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Chapter 11 Alcohols & Ethers 1. Structure & Nomenclature Alcohols have a hydroxyl (–OH) group bonded to a saturated carbon atom (sp3 hybridized) 1o OH Ethanol 2o OH 2-Propanol (isopropyl alcohol) 3o OH 2-Methyl2-propanol (tert-butyl alcohol) © 2014 by John Wiley & Sons, Inc. All rights reserved. OH 2-Propenol (allyl alcohol) OH 2-Propynol (propargyl alcohol) OH Benzyl alcohol © 2014 by John Wiley & Sons, Inc. All rights reserved. Phenols • Compounds that have a hydroxyl group attached directly to a benzene ring © 2014 by John Wiley & Sons, Inc. All rights reserved. Ethers • The oxygen atom of an ether is bonded to two carbon atoms O O Diethyl ether tert-Butyl methyl ether O O Divinyl ether CH3 Ethyl phenyl ether © 2014 by John Wiley & Sons, Inc. All rights reserved. 1A. Nomenclature of Alcohols Rules of naming alcohols • Identify the longest carbon chain that includes the carbon to which the –OH group is attached • Use the lowest number for the carbon to which the –OH group is attached • Alcohol as parent (suffix) ending with “ol” © 2014 by John Wiley & Sons, Inc. All rights reserved. Examples OH OH OH OH 2-Propanol (isopropyl alcohol) 1,2,3-Butanetriol © 2014 by John Wiley & Sons, Inc. All rights reserved. Example OH 3-Propyl-2-heptanol 8 or 1 5 2 OH 3 4 6 7 wrong 3 7 6 5 OH © 2014 by John Wiley & Sons, Inc. All rights reserved. 4 1 2 1B. Nomenclature of Ethers Rules of naming ethers • Similar to those with alkyl halides CH3O– Methoxy CH3CH2O– Ethoxy Example O Ethoxyethane (diethyl ether) © 2014 by John Wiley & Sons, Inc. All rights reserved. Cyclic ethers © 2014 by John Wiley & Sons, Inc. All rights reserved. 2. Physical Properties of Alcohols and Ethers Ethers have boiling points that are roughly comparable with those of hydrocarbons of the same molecular weight (MW) Alcohols have much higher boiling points than comparable ethers or hydrocarbons © 2014 by John Wiley & Sons, Inc. All rights reserved. For example O OH Diethyl ether Pentane 1-Butanol (MW = 74) (MW = 72) (MW = 74) o b.p. = 36oC b.p. = 117.7oC b.p. = 34.6 C Alcohol molecules can associate with each other through hydrogen bonding, whereas those of ethers and hydrocarbons cannot © 2014 by John Wiley & Sons, Inc. All rights reserved. Water solubility of ethers and alcohols • Both ethers and alcohols are able to form hydrogen bonds with water • Ethers have solubilities in water that are similar to those of alcohols of the same molecular weight and that are very different from those of hydrocarbons • The solubility of alcohols in water gradually decreases as the hydrocarbon portion of the molecule lengthens; longchain alcohols are more “alkane-like” and are, therefore, less like water © 2014 by John Wiley & Sons, Inc. All rights reserved. Physical Properties of Ethers Name Formula mp (oC) bp (oC) (1 atm) Dimethyl ether CH3OCH3 -138 -24.9 Diethyl ether CH3CH2OCH2CH3 -116 34.6 Diisopropyl ether (CH3)2CHOCH(CH 3)2 -86 68 1,2-Dimethoxyethane (DME) CH3OCH2CH2OCH3 -68 83 -112 12 -108 65.4 O Oxirane Tetrahydrofuran (THF) O © 2014 by John Wiley & Sons, Inc. All rights reserved. Physical Properties of Alcohols Name Formula Methanol CH3OH Ethanol CH3CH2OH Isopropyl alcohol CH3CH(OH)CH 3 tert-Butyl alcohol (CH3)3COH Hexyl alcohol CH3(CH2)4CH2OH OH Cyclohexanol Ethylene glycol HO OH mp (oC) bp (oC) (1 atm) * -97 64.7 inf. -117 78.3 inf. -88 82.3 inf. 25 82.5 inf. -52 156.5 0.6 24 161.5 3.6 -12.6 197 inf. * Water solubility (g/100 mL H2O) © 2014 by John Wiley & Sons, Inc. All rights reserved. 