversion of configuration CH3 H3C H O trans-2, 3-epoxybutane ALCOHOL, PHENOL & ETHER Reaction of Epoxides : With Grignard reagent : (i) diethylether + (ii) H3O RMgX + H2C––CH2 O CH2MgCl + RCH2CH2OH (primary alcohol) (i) diethylether H2C––CH2 (ii) H3O O Benzyl magnesium chloride CH2CH2CH2OH + Ethylene oxide 3-phenyl-1-propanol Nucleophilic ring opening reactions of epoxides : Y Y R2 C––– CR2 Y:– + R2C– –––CR 2 R2C––– CR2 O .. KSCH2CH 2CH3 H 2C–––CH 2 CH3–– CH2CH2–– CH2SCH2CH2OH ethanol water, 0°C O .. .. .. OH .. .. O H 2O   2-(butylthio) ethanol Note : Nucleophilic ring opening reactions of epoxides is the characteristic feature of S N2 reaction. OCH2CH3 H (i) O NaOCH2CH 3 H H H OH CH3 CH2OH CH 3 H3C H H3C O CH3 O OH H CH 3 CH3O NaOCH3 CH3OH C––––C H H H 2N H H3C (ii) NH3/H2O CH3 CH3 CH3––CH––C––CH3 OH Nucleophilic ring opening of epoxides : C6H5MgBr + H 2C ––– CHCH 3 1, diethyl ether + 2 H3O C6H5CH2CH––CH3 O OH 1. LiAlH4 + 2. H2O CH2–––CH(CH2)7CH3 CH3––CH––(CH2)7 CH3 OH O (i) HBr 10°C H2C––– CH2 BrCH2CH2OH O (ii) CH3CH2OH H2SO4, 25°C H2C––– CH2 O H2C–––CH2 + H2O H3O CH3–CH2OCH2CH2OH + HOCH2CH2OH O MECHANISM + H2C–––CH2 + H—O : Step-1 : :O: .. H2 C––– CH2 + H2O: H H :O+ H H Slow + H—O .. H H + :O: + H—O H .. CH2–CH2–O–H .. .. CH2–CH2–O–H .. H + H .. .. O —H + HOCH OH .. 2CH2.. .. H O+ .. Step-3 : H .. :O: + .. H Step-2 : H2C––– CH2 ALCOHOL, PHENOL & ETHER Ex. : H H OH HBr O OH H H 1,2-Epoxycyclohexane CH3 H3C C––– C H trans -2-bromo cyclohexanol O CH3 2,2,3-trimethyl oxirane OCH3 CH 3OH H 2SO 4 CH3––CH––C––CH3 HO CH3 3-methoxy-3-methyl-2-butanol 1. Preparation and reactions of glycol : (ii) aq. NaHCO3 CH2OCOCH3 HIO4 –CH2–OH 2. Gener al Reactions of Glycer ol :