15.5 Preparation of diols 15.5 PREPARATION OF DIOLS Much of the chemistry of diols--compounds that bear two hydroxyl groups-is analo- gous to that of alcohols. Diols may be prepared, for example, from compounds that con- tain two carbonyl groups, using the same reducing agents employed in the preparation of alcohols. The following example shows the conversion of a dialdehyde to a diol by catalytic hydrogenation. Alternatively, the same transformation can be achieved by reduc tion with sodium borohydride or lithium aluminum hydride. CCh CHCh CH HO HOCH.CHCHCH,CH,OH 3-Methylpentanedial 3-Methyl-1, 5-pentanediol(81-83%) Diols are almost always given substitutive IUPAC names. As the name of the prod- uct in the example indicates, the substitutive nomenclature of diols is similar to that of alcohols. The suffix -diol replaces -ol, and two locants, one for each hydroxyl group, are required. Note that the final -e of the alkane basis name is retained when the suffix begins with a consonant (-diol), but dropped when the suffix begins with a vowel (-ol) PROBLEM 15.5 Write equations showing how 3-methyl-1, 5-pentanediol could be prepared from a dicarboxylic acid or a diester. commonly encountered vicinal diols are 1, 2-ethanediol and 1, 2-propanedior 3 Vicinal diols are diols that have their hydroxyl groups on adjacent carbons. Two HOCH, CH,OH CH3CHCH,OH 1. 2-Ethanediol 1, 2-Propanedio their epoxides. Some appl (ethylene glycol) (propylene glycol) tions were given in the box Ethylene glycol and propylene glycol are common names for these two diols and are acceptable IUPAC names. Aside from these two compounds, the IUPAC system does no use the word"" for naming diols. In the laboratory, vicinal diols are normally prepared from alkenes using the reagent osmium tetraoxide( OsO4). Osmium tetraoxide reacts rapidly with alkenes to give cyclic osmate esters R2C-CR2+ OsO4 -> R2C-CR Alkene Osmium Cyclic osmate ester Osmate esters are fairly stable but are readily cleaved in the pre resence of an oxi- dizing agent such as tert-butyl hydroperoxide Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
15.5 PREPARATION OF DIOLS Much of the chemistry of diols—compounds that bear two hydroxyl groups—is analogous to that of alcohols. Diols may be prepared, for example, from compounds that contain two carbonyl groups, using the same reducing agents employed in the preparation of alcohols. The following example shows the conversion of a dialdehyde to a diol by catalytic hydrogenation. Alternatively, the same transformation can be achieved by reduction with sodium borohydride or lithium aluminum hydride. Diols are almost always given substitutive IUPAC names. As the name of the product in the example indicates, the substitutive nomenclature of diols is similar to that of alcohols. The suffix -diol replaces -ol, and two locants, one for each hydroxyl group, are required. Note that the final -e of the alkane basis name is retained when the suffix begins with a consonant (-diol), but dropped when the suffix begins with a vowel (-ol). PROBLEM 15.5 Write equations showing how 3-methyl-1,5-pentanediol could be prepared from a dicarboxylic acid or a diester. Vicinal diols are diols that have their hydroxyl groups on adjacent carbons. Two commonly encountered vicinal diols are 1,2-ethanediol and 1,2-propanediol. Ethylene glycol and propylene glycol are common names for these two diols and are acceptable IUPAC names. Aside from these two compounds, the IUPAC system does not use the word “glycol” for naming diols. In the laboratory, vicinal diols are normally prepared from alkenes using the reagent osmium tetraoxide (OsO4). Osmium tetraoxide reacts rapidly with alkenes to give cyclic osmate esters. Osmate esters are fairly stable but are readily cleaved in the presence of an oxidizing agent such as tert-butyl hydroperoxide. R2C CR2 Alkene OsO4 Osmium tetraoxide R2C Os O O O O CR2 Cyclic osmate ester CH3CHCH2OH OH 1,2-Propanediol (propylene glycol) HOCH2CH2OH 1,2-Ethanediol (ethylene glycol) H2 (100 atm) Ni, 125°C HCCH2CHCH2CH O O CH3 3-Methylpentanedial HOCH2CH2CHCH2CH2OH CH3 3-Methyl-1,5-pentanediol (81–83%) 15.5 Preparation of Diols 589 Ethylene glycol and propylene glycol are prepared industrially from the corresponding alkenes by way of their epoxides. Some applications were given in the box in Section 6.21. