218 CHAPTER SIX Reactions of alkenes Addition reactions FIGURE 6.6 Electron flow and orbital interaction in the transfer of a oroton from a hydrogen halide to an alkene of the type Hybridization CH2=CHR of carbon from sp- sp--hybridized carbon carbon a) The hydrogen (b) Electrons flow from the i orbital of the alkene CH2=CHR) appre other TI to the hydrogen halide. The T electrons flow in th he hydrogen the site of irection that generates a partial positive charge on attack is the orbital ng the o electrons of the the carbon atom that bears the electron-releasing alkyl double bond group(R). The hydrogen-halogen bond is partially broken and a C-Ho bond is partially formed at the transition state Positively charged arbon is sp-hybridized g bond: carbon is (c) Loss of the halide ion (X )from the halide and c-hg bond formation cor formation of the more stable carbocation Figure 6.6 focuses on the orbitals involved and shows how the T electrons of the double bond flow in the direction that generates the more stable of the two possible car- PROBLEM 6.4 Give a structural formula for the carbocation intermediate that leads to the major product in each of the reactions of Problem 6.3(Section 6.5) SAMPLE SOLUTION (a) Protonation of the double bond of 2-methyl-2-butene can give a tertiary carbocation or a secondary carbocation Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
C C C C H H H H H H R H H H H H H R R sp2 -hybridized carbon Positively charged carbon is sp2 -hybridized sp2 -hybridized carbon sp2 -hybridized carbon Hybridization of carbon changing from sp2 to sp3 A carbon–hydrogen σ bond; carbon is sp3 -hybridized (a) The hydrogen halide (HX) and the alkene (CH2œCHR) approach each other. The electrophile is the hydrogen halide, and the site of electrophilic attack is the orbital containing the σ electrons of the double bond. (b) Electrons flow from the π orbital of the alkene to the hydrogen halide. The π electrons flow in the direction that generates a partial positive charge on the carbon atom that bears the electron-releasing alkyl group (R). The hydrogen–halogen bond is partially broken and a C±H σ bond is partially formed at the transition state. + (c) Loss of the halide ion (X) from the hydrogen halide and C±H σ bond formation complete the formation of the more stable carbocation intermediate CH3CHR. X δ X δ + C C X Figure 6.6 focuses on the orbitals involved and shows how the electrons of the double bond flow in the direction that generates the more stable of the two possible carbocations. PROBLEM 6.4 Give a structural formula for the carbocation intermediate that leads to the major product in each of the reactions of Problem 6.3 (Section 6.5). SAMPLE SOLUTION (a) Protonation of the double bond of 2-methyl-2-butene can give a tertiary carbocation or a secondary carbocation. 218 CHAPTER SIX Reactions of Alkenes: Addition Reactions FIGURE 6.6 Electron flow and orbital interactions in the transfer of a proton from a hydrogen halide to an alkene of the type CH2œCHR. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
6.7 Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes H HaC 2-Methyl-2-butene Protonation Protonation of C-3 (slower) H3C H C一CH2CH2(CH3)2CH一 HaC Tertiary carbocation Secondary carbocation The product of the reaction is derived from the more stable carbocation-in this case, it is a tertiary carbocation that is formed more rapidly than a secondary one. In general, alkyl substituents increase the reactivity of a double bond toward elec- trophilic addition. Alkyl groups are electron-releasing, and the more electron-rich a dou ble bond, the better it can share its T electrons with an electrophile. Along with the observed regioselectivity of addition, this supports the idea that carbocation formation rather than carbocation capture, is rate-determining 6.7 CARBOCATION REARRANGEMENTS IN HYDROGEN HALIDE ADDITION TO ALKENES Our belief that carbocations are intermediates in the addition of hydrogen halides to alkenes is strengthened by the observation that rearrangements sometimes occur. For example, the reaction of hydrogen chloride with 3-methyl-l-butene