CHAPTER 6 REACTIONS OF ALKENES: ADDITION REACTIONS ow that were familiar with the structure and preparation of alkenes, let's look at their chemical reactions. The characteristic reaction of alkenes is addition to the double bond according to the general equation A The range of compounds represented as a-B in this equation is quite large, and their variety offers a wealth of opportunity for converting alkenes to a number of other func- tional group types Alkenes are commonly described as unsaturated hydrocarbons because they have the capacity to react with substances which add to them. Alkanes, on the other hand, are said to be saturated hydrocarbons and are incapable of undergoing addition reactions 6.1 HYDROGENATION OF ALKENES The relationship between reactants and products in addition reactions can be illustrated y the hydrogenation of alkenes to yield alkanes. Hydrogenation is the addition of H to a multiple bond. An example is the reaction of hydrogen with ethylene to form ethane H HH +H一H鸟哑 H△H=-136kJ H H (-32.6kcal) HH Ethylene Hydrogen Ethane 208 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
208 CHAPTER 6 REACTIONS OF ALKENES: ADDITION REACTIONS Now that we’re familiar with the structure and preparation of alkenes, let’s look at their chemical reactions. The characteristic reaction of alkenes is addition to the double bond according to the general equation: The range of compounds represented as A±B in this equation is quite large, and their variety offers a wealth of opportunity for converting alkenes to a number of other functional group types. Alkenes are commonly described as unsaturated hydrocarbons because they have the capacity to react with substances which add to them. Alkanes, on the other hand, are said to be saturated hydrocarbons and are incapable of undergoing addition reactions. 6.1 HYDROGENATION OF ALKENES The relationship between reactants and products in addition reactions can be illustrated by the hydrogenation of alkenes to yield alkanes. Hydrogenation is the addition of H2 to a multiple bond. An example is the reaction of hydrogen with ethylene to form ethane. Pt, Pd, Ni, or Rh H° 136 kJ (32.6 kcal) H H H H H C C H Ethane H H Hydrogen C H H H H C Ethylene A B C C A C C B Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
Heats of Hydrogenation The bonds in the product are stronger than the bonds in the reactants; two C-H o bonds of an alkane are formed at the expense of the H-H o bond and the T component of the alkene s double bond. The overall reaction is exothermic, and the heat evolved on hydrogenation of one mole of an alkene is its heat of hydrogenation Heat of hydro- genation is a positive quantity equal to-AHo for the reaction tain finely divided metal catalysts. Platinum is the hydrogenation catalyst most often used, although palladium, nickel, and rhodium are also effective. Metal-catalyzed addi- The french chemist paul tion of hydrogen is normally rapid at room temperature, and the alkane is produced in Sabatier received the 1912 high yield, usually as the only product. his discovery that finely di- vided nickel is an effective CH3)2C=CHCH3+ H (CH3)2CHCH,CH3 hydrogenation catalyst. 2-Methyl-2-butene Hydroger 2-Methylbutane(100%) CH3 H3C 5.5-Dimethyl( methylene)cyclononane Hydrogen 1, 1, 5-Trimethylcyclononane(73%) PROBLEM 6.1 What three alkenes yield 2-methylbutane on catalytic hydro- genation? The solvent used in catalytic hydrogenation is chosen for its ability to dissolve the alkene and is typically ethanol, hexane, or acetic acid. The metal catalysts are insoluble in these solvents(or, indeed, in any solvent). Two phases, the solution and the metal, are present, and the reaction takes place at the interface between them. Reactions involving a substance in one phase with a different substance in a second phase are called eterogeneous reactions. Catalytic hydrogenation of an alkene is believed to proceed by the series of steps shown in Figure 6.1. As already noted, addition of hydrogen to the alkene is very slow in the absence of a metal catalyst, meaning that any uncatalyzed mechanism must have a very high activation energy. The metal catalyst accelerates