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER FIFTEEN Alcohols, Diols, and Thiols R2C一CR2+2(CH3)3COOH R, 2+ OsO4 2(CH3)3COH O Vicinal terf-But tetraoxide Since osmium tetraoxide is regenerated in this step, alkenes can be converted to vicinal diols using only catalytic amounts of osmium tetraoxide, which is both toxic and expe sive. The entire process is performed in a single operation by simply allowing a solu- tion of the alkene and tert-butyl hydroperoxide in tert-butyl alcohol containing a small amount of osmium tetraoxide and base to stand for several hours CH3(CH2),CH=CH23-10 40,CH3(CH2),CHCH,OH l-Decene 1, 2-Decanediol (73%) Overall, the reaction leads to addition of two hydroxyl groups to the double bond and is referred to as hydroxylation. Both oxygens of the diol come from osmium tetraox ide via the cyclic osmate ester. The reaction of OsOa with the alkene is a syn addition and the conversion of the cyclic osmate to the diol involves cleavage of the bonds between oxygen and osmium. Thus, both hydroxyl groups of the diol become attached to the same face of the double bond; syn hydroxylation of the alkene is observed. Construct a molecular (CH3)COOH, Oso( What is the orientation of the butyl alcohoL. OH groups, axial or equ Cyclohexene cis-1, 2-Cyclohexan PROBLEM 15.6 Give the structures, including stereochemistry, for the diols obtained by hydroxylation of cis-2-butene and trans-2-butene A complementary method, one that gives anti hydroxylation of alkenes by way of the hydrolysis of epoxides, will be described in Section 16.13 15.6 REACTIONS OF ALCOHOLS: A REVIEW AND A PREVIEW Alcohols are versatile starting materials for the preparation of a variety of organic func- tional groups. Several reactions of alcohols have already been seen in earlier chapters and are summarized in Table 15. 2. The remaining sections of this chapter add to the list. 15.7 CONVERSION OF ALCOHOLS TO ETHERS Primary alcohols are converted to ethers on heating in the presence of an acid catalyst, usually sulfuric acid. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Since osmium tetraoxide is regenerated in this step, alkenes can be converted to vicinal diols using only catalytic amounts of osmium tetraoxide, which is both toxic and expensive. The entire process is performed in a single operation by simply allowing a solution of the alkene and tert-butyl hydroperoxide in tert-butyl alcohol containing a small amount of osmium tetraoxide and base to stand for several hours. Overall, the reaction leads to addition of two hydroxyl groups to the double bond and is referred to as hydroxylation. Both oxygens of the diol come from osmium tetraoxide via the cyclic osmate ester. The reaction of OsO4 with the alkene is a syn addition, and the conversion of the cyclic osmate to the diol involves cleavage of the bonds between oxygen and osmium. Thus, both hydroxyl groups of the diol become attached to the same face of the double bond; syn hydroxylation of the alkene is observed. PROBLEM 15.6 Give the structures, including stereochemistry, for the diols obtained by hydroxylation of cis-2-butene and trans-2-butene. A complementary method, one that gives anti hydroxylation of alkenes by way of the hydrolysis of epoxides, will be described in Section 16.13. 