is expected to pro- duce 2-chloro-3-methylbutane. Instead, a mixture of 2-chloro-3-methylbutane and 2 chloro-2-methylbutane results CH2CHCH(CH3)2 noc> CH3 CHCH(CH3) CH3 CH,C(CH3) 3-Methyl-1-butene 2-Chloro-3-methylbutane 2-Chloro-2-methylbutane Addition begins in the usual way, by protonation of the double bond to give, in this case, a secondary carbocation. This carbocation can be captured by chloride to give 2-chloro- 3-methylbutane (40%) or it can rearrange by way of a hydride shift to give a tertiary carbocation. The tertiary carbocation reacts with chloride ion to give 2-chloro-2 methylbutane(60%). CH3CH—C(CH3)2 CH3CH—C(CH3)2 1, 2-Dimethylpropyl cation(secondary) 1, 1-Dimethylpropyl cation(tertiary) The similar yields of the two alkyl chloride products indicate that the rate of attack by chloride on the secondary carbocation and the rate of rearrangement must be very Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
The product of the reaction is derived from the more stable carbocation—in this case, it is a tertiary carbocation that is formed more rapidly than a secondary one. In general, alkyl substituents increase the reactivity of a double bond toward electrophilic addition. Alkyl groups are electron-releasing, and the more electron-rich a double bond, the better it can share its electrons with an electrophile. Along with the observed regioselectivity of addition, this supports the idea that carbocation formation, rather than carbocation capture, is rate-determining. 6.7 CARBOCATION REARRANGEMENTS IN HYDROGEN HALIDE ADDITION TO ALKENES Our belief that carbocations are intermediates in the addition of hydrogen halides to alkenes is strengthened by the observation that rearrangements sometimes occur. For example, the reaction of hydrogen chloride with 3-methyl-1-butene is expected to produce 2-chloro-3-methylbutane. Instead, a mixture of 2-chloro-3-methylbutane and 2- chloro-2-methylbutane results. Addition begins in the usual way, by protonation of the double bond to give, in this case, a secondary carbocation. This carbocation can be captured by chloride to give 2-chloro- 3-methylbutane (40%) or it can rearrange by way of a hydride shift to give a tertiary carbocation. The tertiary carbocation reacts with chloride ion to give 2-chloro-2- methylbutane (60%). The similar yields of the two alkyl chloride products indicate that the rate of attack by chloride on the secondary carbocation and the rate of rearrangement must be very similar. CH3CH C(CH3)2 H 1,2-Dimethylpropyl cation (secondary) CH3CH C(CH3)2 H 1,1-Dimethylpropyl cation (tertiary) hydride shift 2-Chloro-3-methylbutane (40%) CH3CHCH(CH3)2 Cl 2-Chloro-2-methylbutane (60%) CH3CH2C(CH3)2 Cl 3-Methyl-1-butene CH2 CHCH(CH3)2 HCl 0°C Protonation of C-3 Protonation (faster) (slower) of C-2 C H3C H3C CH3 H 1 2 3 4 C 2-Methyl-2-butene C H3C H3C CH2CH3 Tertiary carbocation (CH3)2CH CH3 H C Secondary carbocation 6.7 Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes 219 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
220 CHAPTER SIX Reactions of alkenes Addition reactions PROBLEM 6.5 Addition of hydrogen chloride to 3, 3-dimethyl-1-butene gives a mixture of two es for ric chlorides in approximately equal amounts. Suggest rea- sonable structure these two compounds, and offer a mechanistic explanation for their formation 6.8 FREE-RADICAL ADDITION OF HYDROGEN BROMIDE TO ALKENES For a long time the regioselectivity of addition of hydrogen bromide to alkenes was unpredictable. Sometimes addition occurred according to Markovnikov's rule, but at other times, seemingly under e same co nditions,the opposite regioselectivity(anti Markovnikov addition) was observed. In 1929, Morris S. Kharasch and his students at the University of Chicago began a systematic investigation of this puzzle. After hundreds of experiments, Kharasch concluded that anti-Markovnikov addition occurred when per- oxides, that is, organic compounds of the type ROOR, were present in the reaction mix ture. He and his colleagues found, for example, that carefully purified 1-butene reacted with hydrogen bromide to give only 