6.9 Addition of sulfuric Acid to alkenes Under free-radical conditions in the presence of peroxides, addition takes place with a regioselectivity opposite to that of Markovnikov's rule HaC CH3-C—CHCH HaC 2-Methyl-2-butene Hydrogen bromide 2-Bromo-3-methylbutane Although the possibility of having two different reaction paths available to an alkene and hydrogen bromide may seem like a complication, it can be an advantage in organic synthesis From a single alkene one may prepare either of two different alkyl bromides, with control of regioselectivity, simply by choosing reaction conditions that favor ionic addition or free-radical addition of hydrogen bromide 6.9 ADDITION OF SULFURIC ACID TO ALKENES Acids other than hydrogen halides also add to the carbon-carbon bond of alkenes. Con centrated sulfuric acid, for example, reacts with certain alkenes to form alkyl hydrogen +H-OSO2OH→H-C-C—0SO2OH Alkene Sulfuric acid hydrogen sulfate Notice in the following example that a proton adds to the carbon that has the greater number of hydrogens, and the hydrogen sulfate anion( OSO2OH) adds to the carbon that has the fewer hydrogens CH3CH=CH2+HOsO2OH— CH2CHCH OSO,OH Propene Sulfuric acid Isopropyl hydrogen sulfate Markovnikov's rule is obeyed because the mechanism of sulfuric acid addition to alkenes, illustrated for the case of propene in Figure 6.8, is analogous to that described earlier for the ionic addition of hydrogen halides Alkyl hydrogen sulfates can be converted to alcohols by heating them with water or steam. This is called a hydrolysis reaction, because a bond is cleaved by reaction with water.(The suffix-lysis indicates cleavage. It is the oxygen-sulfur bond that is broken when an alkyl hydrogen sulfate undergoes hydrolysis Cleavage occurs here during hydrolysis H—C—C—0+SO2OH+H2O→>HC-C—OH+HOSO2OH Alkyl hydrogen sulfate Alcohol Sulfuric acid Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Under free-radical conditions in the presence of peroxides, addition takes place with a regioselectivity opposite to that of Markovnikov’s rule. Although the possibility of having two different reaction paths available to an alkene and hydrogen bromide may seem like a complication, it can be an advantage in organic synthesis. From a single alkene one may prepare either of two different alkyl bromides, with control of regioselectivity, simply by choosing reaction conditions that favor ionic addition or free-radical addition of hydrogen bromide. 6.9 ADDITION OF SULFURIC ACID TO ALKENES Acids other than hydrogen halides also add to the carbon–carbon bond of alkenes. Concentrated sulfuric acid, for example, reacts with certain alkenes to form alkyl hydrogen sulfates. Notice in the following example that a proton adds to the carbon that has the greater number of hydrogens, and the hydrogen sulfate anion (OSO2OH) adds to the carbon that has the fewer hydrogens. Markovnikov’s rule is obeyed because the mechanism of sulfuric acid addition to alkenes, illustrated for the case of propene in Figure 6.8, is analogous to that described earlier for the ionic addition of hydrogen halides. Alkyl hydrogen sulfates can be converted to alcohols by heating them with water or steam. This is called a hydrolysis reaction, because a bond is cleaved by reaction with water. (The suffix -lysis indicates cleavage.) It is the oxygen–sulfur bond that is broken when an alkyl hydrogen sulfate undergoes hydrolysis. H2O Water H C C O SO2OH Alkyl hydrogen sulfate HOSO2OH Sulfuric acid H C C OH Alcohol heat Cleavage occurs here during hydrolysis Sulfuric acid HOSO2OH Isopropyl hydrogen sulfate CH3CHCH3 OSO2OH Propene CH3CH CH2 C C Alkene H OSO2OH Sulfuric acid H C C OSO2OH Alkyl hydrogen sulfate C H CH3 H3C H3C C 2-Methyl-2-butene HBr Hydrogen bromide CH3 CH3 H C Br CHCH3 2-Bromo-3-methylbutane peroxides 6.9 Addition of Sulfuric Acid to Alkenes 223 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
224 CHAPTER SIX Reactions of alkenes Addition reactions The overall reaction CH3CH=CH HOSO,OH (CH3)2CHOSO,OH Sulfuric acid The mechanism: Step 1: Protonation of the carbon-carbon double bond in the direction that leads to the more stable carbocation: CH CH-CH +HCOSO,OH CH-CHCH+ :OSO,OH Sulfuric acid Step 2: Carbocation-anion combination CH3CHCH3 :OSO,OH,CH3CHCH3 OSO,OH rogen sulfate FIGURE 6.8 Mechanism of addition of sulfuric acid to The combination of sulfuric acid addition to propene, followed by hydrolysis of the resulting isopropyl hydrogen sulfate, is the major method by which over 10 Ib of isopropyl alcohol is prepared each year in the United States CH3CH=CH2—3CH3CHCH3 Hy o,CHCHCH3 OSO,OH Isopropyl hydrogen sulfate We say that propene has undergone hydration. Overall, H and OH have added across the carbon-carbon double bond. In the same manner, cyclohexanol has been prepared by nore than hydration of cyclohexene OH thetic steps are indicated by dividual steps is essential so hat everything is added to Cyclohex Cyclohexanol (75%) PROBLEM 6.8 Write a structural formula for the compound formed on elec trophilic addition of sulfuric acid to cyclohexene (step 1 in the two-step transfor mation sho he preceding equation Hydration of alkenes by this method, however, is limited to monosubstituted alkenes and disubstituted alkenes of the type RCH=CH. Disubstituted alkenes of the Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
The combination of sulfuric acid addition to propene, followed by hydrolysis of the resulting isopropyl hydrogen sulfate, is the major method by which over 109 lb of isopropyl alcohol is prepared each year in the United States. We say that propene has undergone hydration. Overall, H and OH have added across the carbon–carbon double bond. In the same manner, cyclohexanol has been prepared by hydration of cyclohexene: PROBLEM 6.8 Write a structural formula for the compound formed on electrophilic addition of sulfuric acid to cyclohexene (step 1 in the two-step transformation shown in the preceding equation). Hydration of alkenes by this method, however, is limited to monosubstituted alkenes and disubstituted alkenes of the type RCHœCHR. Disubstituted alkenes of the OH Cyclohexene Cyclohexanol (75%) 1. H2SO4 2. H2O, heat Isopropyl hydrogen sulfate CH3CHCH3 OSO2OH Isopropyl alcohol CH3CHCH3 OH Propene CH3CH CH2 H2SO4 H2O heat 224 CHAPTER SIX Reactions of Alkenes: Addition Reactions It is convenient in synthetic transformations involving more than one step simply to list all the reagents with a single arrow. Individual synthetic steps are indicated by number. Numbering the individual steps is essential so as to avoid the implication that everything is added to the reaction mixture at the same time. The overall reaction: Propene Sulfuric acid Isopropyl hydrogen sulfate The mechanism: Step 1: Protonation of the carbon–carbon double bond in the direction that leads to the more stable carbocation: Propene Sulfuric acid slow Hydrogen sulfate ion Step 2: Carbocation–anion combination fast OSO2OH Isopropyl cation Isopropyl hydrogen sulfate Isopropyl cation Hydrogen sulfate ion CH3CHœCH2 HOSO2OH ±£ (CH3)2CHOSO2OH CH3CHœCH2 H±OSO2OH CH3CHCH3 OSO2OH CH3CHCH3 OSO2OH ±£ CH3CHCH3 FIGURE 6.8 Mechanism of addition of sulfuric acid to propene. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website