15.6 REACTIONS OF ALCOHOLS: A REVIEW AND A PREVIEW Alcohols are versatile starting materials for the preparation of a variety of organic functional groups. Several reactions of alcohols have already been seen in earlier chapters and are summarized in Table 15.2. The remaining sections of this chapter add to the list. 15.7 CONVERSION OF ALCOHOLS TO ETHERS Primary alcohols are converted to ethers on heating in the presence of an acid catalyst, usually sulfuric acid. H H Cyclohexene (CH3)3COOH, OsO4(cat) tert-butyl alcohol, HO cis-1,2-Cyclohexanediol (62%) H H HO HO CH3 CH2 (CH2)7CH 1-Decene OH CH3(CH2)7CHCH2OH 1,2-Decanediol (73%) (CH3)3COOH, OsO4(cat) tert-butyl alcohol, HO R2C Os O O O O CR2 2(CH3)3COOH tert-Butyl hydroperoxide HO OH R2C CR2 Vicinal diol Osmium tetraoxide OsO4 2(CH3)3COH tert-Butyl alcohol HO tert-butyl alcohol 590 CHAPTER FIFTEEN Alcohols, Diols, and Thiols Construct a molecular model of cis-1,2-cyclohexanediol. What is the orientation of the OH groups, axial or equatorial? Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
TABLE 15.2 Summary of Reactions of Alcohols Discussed in Earlier Chapters Reaction(section) and comments General equation and specific example Reaction with hydrogen halides(Sec- ROH HX RX+ H2O tion 4.8)The order of alcohol reactivI- Alcohol Hydrogen halide ty parallels the order of carbocation Alkyl halide Water stability: R3C >R2CH> RCH2> CHRo CH3O CH3. Benzylic alcohols react readily CHOH CHBr m-Methoxybenzyl alcohol m-Methoxybenzyl bromide(98%) Reaction with thionyl chloride(Sec ROH SOCI2- RCI+ SO2+ HCI tion 4. 14) Thionyl chloride converts alcohols to alkyl chlorides Alcohol Thionyl Sulfur Hydrogen chloride (CH3)2C=CHCH2CH2 CHCH3 (CH3)2C=CHCH2 CH2 CHCH 6-Methyl-5-hepten-2-oI oro-2-methyl- eptene(67%) Reaction with phosphorus trihalides 3ROH+ PX3 3RX+ H3PO3 and phosphorus tribromide convert Alcohol Phosphorus trihalide Alkyl halide Phosphorous acid alcohols to alkyl halides. CHOH 5.9)This is a frequently used proce- R2CCHR2 y Acid-catalyzed dehydration( Section . R2C=CR2 H2o dure for the preparation of alkenes The order of alcohol reactivity paral lels the order of carbocation stability: Alcohol R3c>R2CH alcohols react readily Rearrange ments are sometimes observed CHCH2 CH CH=CHCH 1-(m-Bromophenyl)-1-propanol 1-(m-Bromophenyl)propene(71%) Conversion to p-toluenesulfonate esters(Section 8. 14) Alcohols react with p-toluenesulfonyl chloride to ROH H3C So2C→>RC CH3+ HCI give p-toluenesulfonate esters Sulfo nate esters are reactive substrates for nucleophilic substitution and elimina- Alcohol tion reactions. The p-toluenesulfo- chloride p-toluenesulfonate nate group is often abbreviated pyridine OTs Cycloheptanol Cycloheptyl toluenesulfonate(83% Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
15.7 Conversion of Alcohols to Ethers 591 TABLE 15.2 Summary of Reactions of Alcohols Discussed in Earlier Chapters Reaction (section) and comments Reaction with hydrogen halides (Section 4.8) The order of alcohol reactivity parallels the order of carbocation stability: R3C R2CH RCH2 CH3 . Benzylic alcohols react readily. Reaction with thionyl chloride (Section 4.14) Thionyl chloride converts alcohols to alkyl chlorides. Reaction with phosphorus trihalides (Section 4.14) Phosphorus trichloride and phosphorus tribromide convert alcohols to alkyl halides. Acid-catalyzed dehydration (Section 5.9) This is a frequently used procedure for the preparation of alkenes. The order of alcohol reactivity parallels the order of carbocation stability: R3C R2CH RCH2 . Benzylic alcohols react readily. Rearrangements are sometimes observed. Conversion to p-toluenesulfonate esters (Section 8.14) Alcohols react with p-toluenesulfonyl chloride to give p-toluenesulfonate esters. Sulfonate esters are reactive substrates for nucleophilic substitution and elimination reactions. The p-toluenesulfonate group is often abbreviated ±OTs. H heat Alcohol R2CCHR2 W OH Alkene R2CœCR2 Water H2O General equation and specific example SOCl2, pyridine diethyl ether 6-Methyl-5-hepten-2-ol (CH3)2CœCHCH2CH2CHCH3 W OH 6-Chloro-2-methyl- 2-heptene (67%) (CH3)2CœCHCH2CH2CHCH3 W Cl Alcohol ROH Hydrogen halide HX Alkyl halide RX Water H2O CH3O CH2OH m-Methoxybenzyl alcohol CH3O CH2Br m-Methoxybenzyl bromide (98%) HBr Alcohol ROH Thionyl chloride SOCl2 Alkyl chloride RCl Sulfur dioxide SO2 Hydrogen chloride HCl Alcohol 3ROH Phosphorus trihalide PX3 Alkyl halide 3RX Phosphorous acid H3PO3 PBr3 CH2OH