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
6.4 Electrophilic Addition of Hydrogen Halides to Alkenes 213 This methyl group block the double bond to th IGURE 6.3 The catalyst surface sa methyl group that lies over he double bond of a-pinene e surface of the catalyst. Hydrogenation of a-pinene he bottom face of the dou Hydrogen is transferred from yst surface to the bottom he double bond--this is hindered side hydrogen can in principle lead to either cis-pinane or trans-pinane, depending on which face of the double bond accepts the hydrogen atoms(shown in red in the equation) CH3 CH cis-Pinane ch are common na tionship be tween the pair of methyl groups on the bridge and the third methyl group a-Pinene cis-Pinane rans-Pinane ( not formed) product obtained is cis-pinane. None of the stereoisomeric trans-pinane is forme only In practice, hydrogenation of a-pinene is observed to be 100% stereoselective. The The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface. As the molecular model in Figure 6.3 shows, one of the methyl group on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst. The bottom face of the double bond is more exposed, and both hydrogens are transferred from the catalyst surface to that face. Reactions such as catalytic hydrogenation that take place at the"less hindered side of a reactant are common in organic chemistry and are examples of steric effects on reactivity. We have previously seen steric effects on structure and stability in the case of cis and trans stereoisomers and in the preference for equatorial substituents on cyclo- hexane rings 6.4 ELECTROPHILIC ADDITION OF HYDROGEN HALIDES TO ALKENES In many addition reactions the attacking reagent, unlike H2, is a polar molecule. Hydro- gen halides are among the simplest examples of polar substances that add to alkenes Alkene Alkyl halid Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
hydrogen can in principle lead to either cis-pinane or trans-pinane, depending on which face of the double bond accepts the hydrogen atoms (shown in red in the equation). In practice, hydrogenation of -pinene is observed to be 100% stereoselective. The only product obtained is cis-pinane. None of the stereoisomeric trans-pinane is formed. The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface. As the molecular model in Figure 6.3 shows, one of the methyl groups on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst. The bottom face of the double bond is more exposed, and both hydrogens are transferred from the catalyst surface to that face. Reactions such as catalytic hydrogenation that take place at the “less hindered” side of a reactant are common in organic chemistry and are examples of steric effects on reactivity. We have previously seen steric effects on structure and stability in the case of cis and trans stereoisomers and in the preference for equatorial substituents on cyclohexane rings. 6.4 ELECTROPHILIC ADDITION OF HYDROGEN HALIDES TO ALKENES In many addition reactions the attacking reagent, unlike H2, is a polar molecule. Hydrogen halides are among the simplest examples of polar substances that add to alkenes. H C C X Alkyl halide H X Hydrogen halide C C Alkene H2 Ni H H H CH3 CH3 CH3 trans-Pinane (not formed) CH3 H H H H CH3 CH3 cis-Pinane (only product) CH3 H CH3 CH3 -Pinene 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes 213 This methyl group blocks approach of top face of the double bond to the catalyst surface Hydrogen is transferred from the catalyst surface to the bottom face of the double bond—this is the “less hindered side” FIGURE 6.3 The methyl group that lies over the double bond of -pinene shields one face of it, preventing a close approach to the surface of the catalyst. Hydrogenation of -pinene occurs preferentially from the bottom face of the double bond. cis-Pinane and trans-pinane are common names that denote the relationship between the pair of methyl groups on the bridge and the third methyl group. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