3. Important Alcohols & Ethers 3A. Methanol Methanol is highly toxic Ingestion of even small quantities of methanol can cause blindness Large quantities cause death Methanol poisoning can also occur by inhalation of the vapors or by prolonged exposure to the skin © 2014 by John Wiley & Sons, Inc. All rights reserved. 3B. Ethanol Ethanol can be produced by • Fermentation • Acid-catalyzed hydration of ethene © 2014 by John Wiley & Sons, Inc. All rights reserved. 3D. Diethyl Ether Diethyl ether is a very low boiling, highly flammable liquid Most ethers react slowly with oxygen by a radical process called autoxidation to form hydroperoxides and peroxides © 2014 by John Wiley & Sons, Inc. All rights reserved. 4. Synthesis of Alcohols from Alkenes Acid-catalyzed Hydration of Alkenes C C ⊕ H H H2O C C H OH H2O C H C H2O © 2014 by John Wiley & Sons, Inc. All rights reserved. C C H O H H Acid-Catalyzed Hydration of Alkenes • Markovnikov regioselectivity • Free carbocation intermediate • Rearrangement of carbocation possible © 2014 by John Wiley & Sons, Inc. All rights reserved. Oxymercuration–Demercuration C C Hg(OAc) 2 H2O, THF OH C C HgOAc NaBH4 NaOH • Markovnikov regioselectivity • Anti stereoselectivity • Generally takes place without the complication of rearrangements • Mechanism Discussed in Section 8.5 © 2014 by John Wiley & Sons, Inc. All rights reserved. OH C C H Hydroboration–Oxidation • Anti-Markovnikov regioselectivity • Syn-stereoselectivity • Mechanism Discussed in Section 8.7 © 2014 by John Wiley & Sons, Inc. All rights reserved. Markovnikov regioselectivity OH + H , H2O or 1. Hg(OAc)2, H2O, THF 2. NaBH4, NaOH R H R 1. BH3 • THF 2. H2O2, NaOH H R OH Anti-Markovnikov regioselectivity © 2014 by John Wiley & Sons, Inc. All rights reserved. Example Synthesis? (1) OH OH Synthesis? (2) © 2014 by John Wiley & Sons, Inc. All rights reserved. Synthesis (1) H OH • Need anti-Markovnikov addition of H–OH • Use hydroboration-oxidation H OH 1. BH3 • THF 2. H2O2, NaOH © 2014 by John Wiley & Sons, Inc. All rights reserved. Synthesis (2) OH H • Need Markovnikov addition of H–OH • Thus, could potentially use either acid-catalyzed hydration or oxymercuration-demercuration • However, acid-catalyzed hydration is NOT reasonable here due to likely rearrangement of carbocation © 2014 by John Wiley & Sons, Inc. All rights reserved. Oxymercuration-demercuration OH H Hg(OAc) 2 H2O, THF NaBH4 NaOH OH HgOAc © 2014 by John Wiley & Sons, Inc. All rights reserved. 