15.2 Preparation of Alcohols by Reduction of Aldehydes and Ketones Lithium aluminum hydride reacts violently with water and alcohols, so it must be used in solvents such as anhydrous diethyl ether or tetrahydrofuran. Following reduc tion, a separate hydrolysis step is required to liberate the alcohol product 1. LiAlH, diethyl ether →>RCH2OH Aldehyde Primary alcohol CH3(CH,)sCH- LiAIH CH3(CH2)5CH,OH Heptanal I-Heptanol (86%) RCR’ LiAIH,diethyl eth (C6H5)2CHCCH3 (C6H5)2CHCHCH3 1. 1-Diphenyl-2-propanol (84%) Sodium borohydride and lithium aluminum hydride react with carbonyl compounds in much the same way that Grignard reagents do, except that they function as hydride donors rather than as carbanion sources. Borohydride transfers a hydrogen with its pair of bonding electrons to the positively polarized carbon of a carbonyl group. The nega tively polarized oxygen attacks boron. Ultimately, all four of the hydrogens of borohy- dride are transferred and a tetraalkoxyborate is formed H-BH3 H BH3 (R2CHO)4B RC-O RC=O Tetraalkoxyborate Hydrolysis or alcoholysis converts the tetraalkoxyborate intermediate to the corre- sponding alcohol. The following equation illustrates the process for reactions carried out in water. An analogous process occurs in methanol or ethanol and yields the alcohol and (CH3O)4B or(CH3 CH,O)4B R2CHO—B(OCHR2)3 ?R2CHOH+ HOb(OCHR2)3->3R,CHOH +(HO)4B A similar series of hydride transfers occurs when aldehydes and ketones are treated with lithium aluminum hydride Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteLithium aluminum hydride reacts violently with water and alcohols, so it must be used in solvents such as anhydrous diethyl ether or tetrahydrofuran. Following reduc￾tion, a separate hydrolysis step is required to liberate the alcohol product: Sodium borohydride and lithium aluminum hydride react with carbonyl compounds in much the same way that Grignard reagents do, except that they function as hydride donors rather than as carbanion sources. Borohydride transfers a hydrogen with its pair of bonding electrons to the positively polarized carbon of a carbonyl group. The nega￾tively polarized oxygen attacks boron. Ultimately, all four of the hydrogens of borohy￾dride are transferred and a tetraalkoxyborate is formed. Hydrolysis or alcoholysis converts the tetraalkoxyborate intermediate to the corre￾sponding alcohol. The following equation illustrates the process for reactions carried out in water. An analogous process occurs in methanol or ethanol and yields the alcohol and (CH3O)4B or (CH3CH2O)4B. A similar series of hydride transfers occurs when aldehydes and ketones are treated with lithium aluminum hydride. 3H2O B(OCHR2)3  H OH R2CHO R2CHOH HOB(OCHR2)3  3R2CHOH (HO)4B  3R2CœO H BH3  R2C O   BH3  R2C O H  (R2CHO)4B Tetraalkoxyborate 1. LiAlH4, diethyl ether 2. H2O RCH O Aldehyde RCH2OH Primary alcohol CH3(CH2)5CH O Heptanal CH3(CH2)5CH2OH 1-Heptanol (86%) 1. LiAlH4, diethyl ether 2. H2O RCR O Ketone RCHR OH Secondary alcohol 1. LiAlH4, diethyl ether 2. H2O (C6H5)2CHCCH3 O 1,1-Diphenyl-2-propanone (C6H5)2CHCHCH3 OH 1,1-Diphenyl-2-propanol (84%) 1. LiAlH4, diethyl ether 2. H2O 15.2 Preparation of Alcohols by Reduction of Aldehydes and Ketones 585 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website