Cyclopentylmethanol CH2Br (Bromomethyl)cyclopentane (50%) KHSO4 heat Br CHCH2CH3 W OH 1-(m-Bromophenyl)-1-propanol Br CHœCHCH3 1-(m-Bromophenyl)propene (71%) H3C SO2Cl p-Toluenesulfonyl chloride Hydrogen chloride HCl Alkyl p-toluenesulfonate ROS CH3 O X X O Alcohol ROH Cycloheptanol OH Cycloheptyl p-toluenesulfonate (83%) OTs p-toluenesulfonyl chloride pyridine Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER FIFTEEN Alcohols, Diols, and Thiols 2RCH,OH→、 RCHOCHOR+HO Primary alcohol Dialkyl ether Water This kind of reaction is called a condensation a condensation is a reaction in which two molecules combine to form a larger one while liberating a small molecule. In this case two alcohol molecules combine to give an ether and water. 2CH3CH2CH2 CH2OH 0> CH3CH2CH2CH2OCH2CH2CH2CH3+H2O 1-Butanol Dibutyl ether(60 When applied to the synthesis of ethers, the reaction is effective only with primary alcohols. Elimination to form alkenes predominates with secondary and tertiary alcohols. Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140C. At higher temperatures elimination predominates, and ethylene is the hyl ether is outlined in Figure 15.2 Overall reaction 2CH, CHOH CH3CH,OCH- CH3 H2O Ethanol Diethyl et Water Step 1: Proton transfer from the acid catalyst to the oxygen of the alcohol to produce an alkyloxonium ion CH3CH,Q: H-OSO,OH CH3CH,C OSO,OH Ethyl alcohol Sulfuric acid Ethyloxonium ion Hydrogen sulfate ion Step 2: Nucleophilic attack by a molecule of alcohol on the alkyloxonium ion formed in step I H H CH3CH2Q CH2( CH3CH,OCH, CH3 :O: Ethyl alcohol Ethyloxonium ion Diethyloxonium ion Water Step 3: The product of step 2 is the conjugate acid of the dialkyl ether. It is deprotonated in the final step of the process to give the ether. CHCH,0+ Toso.oh at, CH3CH,OCH,CH HOSO,OH CH,CH Diethyloxonium Hydrogen sulfate ion Diethyl ether Sulfuric acid FIGURE 15.2 The mechanism of acid-catalyzed formation of diethyl ether from ethyl alcohol. As an alternative in the third step, the bronsted base that abstracts the proton could be a molecule of the starting alcohol Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
This kind of reaction is called a condensation. A condensation is a reaction in which two molecules combine to form a larger one while liberating a small molecule. In this case two alcohol molecules combine to give an ether and water. When applied to the synthesis of ethers, the reaction is effective only with primary alcohols. Elimination to form alkenes predominates with secondary and tertiary alcohols. Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C. At higher temperatures elimination predominates, and ethylene is the major product. A mechanism for the formation of diethyl ether is outlined in Figure 15.2. 2CH3CH2CH2CH2OH 1-Butanol CH3CH2CH2CH2OCH2CH2CH2CH3 Dibutyl ether (60%) H2O Water H2SO4 130°C 2RCH2OH Primary alcohol RCH2OCH2R Dialkyl ether H2O Water H, heat 592 CHAPTER FIFTEEN Alcohols, Diols, and Thiols CH3CH2O CH2±O ±£ CH3CH2OCH2CH3 O Overall Reaction: 2CH3CH2OH ±±£ CH3CH2OCH2CH3 H2O Step 1: Proton transfer from the acid catalyst to the oxygen of the alcohol to produce an alkyloxonium ion CH3CH2O H±OSO2OH ±£ CH3CH2O OSO2OH H Ethyl alcohol Sulfuric acid Ethyloxonium ion Hydrogen sulfate ion Step 2: Nucleophilic attack by a molecule of alcohol on the alkyloxonium ion formed in step 1 Ethyl alcohol CH3 H H Ethyloxonium ion Diethyloxonium ion Water Step 3: The product of step 2 is the conjugate acid of the dialkyl ether. It is deprotonated in the final step of the process to give the ether. CH3CH2O OSO2OH ±£ CH3CH2OCH2CH3 HOSO2OH Diethyloxonium ion Hydrogen sulfate ion Diethyl ether Sulfuric acid H2SO4 140C fast H H slow SN2 fast H H H H H CH2CH3 Ethanol Diethyl ether Water FIGURE 15.2 The mechanism of acid-catalyzed formation of diethyl ether from ethyl alcohol. As an alternative in the third step, the Brønsted base that abstracts the proton could be a molecule of the starting alcohol. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website