214 CHAPTER SIX Reactions of alkenes Addition reactions Addition occurs rapidly in a variety of solvents, including pentane, benzene, dichloro- CH3CH2 CH,CH HBr CH3CH, CH,CHCH, CH cis-3-Hexene Hydrogen bromide 3-Bromohexane(76%) The reactivity of the hydrogen halides reflects their ability to donate a proton. Hydrogen iodide is the strongest acid of the hydrogen halides and reacts with alkenes at the fastest rate Increasing reactivity of hydrogen halides n addition to alkenes HF < HCI< HBr< hI Slowest rate of addition Fastest rate of addition least acidic most acidic We can gain a general understanding of the mechanism of hydrogen halide addi tion to alkenes by extending some of the principles of reaction mechanisms introduced earlier. In Section 5. 12 we pointed out that carbocations are the conjugate acids of alkenes. Acid-base reactions are reversible processes. An alkene, therefore, can accept a proton from a hydrogen halide to form a carbocation H R2C=CR2+“H-x:、R2C一CR2 Alkene Anion (conjugate acid) (conjugate base) Figure 6. 4 shows the complementary nature of the electrostatic potentials of an alkene and a hydrogen halide. We've also seen( Section 4.9) that carbocations, when generated in the presence of halide anions, react with them to form alkyl halides H H RC—CR, R,C—CR Carbocation(electrophile) Halide ion(nucleophile) Alkyl halide Both steps in this general mechanism are based on precedent. It is called FIGURE 6.4 Electro electrophilic addition because the reaction is triggered by the attack of an electrophile static potential maps of HCl (an acid)on the T electrons of the double bond. Using the two TT electrons to form a and ethylene. When the two bond to an electrophile generates a carbocation as a reactive intermediate; normally this tween the electron-rich site is the rate-determining step (red)of ethylene and the electron-poor region(blue) 6.5 REGIOSELECTIVITY OF HYDROGEN HALIDE ADDITION gion of ethylene is associated MARKOVNIKOV'S RULE with the I electrons of the In principle a hydrogen halide can add to an unsymmetrical alkene(an alkene in which double bond, while h is the electron-poor atom(blue)of the two carbons of the double bond are not equivalently substituted)in either of two direc- HCI tions. In practice, addition is so highly regioselective as to be considered regiospecific Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Addition occurs rapidly in a variety of solvents, including pentane, benzene, dichloromethane, chloroform, and acetic acid. The reactivity of the hydrogen halides reflects their ability to donate a proton. Hydrogen iodide is the strongest acid of the hydrogen halides and reacts with alkenes at the fastest rate. We can gain a general understanding of the mechanism of hydrogen halide addition to alkenes by extending some of the principles of reaction mechanisms introduced earlier. In Section 5.12 we pointed out that carbocations are the conjugate acids of alkenes. Acid–base reactions are reversible processes. An alkene, therefore, can accept a proton from a hydrogen halide to form a carbocation. Figure 6.4 shows the complementary nature of the electrostatic potentials of an alkene and a hydrogen halide. We’ve also seen (Section 4.9) that carbocations, when generated in the presence of halide anions, react with them to form alkyl halides. Both steps in this general mechanism are based on precedent. It is called electrophilic addition because the reaction is triggered by the attack of an electrophile (an acid) on the electrons of the double bond. Using the two electrons to form a bond to an electrophile generates a carbocation as a reactive intermediate; normally this is the rate-determining step. 6.5 REGIOSELECTIVITY OF HYDROGEN HALIDE ADDITION: MARKOVNIKOV’S RULE In principle a hydrogen halide can add to an unsymmetrical alkene (an alkene in which the two carbons of the double bond are not equivalently substituted) in either of two directions. In practice, addition is so highly regioselective as to be considered regiospecific. X Halide ion (nucleophile) R2C CR2 H Carbocation (electrophile) Alkyl halide R2C X CR2 H X Anion (conjugate base) R2C CR2 H Carbocation (conjugate acid) H X Hydrogen halide (acid) R2C CR2 Alkene (base) Increasing reactivity of hydrogen halides in addition to alkenes HF HCl HBr HI Fastest rate of addition; most acidic Slowest rate of addition; least acidic C CH3CH2 H H C CH2CH3 cis-3-Hexene HBr Hydrogen bromide 3-Bromohexane (76%) CH3CH2CH2CHCH2CH3 Br 30°C CHCl3 214 CHAPTER SIX Reactions of Alkenes: Addition Reactions FIGURE 6.4 Electrostatic potential maps of HCl and ethylene. When the two react, the interaction is between the electron-rich site (red) of ethylene and the electron-poor region (blue) of HCl. The electron-rich region of ethylene is associated with the π electrons of the double bond, while H is the electron-poor atom (blue) of HCl. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