2-bromobutane--the product expected on the basis of markovnikoy' s rule CH2=,CH3 HBr CH3CHCH, CH3 1-Butene (only product: 90%o yield On the other hand, when the same reaction was performed in the presence of an added peroxide, only 1-bromobutane was formed. CH,,CH3 HBr BrCH, CH,, CH3 1-Butene orogen (only product: 95%o yield Kharasch termed this phenomenon the peroxide effect and demonstrated that it could occur even if peroxides were not deliberately added to the reaction mixture. Unless alkenes are protected from atmospheric oxygen, they become contaminated with small amounts of alkyl hydroperoxides, compounds of the type ROOH. These alkyl hydroper oxides act in the same way as deliberately added peroxides to promote addition in the direction opposite to that predicted by Markovnikov's rule PRobLEM 6.6 Kharasch's earliest studies in this area were carried out in collab- oration with graduate student Frank R. Mayo. Mayo performed over 400 experi ments in which allyl bromide (3-bromo-1-propene) was treated with hydrogen bromide under a variety of conditions, and determined the distribution of the ormal"and"abnormal "products formed during the reaction. What two prod ucts were formed? Which is the product of addition in accordance with Markovnikov's rule? Which one corresponds to addition opposite to the rule? Kharasch proposed that hydrogen bromide can add to alkenes by two different mechanisms, both of which are, in modern terminology, regiospecific. The first mecha- nism is the one we discussed in the preceding section, electrophilic addition, and fol Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
PROBLEM 6.5 Addition of hydrogen chloride to 3,3-dimethyl-1-butene gives a mixture of two isomeric chlorides in approximately equal amounts. Suggest reasonable structures for these two compounds, and offer a mechanistic explanation for their formation. 6.8 FREE-RADICAL ADDITION OF HYDROGEN BROMIDE TO ALKENES For a long time the regioselectivity of addition of hydrogen bromide to alkenes was unpredictable. Sometimes addition occurred according to Markovnikov’s rule, but at other times, seemingly under the same conditions, the opposite regioselectivity (antiMarkovnikov addition) was observed. In 1929, Morris S. Kharasch and his students at the University of Chicago began a systematic investigation of this puzzle. After hundreds of experiments, Kharasch concluded that anti-Markovnikov addition occurred when peroxides, that is, organic compounds of the type ROOR, were present in the reaction mixture. He and his colleagues found, for example, that carefully purified 1-butene reacted with hydrogen bromide to give only 2-bromobutane—the product expected on the basis of Markovnikov’s rule. On the other hand, when the same reaction was performed in the presence of an added peroxide, only 1-bromobutane was formed. Kharasch termed this phenomenon the peroxide effect and demonstrated that it could occur even if peroxides were not deliberately added to the reaction mixture. Unless alkenes are protected from atmospheric oxygen, they become contaminated with small amounts of alkyl hydroperoxides, compounds of the type ROOH. These alkyl hydroperoxides act in the same way as deliberately added peroxides to promote addition in the direction opposite to that predicted by Markovnikov’s rule. PROBLEM 6.6 Kharasch’s earliest studies in this area were carried out in collaboration with graduate student Frank R. Mayo. Mayo performed over 400 experiments in which allyl bromide (3-bromo-1-propene) was treated with hydrogen bromide under a variety of conditions, and determined the distribution of the “normal” and “abnormal” products formed during the reaction. What two products were formed? Which is the product of addition in accordance with Markovnikov’s rule? Which one corresponds to addition opposite to the rule? Kharasch proposed that hydrogen bromide can add to alkenes by two different mechanisms, both of which are, in modern terminology, regiospecific. The first mechanism is the one we discussed in the preceding section, electrophilic addition, and fol- Hydrogen bromide HBr 1-Bromobutane (only product; 95% yield) BrCH2CH2CH2CH3 1-Butene CH2 CHCH2CH3 peroxides Hydrogen bromide HBr 2-Bromobutane (only product; 90% yield) CH3CHCH2CH3 Br 1-Butene CH2 CHCH2CH3 no peroxides 220 CHAPTER SIX Reactions of Alkenes: Addition Reactions Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