5. Reactions of Alcohols The reactions of alcohols have mainly to do with the following: • The oxygen atom of the –OH group is nucleophilic and weakly basic • The hydrogen atom of the –OH group is weakly acidic • The –OH group can be converted to a leaving group so as to allow substitution or elimination reactions © 2014 by John Wiley & Sons, Inc. All rights reserved. O H C–O & O–H bonds of an alcohol are polarized Protonation of the alcohol converts a ⊖ poor leaving group (HO ) into a good one (H2O) © 2014 by John Wiley & Sons, Inc. All rights reserved. Once the alcohol is protonated substitution reactions become possible H Nu + C O protonated alcohol H SN2 H Nu C + O The protonated –OH group is a good leaving group (H2O) © 2014 by John Wiley & Sons, Inc. All rights reserved. H 6. Alcohols as Acids Alcohols have acidities similar to that of water pKa Values for Some Weak Acids Acid pKa CH3OH 15.5 H2O 15.74 CH3CH2OH 15.9 (CH3)3COH 18.0 © 2014 by John Wiley & Sons, Inc. All rights reserved. Relative Acidity H2O & alcohols are the strongest acids in this series H2O > ROH > RC CH > H2 > NH3 > RH Increasing acidity Relative Basicity R > NH2 > H > RC ⊖ HO is the weakest acid in this series C > RO > HO Increasing basicity © 2014 by John Wiley & Sons, Inc. All rights reserved. 7. Conversion of Alcohols into Alkyl Halides R OH R • HX (X = Cl, Br, I) • PBr3 • SOCl2 © 2014 by John Wiley & Sons, Inc. All rights reserved. X Examples OH conc. HCl Cl o 25 C + HOH (94%) OH PBr3 Br (63%) © 2014 by John Wiley & Sons, Inc. All rights reserved. 8. Alkyl Halides from the Reaction of Alcohols with Hydrogen Halides R OH + HX R X + H2O The order of reactivity of alcohols • 3o > 2o > 1o < methyl The order of reactivity of the hydrogen halides • HI > HBr > HCl (HF is generally unreactive) © 2014 by John Wiley & Sons, Inc. All rights reserved. R OH + NaX No Reaction! ⊖ HO is a poor leaving group ⊕ H3O is a good leaving group © 2014 by John Wiley & Sons, Inc. All rights reserved. 11. Synthesis of Ethers 11A. Ethers by Intermolecular Dehydration of Alcohols H2SO4 180oC Ethene OH H2SO4 o 140 C © 2014 by John Wiley & Sons, Inc. All rights reserved. O Diethyl ether Mechanism OH + H OSO3H O H + OSO3H H OH O O H 2O H • This method is only good for the synthesis of symmetrical ethers © 2014 by John Wiley & Sons, Inc. All rights reserved. + H2O For unsymmetrical ethers ROH + R'OH H2SO4 R R' + o 1 alcohols O R O R + R' O R' © 2014 by John Wiley & Sons, Inc. All rights reserved. Mixture of ethers R Exception OH + cat. H2SO4 HO R O + HO (good yield) H R H OH © 2014 by John Wiley & Sons, Inc. All rights reserved. 11B. The Williamson Ether Synthesis R X R'O R O R' (SN2) Via SN2 reaction, thus R is limited to 1o and some 2o (but R' can be 1o, 2o or 3o) © 2014 by John Wiley & Sons, Inc. All rights reserved. Example 1 Na H O O Na + H2 H Br O © 2014 by John Wiley & Sons, Inc. All rights reserved. Example 2 Cl HO Cl NaOH H2O O O © 2014 by John Wiley & Sons, Inc. All rights reserved. Example 3 I NaOH OH O H2O However I NaOH OH H2O No epoxide observed! © 2014 by John Wiley & Sons, Inc. All rights reserved. 