8Chapter 06

8Chapter 06 - Chapter 6 Reactions of Alkenes: Addition...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Chapter 6 Reactions of Alkenes: Addition Reactions Dr. Wolf's CHM 201 & 202 6-1 Reactions of Alkenes Reactions of Alkenes The characteristic reaction of alkenes is addition to the double bond. C C Dr. Wolf's CHM 201 & 202 + A—B A C CB 6-2 6.1 Hydrogenation of Alkenes Dr. Wolf's CHM 201 & 202 6-3 Hydrogenation of ethylene Hydrogenation of ethylene H C H π H + C σ H—H H H H σ H C C H σ H H exothermic ∆ H° = –136 kJ/mol exothermic catalyzed by finely divided Pt, Pd, Rh, Ni Dr. Wolf's CHM 201 & 202 6-4 Example Example H3C CH2 H3C H2, Pt CH3 H3C H H3C (73%) Dr. Wolf's CHM 201 & 202 6-5 Problem 6.1 Problem 6.1 What three alkenes yield 2-methylbutane on What catalytic hydrogenation? catalytic Dr. Wolf's CHM 201 & 202 6-6 Problem 6.1 Problem 6.1 What three alkenes yield 2-methylbutane on What catalytic hydrogenation? catalytic H2, Pt Dr. Wolf's CHM 201 & 202 6-7 Mechanism of catalytic hydrogenation. Mechanism of catalytic hydrogenation. Figure 6.1 Figure 6.1 HH Dr. Wolf's CHM 201 & 202 B A Y C HH C X 6-8 Mechanism of catalytic hydrogenation. Mechanism of catalytic hydrogenation. Figure 6.1 Figure 6.1 B A H Y C C X H H H Dr. Wolf's CHM 201 & 202 6-9 Mechanism of catalytic hydrogenation. Mechanism of catalytic hydrogenation. Figure 6.1 Figure 6.1 H A H B Y C X C H H Dr. Wolf's CHM 201 & 202 6-10 Mechanism of catalytic hydrogenation. Mechanism of catalytic hydrogenation. Figure 6.1 Figure 6.1 H A H B Y C X C H H Dr. Wolf's CHM 201 & 202 6-11 Mechanism of catalytic hydrogenation. Mechanism of catalytic hydrogenation. Figure 6.1 Figure 6.1 H A B C H Dr. Wolf's CHM 201 & 202 Y X C H H 6-12 Mechanism of catalytic hydrogenation. Mechanism of catalytic hydrogenation. Figure 6.1 Figure 6.1 A B Y C H Dr. Wolf's CHM 201 & 202 H X C H H 6-13 6.2 Heats of Hydrogenation can be used to measure relative stability can of isomeric alkenes of correlation with structure is same as correlation when heats of combustion are measured when Dr. Wolf's CHM 201 & 202 6-14 126 126 119 119 115 115 CH3CH2CH2CH3 Dr. Wolf's CHM 201 & 202 6-15 Heats of Hydrogenation (kJ/mol) Ethylene Ethylene 136 136 Monosubstituted Monosubstituted 125-126 125-126 cis-Disubstituted cis-Disubstituted 1117-119 17-119 117-119 117-119 trans-Disubstituted ttrans-Disubstituted rans trans 114-115 114-115 Terminally disubstituted Terminally disubstituted 116-117 116-117 Trisubstituted Trisubstituted 112 112 Tetrasubstituted Tetrasubstituted 110 110 Dr. Wolf's CHM 201 & 202 6-16 Problem 6.2 Problem 6.2 Match each alkene of Problem 6.1 with its correct heat of hydrogenation. 126 kJ/mol 118 kJ/mol 112 kJ/mol Dr. Wolf's CHM 201 & 202 6-17 Problem 6.2 Problem 6.2 Match each alkene of Problem 6.1 with its correct heat of hydrogenation. 