15.4 Preparation of Alcohols from Epoxides (CH3)C=CHCH, CH-CCH 2.H2O CH3)2C=CHCH,CH,CHCH3 this transformation because 6-Methyl-5-hepten-2-one 6-Methyl-5-hepten-2-ol(90%) double bonds faster than it reduces carbonyl groups. 15.3 PREPARATION OF ALCOHOLS BY REDUCTION OF CARBOXYLIC ACIDS AND ESTERS Carboxylic acids are exceedingly difficult to reduce. Acetic acid, for example, is often used as a solvent in catalytic hydrogenations because it is inert under the reaction con- ditions. A very powerful reducing agent is required to convert a carboxylic acid to a pri mary alcohol. Lithium aluminum hydride is that reducing agent RCOH RCH,OH Carboxylic acid Primary alcohol Cyclopropylmethar Sodium borohydride is not nearly as potent a hydride donor as lithium aluminum hydride and does not reduce carboxylic acids Esters are more easily reduced than carboxylic acids. Two alcohols are formed from each ester molecule. The acyl group of the ester is cleaved, giving a primary alcohol. RCOR RCH,OH R'OH Ester Primary alcohol Alcohol Lithium aluminum hydride is the reagent of choice for reducing esters to alcohols LiAlH4, diethyl etl COCH, CH -CH,OH CH3 CH,OH Ethyl benzoate Benzyl alcohol (90%) Ethanol PROBLEM 15.3 Give the structure of an ester that will yield a mixture contain- ing equimolar amounts of 1-propanol and 2-propanol on reduction with lithium aluminum hydride. Sodium borohydride reduces esters, but the reaction is too slow to be useful Hydrogenation of esters requires a special catalyst and extremely high pressures and tem- peratures; it is used in industrial settings but rarely in the laboratory 15.4 PREPARATION OF ALCOHOLS FROM EPOXIDES Although the chemical reactions of epoxides will not be covered in detail until the fol lowing chapter, we shall introduce their use in the synthesis of alcohols here Back Forward Main MenuToc Study Guide ToC Student o MHHE Website15.3 PREPARATION OF ALCOHOLS BY REDUCTION OF CARBOXYLIC ACIDS AND ESTERS Carboxylic acids are exceedingly difficult to reduce. Acetic acid, for example, is often used as a solvent in catalytic hydrogenations because it is inert under the reaction con￾ditions. A very powerful reducing agent is required to convert a carboxylic acid to a pri￾mary alcohol. Lithium aluminum hydride is that reducing agent. Sodium borohydride is not nearly as potent a hydride donor as lithium aluminum hydride and does not reduce carboxylic acids. Esters are more easily reduced than carboxylic acids. Two alcohols are formed from each ester molecule. The acyl group of the ester is cleaved, giving a primary alcohol. Lithium aluminum hydride is the reagent of choice for reducing esters to alcohols. PROBLEM 15.3 Give the structure of an ester that will yield a mixture contain￾ing equimolar amounts of 1-propanol and 2-propanol on reduction with lithium aluminum hydride. Sodium borohydride reduces esters, but the reaction is too slow to be useful. Hydrogenation of esters requires a special catalyst and extremely high pressures and tem￾peratures; it is used in industrial settings but rarely in the laboratory. 15.4 PREPARATION OF ALCOHOLS FROM EPOXIDES Although the chemical reactions of epoxides will not be covered in detail until the fol￾lowing chapter, we shall introduce their use in the synthesis of alcohols here. 1. LiAlH4, diethyl ether 2. H2O COCH2CH3 O Ethyl benzoate CH2OH Benzyl alcohol (90%) CH3CH2OH Ethanol RCOR O Ester RCH2OH Primary alcohol ROH Alcohol 1. LiAlH4, diethyl ether 2. H2O RCOH O Carboxylic acid RCH2OH Primary alcohol 1. LiAlH4, diethyl ether 2. H2O CO2H Cyclopropanecarboxylic acid CH2OH Cyclopropylmethanol (78%) CHCH2CH2CCH3 (CH3)2C O 6-Methyl-5-hepten-2-one CHCH2CH2CHCH3 (CH3)2C OH 6-Methyl-5-hepten-2-ol (90%) 1. LiAlH4, diethyl ether 2. H2O 15.4 Preparation of Alcohols from Epoxides 587 Catalytic hydrogenation would not be suitable for this transformation, because H2 adds to carbon–carbon double bonds faster than it reduces carbonyl groups. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website