11C. Synthesis of Ethers by Alkoxymercuration–Demercuration Markovnikov regioselectivity OR' 1. Hg(O2CCF3)2, R'OH R 2. NaBH4, NaOH (1) R (2) OR' R Hg(O2CCF3) © 2014 by John Wiley & Sons, Inc. All rights reserved. Example i 1. Hg(O2CCF3)2, PrOH 2. NaBH4, NaOH © 2014 by John Wiley & Sons, Inc. All rights reserved. O 12. Reactions of Ethers Dialkyl ethers react with very few reagents other than acids O + HBr O H an oxonium salt © 2014 by John Wiley & Sons, Inc. All rights reserved. + Br 12A. Cleavage of Ethers Heating dialkyl ethers with very strong acids (HI, HBr, and H2SO4) causes them to undergo reactions in which the carbon–oxygen bond breaks O + 2 HBr 2 Cleavage of an ether © 2014 by John Wiley & Sons, Inc. All rights reserved. Br + H2O Mechanism + H O Br + Br O H Br H + Br O O H H O H + Br © 2014 by John Wiley & Sons, Inc. All rights reserved. H + Br 13. Epoxides Epoxide (oxirane) • A 3-membered ring containing an oxygen © 2014 by John Wiley & Sons, Inc. All rights reserved. 13A. Synthesis of Epoxides: Epoxidation Electrophilic epoxidation C C O peroxy acid C © 2014 by John Wiley & Sons, Inc. All rights reserved. C Peroxy acids (peracids) O R C O OH • Common peracids © 2014 by John Wiley & Sons, Inc. All rights reserved. Mechanism peroxy acid O O O H R O O O O carboxylic acid R R O H O H epoxide alkene concerted transition state © 2014 by John Wiley & Sons, Inc. All rights reserved. 14. Reactions of Epoxides The highly strained three-membered ring of epoxides makes them much more reactive toward nucleophilic substitution than other ethers © 2014 by John Wiley & Sons, Inc. All rights reserved. Acid-catalyzed ring opening of epoxide C + H C O O C H + C O H H H H O C H O O H H + H H O H O C C H O © 2014 by John Wiley & Sons, Inc. All rights reserved. O C H H H Base-catalyzed ring opening of epoxide RO R O + C C C C O O R O H RO C + R C OH © 2014 by John Wiley & Sons, Inc. All rights reserved. O If the epoxide is unsymmetrical, in the base-catalyzed ring opening, attack by the alkoxide ion occurs primarily at the less substituted carbon atom EtO Et O + O O o 1 carbon atom is less hindered © 2014 by John Wiley & Sons, Inc. All rights reserved. In the acid-catalyzed ring opening of an unsymmetrical epoxide the nucleophile attacks primarily at the more substituted carbon atom cat. HA MeOH + O MeO OH O MeO OH MeOH + (protonated epoxide) H H o This carbon resembles a 3 carbocation © 2014 by John Wiley & Sons, Inc. All rights reserved. 15. Anti 1,2-Dihydroxylation of Alkenes via Epoxides Synthesis of 1,2-diols © 2014 by John Wiley & Sons, Inc. All rights reserved. Anti-Dihydroxylation • A 2-step procedure via ring-opening of epoxides OH OH © 2014 by John Wiley & Sons, Inc. All rights reserved. H2O 17. Summary of Reactions of Alkenes, Alcohols, and Ethers Synthesis of alcohols O 1. 2. H2O OH (1o alcohol) MgBr © 2014 by John Wiley & Sons, Inc. All rights reserved. 1. BH3 THF 2. H2O2, NaOH Synthesis of alcohols 1. BH3 THF 2. H2O2, NaOH + H , H2O OH (2o alcohol) 1. Hg(OAc)2, H2O 2. NaBH4 or © 2014 by John Wiley & Sons, Inc. All rights reserved. Synthesis of alcohols OH 1. Hg(OAc)2, H2O 2. NaBH4 (3o alcohol) © 2014 by John Wiley & Sons, Inc. All rights reserved. Reaction of alcohols OR Br PBr3 1. base 2. R-X OH SOCl 2 (1o alcohol) Cl © 2014 by John Wiley & Sons, Inc. All rights reserved. Synthesis of ethers R RO R R O conc. H2SO4 R 140oC R X OH Cleavage reaction of ethers O R' H X R O R' X H © 2014 by John Wiley & Sons, Inc. All rights reserved. ROH + R'X END OF CHAPTER 11 © 2014 by John Wiley & Sons, Inc. All rights reserved.