the rate of hydrogenation by providing an alternative pathway that involves a sequence of several low activation energy steps 6.2 HEATS OF HYDROGENATION Heats of hydrogenation are used to compare the relative stabilities of alkenes in much Remember that a ca the same way as heats of combustion. Both methods measure the differences in the fe rects the rate of a reaction energy of isomers by converting them to a product or products common to all. Catalytic but not the energy relation hydrogenation of 1-butene, cis-2-butene, or trans-2-butene yields the same product- oducts Thus. the heat of butane. As Figure 6.2 shows, the measured heats of hydrogenation reveal that trans-2- hydrogenation of a particu- butene is 4 J/mol (1.0 kcal/mol) lower in energy than cis-2-butene and that cis-2-butene lar alkene is the same irre is 7 k/mol(1.7 kcal/mol) lower in energy than 1-butene spective of what catalyst is Heats of hydrogenation can be used to estimate the stability of double bonds as structural units, even in alkenes that are not isomers. Table 6. 1 lists the heats of hydro- genation for a representative collection of alkenes Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
The bonds in the product are stronger than the bonds in the reactants; two C±H bonds of an alkane are formed at the expense of the H±H bond and the component of the alkene’s double bond. The overall reaction is exothermic, and the heat evolved on hydrogenation of one mole of an alkene is its heat of hydrogenation. Heat of hydrogenation is a positive quantity equal to H° for the reaction. The uncatalyzed addition of hydrogen to an alkene, although exothermic, is very slow. The rate of hydrogenation increases dramatically, however, in the presence of certain finely divided metal catalysts. Platinum is the hydrogenation catalyst most often used, although palladium, nickel, and rhodium are also effective. Metal-catalyzed addition of hydrogen is normally rapid at room temperature, and the alkane is produced in high yield, usually as the only product. PROBLEM 6.1 What three alkenes yield 2-methylbutane on catalytic hydrogenation? The solvent used in catalytic hydrogenation is chosen for its ability to dissolve the alkene and is typically ethanol, hexane, or acetic acid. The metal catalysts are insoluble in these solvents (or, indeed, in any solvent). Two phases, the solution and the metal, are present, and the reaction takes place at the interface between them. Reactions involving a substance in one phase with a different substance in a second phase are called heterogeneous reactions. Catalytic hydrogenation of an alkene is believed to proceed by the series of steps shown in Figure 6.1. As already noted, addition of hydrogen to the alkene is very slow in the absence of a metal catalyst, meaning that any uncatalyzed mechanism must have a very high activation energy. The metal catalyst accelerates the rate of hydrogenation by providing an alternative pathway that involves a sequence of several low activation energy steps. 6.2 HEATS OF HYDROGENATION Heats of hydrogenation are used to compare the relative stabilities of alkenes in much the same way as heats of combustion. Both methods measure the differences in the energy of isomers by converting them to a product or products common to all. Catalytic hydrogenation of 1-butene, cis-2-butene, or trans-2-butene yields the same product— butane. As Figure 6.2 shows, the measured heats of hydrogenation reveal that trans-2- butene is 4 kJ/mol (1.0 kcal/mol) lower in energy than cis-2-butene and that cis-2-butene is 7 kJ/mol (1.7 kcal/mol) lower in energy than 1-butene. Heats of hydrogenation can be used to estimate the stability of double bonds as structural units, even in alkenes that are not isomers. Table 6.1 lists the heats of hydrogenation for a representative collection of alkenes. (CH3)2C CHCH3 2-Methyl-2-butene H2 Hydrogen (CH3)2CHCH2CH3 2-Methylbutane (100%) Pt Pt CH3 H3C CH3 H 1,1,5-Trimethylcyclononane (73%) H2 Hydrogen CH3 H3C CH2 5,5-Dimethyl(methylene)cyclononane 6.2 Heats of Hydrogenation 209 The French chemist Paul Sabatier received the 1912 Nobel Prize in chemistry for his discovery that finely divided nickel is an effective hydrogenation catalyst. Remember that a catalyst affects the rate of a reaction but not the energy relationships between reactants and products. Thus, the heat of hydrogenation of a particular alkene is the same irrespective of what catalyst is used. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