6.5 Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule RCH=CH+H一X—> RCH-CH, rather than RCh-CH H X R2C=CH2+H一x→>R2C一CH2 rather than r2C一CH R,C=CHR +H-X->R,C-CHR rather than R,C-CHR In 1870, Vladimir Markovnikov, a colleague of Alexander Zaitsev at the univer- sity of Kazan, noticed a pattern in the hydrogen halide addition to alkenes and assem- 1988 issue of the Journal of bled his observations into a simple statement. Markovnikov's rule states that when an Chemical Education trace velopment of unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to Markoynikoy's rule In that the carbon that has the greater number of hydrogen substituents, and the halogen adds article Markovnikov's name to the carbon having fewer hydrogen substituents. The preceding general equations illus- is spelled Markownikoft, trate regioselective addition according to Markovnikov's rule, and the equations that fol- which is the way it appeared low provide some examples CH3 CH,CH-CH2 HBr >CHaCH,CHCH 1-Butene Hydrogen bromide 2-Bromobutane(80%) CH2-C-B C CH3 2-Methylpropene Hydrogen bromide 2-Bromo-2-methylpropane (90%0) CH HCI 1-Methylcyclopentene Hydrogen chloride 1-Chloro-1-methylcyclopentane (100%0) PROBLEM 6.3 Write the structure of the major organic product formed in the reaction of hydrogen chloride with each of the following (a)2-Methyl-2-butene (c cis-2-Butene (b)2-Methyl-1-butene CH3CH SAMPLE SOLUTION (a)Hydrogen chloride adds to the double bond of 2 methyl-2-butene in accordance with Markovnikov's rule. The proton adds to the carbon that has one attached hydrogen, chlorine to the carbon that has none Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
In 1870, Vladimir Markovnikov, a colleague of Alexander Zaitsev at the University of Kazan, noticed a pattern in the hydrogen halide addition to alkenes and assembled his observations into a simple statement. Markovnikov’s rule states that when an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon having fewer hydrogen substituents. The preceding general equations illustrate regioselective addition according to Markovnikov’s rule, and the equations that follow provide some examples. PROBLEM 6.3 Write the structure of the major organic product formed in the reaction of hydrogen chloride with each of the following: (a) 2-Methyl-2-butene (c) cis-2-Butene (b) 2-Methyl-1-butene (d) SAMPLE SOLUTION (a) Hydrogen chloride adds to the double bond of 2- methyl-2-butene in accordance with Markovnikov’s rule. The proton adds to the carbon that has one attached hydrogen, chlorine to the carbon that has none. CH3CH CH3 1-Methylcyclopentene HCl Hydrogen chloride CH3 Cl 1-Chloro-1-methylcyclopentane (100%) 0°C C H3C H3C CH2 2-Methylpropene HBr Hydrogen bromide CH3 C Br CH3 CH3 2-Bromo-2-methylpropane (90%) acetic acid Hydrogen bromide HBr 2-Bromobutane (80%) CH3CH2CHCH3 Br 1-Butene CH3CH2CH CH2 acetic acid RCHœCH2 H±X RCH±CH2 W X W H RCH±CH2 W H W X rather than R2CœCH2 H±X R2C±CH2 W X W H R2C±CH2 W H W X rather than R2CœCHR H ±X R2C±CHR W X W H R2C±CHR W H W X rather than 6.5 Regioselectivity of Hydrogen Halide Addition: Markovnikov’s Rule 215 An article in the December 1988 issue of the Journal of Chemical Education traces the historical development of Markovnikov’s rule. In that article Markovnikov’s name is spelled Markownikoff, which is the way it appeared in his original paper written in German. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
