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Alcohols
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Alcohols as Acids
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resonance in phenols
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Methanol and ethanol are industrial chemicals: solvents, antifreeze, fuel, antiseptic, social / recreational 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. Alcohols
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Hydration of alkenes Hydroboration-oxidation of alkenes Hydrolysis of alkyl halides Sources of alcohols Reactions discussed in earlier chapters
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Reduction of aldehydes and ketones Reduction of carboxylic acids Reduction of esters Reaction of Grignard reagents Sources of alcohols New methods:
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Preparation of Alcohols by Reduction of Aldehydes and Ketones
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CRHOH H CRH O Reduction of Aldehydes Gives Primary Alcohols
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Pt, ethanol (92%) Example: Catalytic Hydrogenation CH 3 O CH 2 OH O CH 3 O CH + H2H2H2H2
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CRHOH R' C R R' O Reduction of Ketones Gives Secondary Alcohols
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(93-95%) Example: Catalytic Hydrogenation + H2H2H2H2OPt ethanol HOH
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H:–H:–H:–H:– H:–H:–H:–H:– CRH OHOHOHOH H CRH O CRH OHOHOHOH R' CRR' O Retrosynthetic Analysis
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Sodium borohydride Lithium aluminum hydride Li+ Na+ – B HHHH – Al HHHH Metal Hydride Reducing Agents act as hydride donors
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NaBH 4 (82%) Examples: Sodium Borohydride CH 2 OH O CH O2NO2NO2NO2Nmethanol O2NO2NO2NO2N O HOH(84%) NaBH 4 ethanol Aldehyde Ketone
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Lithium aluminum hydride more reactive than sodium borohydride cannot use water, ethanol, methanol etc. as solvents diethyl ether is most commonly used solvent
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Examples: Lithium Aluminum Hydride (84%) Aldehyde KetoneO CH 3 (CH 2 ) 5 CH CH 3 (CH 2 ) 5 CH 2 OH 1. LiAlH 4 diethyl ether 2. H 2 O O (C 6 H 5 ) 2 CHCCH 3 1. LiAlH 4 diethyl ether 2. H 2 O (86%)OH (C 6 H 5 ) 2 CHCHCH 3
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neither NaBH 4 or LiAlH 4 reduces isolated double bonds HOH O 1. LiAlH 4 diethyl ether 2. H 2 O (90%) Selectivity
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Preparation of Alcohols By Reduction of Carboxylic Acids and Esters
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lithium aluminum hydride is only effective reducing agent Reduction of Carboxylic Acids Gives Primary Alcohols CRHOH H C R HO O
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Example: Reduction of a Carboxylic Acid 1. LiAlH 4 diethyl ether 2. H 2 O COH O CH 2 OH (78%)
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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 Reduction of Esters Gives Primary Alcohols
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Example: Reduction of a Carboxylic Acid 1. LiAlH 4 diethyl ether 2. H 2 O (90%)O COCH 2 CH 3 CH 3 CH 2 OH CH 2 OH +
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Summary
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Synthesis of Alcohols Sodium borohydride / Lithium aluminum hydride Reductions
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