6.8 Free-Radical Addition of Hydrogen Bromide to Alkenes The overall reaction CH3 CH,CH=CH, HBr OOR CH3CH,CH, CH, BI 1-Butene Hydrogen bromide The mechanism (a) Initiation Step 1: Dissociation of a peroxide into two alkoxy radicals: OR +.OR Peroxid Two alkoxy radicals Step 2: Hydrogen atom abstraction from hydrogen bromide by an alkoxy radical: 4 : Hydrogen Bromine radical atom (b)Chain propagation Step 3: Addition of a bromine atom to the alkene: CH3CHCH=CH2 CH3CH2CH--CH Br 1-Butene Bromine atom (1-Bromomethyl)propyl radical Step 4: Abstraction of a hydrogen atom from hydrogen bromide by the free radical formed in step 3 CHCH:H-cHBr,、H:Br CH,, CH Br+ (1-Bromomethyl)propyl Hydrogen 1-Bromobutane FIGURE 6.7 lows markovnikov 's rule. It is the mechanism followed when care is taken to ensure that no peroxides are present. The second mechanism is the free-radical chain process, pre- nted in Figure 6.7. Peroxides are initiators; they are not incorporated into the product but act as a source of radicals necessary to get the chain reaction started. The oxygen-oxygen bond of a peroxide is relatively weak, and the free-radical addition of hydrogen bromide to alkenes begins when a peroxide molecule undergoes homolytic cleavage to two alkoxy radicals. This is depicted in step l of Figure 6.7. a bromine atom is generated in step 2 when one of these alkoxy radicals abstracts a proton from hydrogen bromide. Once a bromine atom becomes available, the propagation phase of the chain reaction begins. In the propagation phase as shown in step 3, a bromine atom adds to the alkene in the direc- tion that produces the more stable alkyl radical Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
lows Markovnikov’s rule. It is the mechanism followed when care is taken to ensure that no peroxides are present. The second mechanism is the free-radical chain process, presented in Figure 6.7. Peroxides are initiators; they are not incorporated into the product but act as a source of radicals necessary to get the chain reaction started. The oxygen–oxygen bond of a peroxide is relatively weak, and the free-radical addition of hydrogen bromide to alkenes begins when a peroxide molecule undergoes homolytic cleavage to two alkoxy radicals. This is depicted in step 1 of Figure 6.7. A bromine atom is generated in step 2 when one of these alkoxy radicals abstracts a proton from hydrogen bromide. Once a bromine atom becomes available, the propagation phase of the chain reaction begins. In the propagation phase as shown in step 3, a bromine atom adds to the alkene in the direction that produces the more stable alkyl radical. 6.8 Free-Radical Addition of Hydrogen Bromide to Alkenes 221 The overall reaction: 1-Butene Hydrogen bromide 1-Bromobutane The mechanism: (a) Initiation Step 1: Dissociation of a peroxide into two alkoxy radicals: light or heat Peroxide Two alkoxy radicals Step 2: Hydrogen atom abstraction from hydrogen bromide by an alkoxy radical: Alcohol (b) Chain propagation Step 3: Addition of a bromine atom to the alkene: 1-Butene Bromine atom (1-Bromomethyl)propyl radical Step 4: Abstraction of a hydrogen atom from hydrogen bromide by the free radical formed in step 3: (1-Bromomethyl)propyl 1-Bromobutane radical Hydrogen bromide Bromine atom Alkoxy radical Hydrogen bromide Bromine atom ROOR light or heat CH3CH2CHœCH2 HBr ±±±£ CH3CH2CH2CH2Br RO OR ±±£ RO OR RO H Br ±£ RO H Br CH3CH2CHœCH2 Br ±£ CH3CH2CH±CH2 Br CH3CH2CH±CH2Br H Br ±£ CH3CH2CH 2CH 2Br Br FIGURE 6.7 Initiation and propagation steps in the free-radical addition of hydrogen bromide to 1-butene. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER SIX Reactions of alkenes Addition reactions Addition of a bromine atom to C-l gives a secondary alkyl radical. HaCH,CH=CH2 CH3CH2CH—CH2 econdary alkyl radical Addition of a bromine atom to C-2 gives a primary alkyl radical CH3CH2CH=CH2—>CH3CH2CH—CH2 A secondary alkyl radical is more stable than a primary radical. Bromine therefore adds to c-1 of 1-butene faster than it adds to c-2. Once the bromine atom has added to the double bond, the regioselectivity of addition is set. The alkyl radical then abstracts a hydrogen atom from hydrogen bromide to give the alkyl bromide product as shown in step 4 of Figure 6.7. The regioselectivity of addition of hydrogen bromide to alkenes under normal (ionic addition) conditions is controlled by the tendency of a proton to add to the dou- ble bond so as to produce the more stable carbocation. Under free-radical conditions the regioselectivity is governed by addition of a bromine atom to give the more stable alkyl radical photochemically, either with or without added peroxide Free-radical addition of hydrogen bromide to the double bond can also be initiated H HBr bon for the carbon that has CH,Br olecular model of the free.rad Methy lenecyclopentane Hydroger (Bromomethyl)cyclopentane(60%) bromide Among the hydrogen halides, only hydrogen bromide reacts with alkenes by both an ionic and a free-radical mechanism. Hydrogen iodide and hydrogen chloride always ydrogen mide normally reacts by the ionic mechanism, but if peroxides are present or if the reac tion is initiated photochemically, the free-radical mechanism is followed PROBLEM 6.7 Give the major organic product formed when hydrogen bromide reacts with each of the alkenes in Problem 6.3 in the absence of peroxides and in their SAMPLE SOLUTION (a) The addition of hydrogen bromide in the absence of peroxides exhibits a regioselectivity just like that of hydrogen chloride addition Markovnikov's rule is followed H3C CH3一C—CH2CH3 2-Methyl-2-butene Hydrogen bromide 2-Bromo-2-methylbutane Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Addition of a bromine atom to C-1 gives a secondary alkyl radical. Addition of a bromine atom to C-2 gives a primary alkyl radical. A secondary alkyl radical is more stable than a primary radical. Bromine therefore adds to C-1 of 1-butene faster than it adds to C-2. Once the bromine atom has added to the double bond, the regioselectivity of addition is set. The alkyl radical then abstracts a hydrogen atom from hydrogen bromide to give the alkyl bromide product as shown in step 4 of Figure 6.7. The regioselectivity of addition of hydrogen bromide to alkenes under normal (ionic addition) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocation. Under free-radical conditions the regioselectivity is governed by addition of a bromine atom to give the more stable alkyl radical. Free-radical addition of hydrogen bromide to the double bond can also be initiated photochemically, either with or without added peroxides. Among the hydrogen halides, only hydrogen bromide reacts with alkenes by both an ionic and a free-radical mechanism. Hydrogen iodide and hydrogen chloride always add to alkenes by an ionic mechanism and follow Markovnikov’s rule. Hydrogen bromide normally reacts by the ionic mechanism, but if peroxides are present or if the reaction is initiated photochemically, the free-radical mechanism is followed. PROBLEM 6.7 Give the major organic product formed when hydrogen bromide reacts with each of the alkenes in Problem 6.3 in the absence of peroxides and in their presence. SAMPLE SOLUTION (a) The addition of hydrogen bromide in the absence of peroxides exhibits a regioselectivity just like that of hydrogen chloride addition; Markovnikov’s rule is followed. C H CH3 H3C H3C C 2-Methyl-2-butene HBr Hydrogen bromide CH3 CH3 Br C CH2CH3 2-Bromo-2-methylbutane no peroxides H CH2Br (Bromomethyl)cyclopentane (60%) HBr Hydrogen bromide CH2 Methylenecyclopentane h Primary alkyl radical CH3CH2CH CH2 Br CH3CH2CH 4 3 2 1 CH2 Br Secondary alkyl radical CH3CH2CH Br CH2 Br CH3CH2CH 4 3 2 1 CH2 222 CHAPTER SIX Reactions of Alkenes: Addition Reactions Using an sp2 -hybridized carbon for the carbon that has the unpaired electron, make a molecular model of the free-radical intermediate in this reaction. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website