216 CHAPTER SIX Reactions of alkenes Addition reactions H3C HaC 2-Methyl-2-butene Chlorine becomes attached Hydrogen becomes attached CH CH3—C—CH2CH3 2-Chloro-2-methylbutane (major product from Markovnikov addition of hydrogen chloride to 2-methyl-2-butene) when we examine the mechanism of electrophilic addition in more detal wort? Markovnikov's rule, like Zaitsev's, organizes experimental observations in a form 6.6 MECHANISTIC BASIS FOR MARKOVNIKOV'S RULE Let's compare the carbocation intermediates for addition of a hydrogen halide(hx)to an unsymmetrical alkene of the type RCH=CH2(a) according to Markovnikov's rule and(b)opposite to Markovnikov's rule (a) Addition according to Markovnikov's rule: RCH=CHa RCH—CH, → RCHCH Secondary Halide Observed product (b) Addition opposite to Markovnikov 's rule: RCH=CH2→>RCH—CH2+:X RCH,CH X-H Halide The transition state for protonation of the double bond has much of the character of a carbocation, and the activation energy for formation of the more stable carbocation (secondary)is less than that for formation of the less stable(primary)one. Figure 6.5 uses a potential energy diagram to illustrate these two competing modes of addition. Both car- bocations are rapidly captured by X to give an alkyl halide, with the major product derived from the carbocation that is formed faster. The energy difference between a pri mary carbocation and a secondary carbocation is so great and their rates of formation are so different that essentially all the product is derived from the secondary carbocation Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Markovnikov’s rule, like Zaitsev’s, organizes experimental observations in a form suitable for predicting the major product of a reaction. The reasons why it works appear when we examine the mechanism of electrophilic addition in more detail. 6.6 MECHANISTIC BASIS FOR MARKOVNIKOV’S RULE Let’s compare the carbocation intermediates for addition of a hydrogen halide (HX) to an unsymmetrical alkene of the type RCHœCH2 (a) according to Markovnikov’s rule and (b) opposite to Markovnikov’s rule. (a) Addition according to Markovnikov’s rule: (b) Addition opposite to Markovnikov’s rule: The transition state for protonation of the double bond has much of the character of a carbocation, and the activation energy for formation of the more stable carbocation (secondary) is less than that for formation of the less stable (primary) one. Figure 6.5 uses a potential energy diagram to illustrate these two competing modes of addition. Both carbocations are rapidly captured by X to give an alkyl halide, with the major product derived from the carbocation that is formed faster. The energy difference between a primary carbocation and a secondary carbocation is so great and their rates of formation are so different that essentially all the product is derived from the secondary carbocation. X Halide ion RCH CH2 H Primary carbocation RCH2CH2 X Not formed X H RCH CH2 X Halide ion RCH CH2 H Secondary carbocation H X RCH CH2 RCHCH3 X Observed product 2-Methyl-2-butene C H3C H3C CH3 H C (major product from Markovnikov addition of hydrogen chloride to 2-methyl-2-butene) 2-Chloro-2-methylbutane CH3 CH3 Cl C CH2CH3 Hydrogen becomes attached to this carbon Chlorine becomes attached to this carbon 216 CHAPTER SIX Reactions of Alkenes: Addition Reactions Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
6.6 Mechanistic Basis for Markovnikov's Rule RULES, LAWS THEORIES, AND THE SCIENTIFIC METHOD s we have just seen, Markovnikov's rule can be Most observations in chemistry come from ex expressed in two ways periments. If we do enough experiments we may see a pattern running through our observations. a law is a mathematical (the law of gravity)or verbal (the lay 1. When a hydrogen halide adds to an alkene, hy- of diminishing returns)description of that pattern. has the greater number of hydrogens attached Establishing a law can lead to the framing of a rule that lets us predict the results of future experiments to it, and the halogen to the carbon that has This is what the 1870 version of Markoynikoy's rule is the fewer hydrogens. a statement based on experimental observations that When a hydrogen halide adds to an alkene, pro- has predictive value tonation of the double bond occurs in the direc a theory is our best present interpretation of tion that gives the more stable carbocation why things happen the way they do. The modern ver sion of markovnikov's rule, which is based on mecha Vladimir Markovnikov expressed it in 1870; the sec- nistic reasoning and carbocation stability, recasts the ond is the way we usually phrase it now These two rule in terms of theoretical ideas. Mechanisms, and planations grounded in them, belong to the theory statements differ in an important way-a way that