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Chapter 15 Alcohols, Diols, and Thiols 15.1 Sources of Alcohols Methanol Methanol is an industrial chemical end uses: solvent, antifreeze, fuel principal use: preparation of formaldehyde Methanol Methanol is an industrial chemical end uses: solvent, antifreeze, fuel principal use: preparation of formaldehyde prepared by hydrogenation of carbon monoxide CO + 2H2 CH3OH Ethanol Ethanol is an industrial chemical Most ethanol comes from fermentation Synthetic ethanol is produced by hydration of ethylene Synthetic ethanol is denatured (made unfit for drinking) by adding methanol, benzene, pyridine, castor oil, gasoline, etc. Other alcohols Isopropyl alcohol is prepared by hydration of propene. All alcohols with four carbons or fewer are readily available. Most alcohols with five or six carbons are readily available. Sources of alcohols Reactions discussed in earlier chapters (Table 15.1) Hydration of alkenes Hydroboration-oxidation of alkenes Hydrolysis of alkyl halides Syntheses using Grignard reagents organolithium reagents Sources of alcohols New methods in Chapter 15 Reduction of aldehydes and ketones Reduction of carboxylic acids Reduction of esters Reaction of Grignard reagents with epoxides Diols by hydroxylation of alkenes 15.2 Preparation of Alcohols by Reduction of Aldehydes and Ketones Reduction of Aldehydes Gives Primary Alcohols R R C H O H C H OH Example: Catalytic Hydrogenation O CH3O CH + H2 Pt, ethanol CH3O CH2OH (92%) Reduction of Ketones Gives Secondary Alcohols R R C R' O H C R' OH Example: Catalytic Hydrogenation H O + H2 OH Pt ethanol (93-95%) Retrosynthetic Analysis R H C R OH H:– R C R' O C O H H H C R OH H:– R' Metal Hydride Reducing Agents H H + Na H – B H Li + Al H H H Sodium borohydride H – Lithium aluminum hydride act as hydride donors Examples: Sodium Borohydride Aldehyde O2N O2N O NaBH4 CH2OH CH methanol (82%) Ketone O NaBH4 ethanol H OH (84%) Lithium aluminum hydride more reactive than sodium borohydride cannot use water, ethanol, methanol etc. as solvents diethyl ether is most commonly used solvent Examples: Lithium Aluminum Hydride Aldehyde O CH3(CH2)5CH 1. LiAlH4 diethyl ether 2. H2O CH3(CH2)5CH2OH (86%) Ketone O (C6H5)2CHCCH3 1. LiAlH4 diethyl ether 2. H2O OH (C6H5)2CHCHCH3 (84%) Selectivity neither NaBH4 or LiAlH4 reduces isolated double bonds O 1. LiAlH4 diethyl ether 2. H2O (90%) H OH 15.3 Preparation of Alcohols By Reduction of Carboxylic Acids and Esters Reduction of Carboxylic Acids Gives Primary Alcohols R R C HO O H C H lithium aluminum hydride is only effective reducing agent OH Example: Reduction of a Carboxylic Acid O COH 1. LiAlH4 diethyl ether 2. H2O CH2OH (78%) Reduction of Esters Gives Primary Alcohols Lithium aluminum hydride preferred for laboratory reductions Sodium borohydride reduction is too slow to be useful Catalytic hydrogenolysis used in industry but conditions difficult or dangerous to duplicate in the laboratory (special catalyst, high temperature, high pressure Example: Reduction of a Carboxylic Acid O COCH2CH3 1. LiAlH4 diethyl ether 2. H2O CH2OH + (90%) CH3CH2OH 15.4 Preparation of Alcohols From Epoxides Reaction of Grignard Reagents with Epoxides R MgX CH2 H2C O R CH2 CH2 OMgX H3O+ RCH2CH2OH Example CH2 CH3(CH2)4CH2MgBr + H2C O 1. diethyl ether 2. H3O+ CH3(CH2)4CH2CH2CH2OH (71%) 15.5 Preparation of Diols Diols are prepared by... reactions used to prepare alcohols hydroxylation of alkenes Example: reduction of a dialdehyde O O HCCH2CHCH2CH CH3 H2 (100 atm) Ni, 125°C HOCH2CH2CHCH2CH2OH CH3 3-Methyl-1,5-pentanediol (81-83%) Hydroxylation of Alkenes Gives Vicinal Diols vicinal diols have hydroxyl groups on adjacent carbons ethylene glycol (HOCH2CH2OH) is most familiar example Osmium Tetraoxide is Key Reagent syn addition of —OH groups to each carbon of double bond C C C C HO C C O O Os O O OH Example CH3(CH2)7CH CH2 (CH3)3COOH OsO4 (cat) tert-Butyl alcohol HO– CH3(CH2)7CHCH2OH OH (73%) Example H (CH3)3COOH OsO4 (cat) H tert-Butyl alcohol HO– H H HO OH (62%)