210 CHAPTER SIX Reactions of alkenes Addition reactions FIGURE 6.1 A mechanisn Step 1: Hydrogen molecules react with Step 2: The alkene reacts with the metal for heterogeneous catalysis metal atoms at the catalyst surface n the hydrogenation of bond between the two carbons is replaced alkenes bond is broken and replaced by two weak by two relatively weak carbon-metal o bonds. 88 Step 3: A hy inferred from the catalyst surface carbons of the double bond 。e。 8 FIGURE 6.2 Heats plotted on a comme All energies are in m HC CH3 H3C H CH,=CHCH, CH3 1-Butene cis-2-Butene trans-2-Butene 119 115 △H° △HP △B° CHCH,CH,CH3 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
210 CHAPTER SIX Reactions of Alkenes: Addition Reactions Step 1: Hydrogen molecules react with metal atoms at the catalyst surface. The relatively strong hydrogen–hydrogen σ bond is broken and replaced by two weak metal–hydrogen bonds. Step 2: The alkene reacts with the metal catalyst. The π component of the double bond between the two carbons is replaced by two relatively weak carbon–metal σ bonds. Step 3: A hydrogen atom is transferred from the catalyst surface to one of the carbons of the double bond. Step 4: The second hydrogen atom is transferred, forming the alkane. The sites on the catalyst surface at which the reaction occurred are free to accept additional hydrogen and alkene molecules. FIGURE 6.1 A mechanism for heterogeneous catalysis in the hydrogenation of alkenes. 1-Butene cis-2-Butene trans-2-Butene Potential energy Alkene CH3CH2CH2CH3 CH2 CHCH2CH3 H3C CH3 C C H H 126 7 119 4 115 H2 H3C C C H CH3 H ∆H ∆H ∆H FIGURE 6.2 Heats of hydrogenation of butene isomers plotted on a common scale. All energies are in kilojoules per mole. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
6.2 Heats of Hydrogenation TABLE 6.1 Heats of Hydrogenation of Some Alkenes Heat of hydrogenation Alkene Structure k/mol kcal/mol Ethylene Monosubstituted alkenes Propene H=CHCH 299 1-Butene CH2-CHCH2 CH 1-Hexene CH2-CHCH? CH2 CH2 CH3 30.2 Cis-disubstituted alkenes H3C Cis-2-Butene 119 Cis-2-Pentene Trans-disubstituted alkenes H3C trans-2-Butene 27.4 CH rans-2-Pentene C=C 27.2 CH2 CH3 Trisubstituted alkenes 2-Methyl-2-pentene (CH3)2C-CHCH2 CH3 26.7 Tetrasubstituted alkenes 2, 3-Dimethyl-2-butene (CH3)2C-C(CH3)2 exact the pattern of alkene stability determined from heats of hydrogenation parallels tly the pattern deduced from heats of combustion. Decreasing heat of hydrogenation and increasing stability of the double bond CH2-CH2 RCHECH RCH=CHR R2C-CHR R2C=CR2 Ethylene Disubstituted Trisubstituted Tetrasubstituted Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
The pattern of alkene stability determined from heats of hydrogenation parallels exactly the pattern deduced from heats of combustion. Decreasing heat of hydrogenation and increasing stability of the double bond R2C CR2 Tetrasubstituted R2C CHR Trisubstituted RCH CHR Disubstituted RCH CH2 Monosubstituted CH2 CH2 Ethylene 6.2 Heats of Hydrogenation 211 TABLE 6.1 Heats of Hydrogenation of Some Alkenes Heat of hydrogenation kcal/mol 29.9 30.1 30.2 28.4 32.6 27.4 27.2 26.7 26.4 28.1 kJ/mol 125 126 126 119 117 136 115 114 112 110 Alkene Propene 1-Butene 1-Hexene cis-2-Butene Monosubstituted alkenes Cis-disubstituted alkenes trans-2-Butene trans-2-Pentene Trans-disubstituted alkenes 2-Methyl-2-pentene Trisubstituted alkenes cis-2-Pentene 2,3-Dimethyl-2-butene Tetrasubstituted alkenes Ethylene Structure CH2 CH2 CH2 CHCH3 (CH3)2C CHCH2CH3 (CH3)2C C(CH3)2 CH2 CHCH2CH3 CH2 CHCH2CH2CH2CH3 H3C C CH3 H H C H3C C CH2CH3 H H C H3C C H H CH3 C H3C C H H CH2CH3 C Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER SIX Reactions of alkenes Addition reactions Ethylene, which has no substituents to stabilize its double bond, has the highest heat of hydrogenation hes that are similar in structure to one another have similar heats of hydrogenation. For example, the heats of hydrogenation of the monosubstituted (terminal) alkenes propene, 1-butene, and 1-hexene are almost identical. Cis-disubsti tuted alkenes have lower heats of hydrogenation than monosubstituted alkenes but higher heats of hydrogenation than their more stable trans stereoisomers. Alkenes with trisub- stituted double bonds have lower heats of hydrogenation than disubstituted alkenes, and tetrasubstituted alkenes have the lowest heats of hydrogenation. PROBLEM 6.2 Match each alkene of problem 6. 