126 kJ/mol highest heat of hydrogenation; least stable isomer 118 kJ/mol 112 kJ/mol Dr. Wolf's CHM 201 & 202 lowest heat of hydrogenation; most stable isomer 6-18 6.3 Stereochemistry of Stereochemistry Alkene Hydrogenation Alkene Dr. Wolf's CHM 201 & 202 6-19 Two spatial (stereochemical) aspects of Two spatial (stereochemical) aspects of alkene hydrogenation: alkene hydrogenation: • syn addition of both H atoms to double bond • hydrogenation is stereoselective, hydrogenation corresponding to addition to less crowded face of double bond of Dr. Wolf's CHM 201 & 202 6-20 syn--Additionversus anti-Addition -Addition synAddition versusanti -Addition anti -Addition anti syn addition Dr. Wolf's CHM 201 & 202 anti addition 6-21 Example of syn-Addition Example of syn-Addition CO2CH3 CO2CH3 H H2, Pt H CO2CH3 CO CO2CH3 (100%) Dr. Wolf's CHM 201 & 202 6-22 Stereoselectivity 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) iis said to be stereoselective. s stereoselective Dr. Wolf's CHM 201 & 202 6-23 H3C CH3 H H3C H3C H H H3C H Dr. Wolf's CHM 201 & 202 Example of Example of stereoselective reaction stereoselective reaction H2, cat CH3 CH Both products correspond to syn addition of H2. H3C H H CH3 CH H CH3 6-24 H3C CH3 H H3C H3C H H H3C Example of Example of stereoselective reaction stereoselective reaction H2, cat CH3 CH But only this one is formed. H Dr. Wolf's CHM 201 & 202 6-25 H3C CH3 H H3C H3C H Example of Example of Example stereoselective reaction stereoselective reaction H2, cat Top face of double Top face of double bond blocked by bond blocked by this methyl group this methyl group CH3 CH H H3C H Dr. Wolf's CHM 201 & 202 6-26 H3C CH3 H H3C H3C H Example of Example of stereoselective reaction stereoselective reaction H2, cat CH3 CH H H3C H Dr. Wolf's CHM 201 & 202 H22 adds to H adds to bottom face of bottom face of double bond. double bond. 6-27 6.4 Electrophilic Addition of Electrophilic Hydrogen Halides to Alkenes Alkenes Dr. Wolf's CHM 201 & 202 6-28 General equation for electrophilic addition General equation for electrophilic addition C C Dr. Wolf's CHM 201 & 202 δ+ δ– + E—Y E C C Y 6-29 When EY is a hydrogen halide When EY is a hydrogen halide C C Dr. Wolf's CHM 201 & 202 δ+ δ– + H—X H C C X 6-30 Example Example CH2CH3 CH3CH2 C H C H HBr CHCl3, -30°C CH3CH2CH2CHCH2CH3 Br (76%) Dr. Wolf's CHM 201 & 202 6-31 Mechanism Mechanism Electrophilic addition of hydrogen halides Electrophilic to alkenes proceeds by rate-determining formation of a carbocation intermediate. formation Electrons flow from the π system of the Electrons alkene (electron rich) toward the positively polarized proton of the hydrogen halide. Dr. Wolf's CHM 201 & 202 6-32 Mechanism Mechanism +C C H .. – : X: .. H C .. X: .. C Dr. Wolf's CHM 201 & 202 6-33 Mechanism Mechanism +C C H .. – : X: .. H C C Dr. Wolf's CHM 201 & 202 .. X: .. .. :X .. C C H 6-34 6.5 Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule Dr. Wolf's CHM 201 & 202 