is part of the scientific method related to the scientific method Adherence to the scientific method is what It is worth remembering that a theory can defines science. The scientific method has four major never be proven correct. It can only be proven incor- elements: observation, law, theory, and hypothesis. rect, incomplete, or inadequate. Thus, theories are always being tested and refined. As important as anything else in the scientific method is the testable hypothesis. Once a theory is proposed, experiments Observation Lav are designed to test its validity. If the results are con- sistent with the theory, our belief in its soundness is strengthened. If the results conflict with it, the theory is flawed and must be modified section 6.7 describes some observations that support the theory that car Theory ocations are intermediates in the addition of hydro- gen halides to alkenes FIGURE 6.5 Energy diagra comparing addition of RCH,CH hydrogen halide to an alkene according to Markovnikov's rule with addition in the Markovnikov,s rule RCHCH3 alkene and hydrogen halide are shown in the center of X the diagram. The lower RCH=CH ponds to markovnikov rule proceeds to the right and shown in red; the higher energy pathway proceeds to RCHCHOX RCHCH Reaction coordinate Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
RULES, LAWS, THEORIES, AND THE SCIENTIFIC METHOD As we have just seen, Markovnikov’s rule can be expressed in two ways: 1. When a hydrogen halide adds to an alkene, hydrogen adds to the carbon of the alkene that has the greater number of hydrogens attached to it, and the halogen to the carbon that has the fewer hydrogens. 2. When a hydrogen halide adds to an alkene, protonation of the double bond occurs in the direction that gives the more stable carbocation. The first of these statements is close to the way Vladimir Markovnikov expressed it in 1870; the second is the way we usually phrase it now. These two statements differ in an important way—a way that is related to the scientific method. Adherence to the scientific method is what defines science. The scientific method has four major elements: observation, law, theory, and hypothesis. Most observations in chemistry come from experiments. If we do enough experiments we may see a pattern running through our observations. A law is a mathematical (the law of gravity) or verbal (the law of diminishing returns) description of that pattern. Establishing a law can lead to the framing of a rule that lets us predict the results of future experiments. This is what the 1870 version of Markovnikov’s rule is: a statement based on experimental observations that has predictive value. A theory is our best present interpretation of why things happen the way they do. The modern version of Markovnikov’s rule, which is based on mechanistic reasoning and carbocation stability, recasts the rule in terms of theoretical ideas. Mechanisms, and explanations grounded in them, belong to the theory part of the scientific method. It is worth remembering that a theory can never be proven correct. It can only be proven incorrect, incomplete, or inadequate. Thus, theories are always being tested and refined. As important as anything else in the scientific method is the testable hypothesis. Once a theory is proposed, experiments are designed to test its validity. If the results are consistent with the theory, our belief in its soundness is strengthened. If the results conflict with it, the theory is flawed and must be modified. Section 6.7 describes some observations that support the theory that carbocations are intermediates in the addition of hydrogen halides to alkenes. 6.6 Mechanistic Basis for Markovnikov’s Rule 217 RCH2CH2 + RCH2CH2X RCH CH2 RCHCH3 X− RCHCH3 + X− X + HX Reaction coordinate Potential energy FIGURE 6.5 Energy diagram comparing addition of a hydrogen halide to an alkene according to Markovnikov’s rule with addition in the direction opposite to Markovnikov’s rule. The alkene and hydrogen halide are shown in the center of the diagram. The lower energy pathway that corresponds to Markovnikov’s rule proceeds to the right and is shown in red; the higher energy pathway proceeds to the left and is shown in blue. Observation Law Hypothesis Theory Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