1 with its correct heat of hydro- genation. Heats of hydrogenation in kJlmol(kcal/mol): 112(26.7):118(28.2): 126(30.2) 6.3 STEREOCHEMISTRY OF ALKENE HYDROGENATION In the mechanism for alkene hydrogenation shown in Figure 6.1, hydrogen atoms are transferred from the catalyst's surface to the alkene. Although the two hydrogens are not transferred simultaneously, it happens that both add to the same face of the double bond, as the following example illustrates H +H2 H CO, CH3 I cyclohexene-1, 2-dicarboxylate The term syn addition describes the stereochemistry of reactions such as catalytic hydro- genation in which two atoms or groups add to the same face of a double bond. When atoms or groups add to opposite faces of the double bond, the process is called anti addition A second stereochemical aspect of alkene hydrogenation concerns its stereoselec- tivity. A reaction in which a single starting material can give two or more stereoisomeric products but yields one of them in greater amounts than the other(or even to the exclu- elimination reactions(Sec sion of the other) is said to be stereoselective. The catalytic hydrogenation of a-pinene (a constituent of turpentine) is an example of a stereoselective reaction. Syn addition of Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Ethylene, which has no alkyl substituents to stabilize its double bond, has the highest heat of hydrogenation. Alkenes that are similar in structure to one another have similar heats of hydrogenation. For example, the heats of hydrogenation of the monosubstituted (terminal) alkenes propene, 1-butene, and 1-hexene are almost identical. Cis- disubstituted alkenes have lower heats of hydrogenation than monosubstituted alkenes but higher heats of hydrogenation than their more stable trans stereoisomers. Alkenes with trisubstituted double bonds have lower heats of hydrogenation than disubstituted alkenes, and tetrasubstituted alkenes have the lowest heats of hydrogenation. PROBLEM 6.2 Match each alkene of Problem 6.1 with its correct heat of hydrogenation. Heats of hydrogenation in kJ/mol (kcal/mol): 112 (26.7); 118 (28.2); 126 (30.2) 6.3 STEREOCHEMISTRY OF ALKENE HYDROGENATION In the mechanism for alkene hydrogenation shown in Figure 6.1, hydrogen atoms are transferred from the catalyst’s surface to the alkene. Although the two hydrogens are not transferred simultaneously, it happens that both add to the same face of the double bond, as the following example illustrates. The term syn addition describes the stereochemistry of reactions such as catalytic hydrogenation in which two atoms or groups add to the same face of a double bond. When atoms or groups add to opposite faces of the double bond, the process is called anti addition. A second stereochemical aspect of alkene hydrogenation concerns its stereoselectivity. A reaction in which a single starting material can give two or more stereoisomeric products but yields one of them in greater amounts than the other (or even to the exclusion of the other) is said to be stereoselective. The catalytic hydrogenation of -pinene (a constituent of turpentine) is an example of a stereoselective reaction. Syn addition of syn addition anti addition Pt CO2CH3 CO2CH3 Dimethyl cyclohexene-1,2-dicarboxylate CO2CH3 CO2CH3 H H Dimethyl cyclohexane-cis-1,2-dicarboxylate (100%) H2 212 CHAPTER SIX Reactions of Alkenes: Addition Reactions Stereoselectivity was defined and introduced in connection with the formation of stereoisomeric alkenes in elimination reactions (Section 5.11). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website