6-35 Markovnikov's Rule 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 that has the fewer hydrogen substituents. Dr. Wolf's CHM 201 & 202 6-36 Markovnikov's Rule CH3CH2CH CH2 HBr acetic acid CH3CH2CHCH3 Br (80%) Example 1 Example 1 Dr. Wolf's CHM 201 & 202 6-37 Markovnikov's Rule CH3 H C C CH3 H CH3 HBr acetic acid CH3 C CH3 Br (90%) Example 2 Example 2 Dr. Wolf's CHM 201 & 202 6-38 Markovnikov's Rule CH3 HCl CH3 0°C Cll C (100%) Example 3 Example 3 Dr. Wolf's CHM 201 & 202 6-39 6.6 Mechanistic Basis for Markovnikov's Rule Protonation of double bond occurs in Protonation direction that gives more stable of two possible carbocations. possible Dr. Wolf's CHM 201 & 202 6-40 Mechanistic Basis for Markovnikov's Rule: Example 1 CH3CH2CH Dr. Wolf's CHM 201 & 202 CH2 HBr acetic acid CH3CH2CHCH3 Br 6-41 Mechanistic Basis for Markovnikov's Rule: Example 1 + CH3CH2CH—CH3 + Br – HBr CH3CH2CH CH2 CH3CH2CHCH3 Br Dr. Wolf's CHM 201 & 202 6-42 Mechanistic Basis for Markovnikov's Rule: Example 1 + CH3CH2CH2—CH2 primary carbocation is less stable: not formed + CH3CH2CH—CH3 + Br – HBr CH3CH2CH CH2 CH3CH2CHCH3 Br Dr. Wolf's CHM 201 & 202 6-43 Mechanistic Basis for Mechanistic Markovnikov's Rule: Markovnikov's Example 3 H CH3 Dr. Wolf's CHM 201 & 202 HCl 0°C CH3 Cll C 6-44 Mechanistic Basis for Mechanistic Markovnikov's Rule: Markovnikov's Example 3 H H + H Cl – HCl CH3 Dr. Wolf's CHM 201 & 202 CH3 CH HCl 0°C CH3 Cll C 6-45 Mechanistic Basis for Mechanistic Markovnikov's Rule: Markovnikov's Example 3 secondary carbocation secondary is less stable: not formed not H + H H CH3 + H H Cl – HCl CH3 Dr. Wolf's CHM 201 & 202 CH3 CH HCl 0°C CH3 Cll C 6-46 6.7 Carbocation Rearrangements in Carbocation Hydrogen Halide Addition to Alkenes to Dr. Wolf's CHM 201 & 202 6-47 Rearrangements sometimes occur Rearrangements sometimes occur H2C CHCH(CH3)2 HCl, 0°C H + CH3CHCH(CH3)2 + CH3CHC(CH3)2 CH3CHCH(CH3)2 CH3CH2C(CH3)2 Cll C (40%) Dr. Wolf's CHM 201 & 202 (60%) Cll C 6-48 6.8 Addition of Sulfuric Acid Addition to Alkenes to Dr. Wolf's CHM 201 & 202 6-49 Addition of H2SO4 Addition of H2SO4 CH3CH CH2 HOSO2OH CH3CHCH3 OSO2OH Isopropyl hydrogen sulfate follows Markovnikov's rule: follows Markovnikov's rule: yields an alkyl hydrogen sulfate yields an alkyl hydrogen sulfate Dr. Wolf's CHM 201 & 202 6-50 Mechanism Mechanism CH3CH CH2 + H .. O .. SO2OH slow + CH3CH CH3 – .. + :O .. fast Dr. Wolf's CHM 201 & 202 SO2OH CH3CHCH3 : OSO2OH .. .. 6-51 Alkyl hydrogen sulfates undergo Alkyl hydrogen sulfates undergo Alkyl Alkyl hydrolysis in hot water hydrolysis in hot water hydrolysis hydrolysis CH3CHCH3 O + H—OH SO2OH heat CH3CHCH3 O Dr. Wolf's CHM 201 & 202 + HO—SO2OH H 6-52 Application: Coversion of alkenes to alcohols Application: Coversion of alkenes to alcohols OH 1. H2SO4 2. H2O, heat (75%) Dr. Wolf's CHM 201 & 202 6-53 But... But... not all alkenes yield alkyl hydrogen sulfates on reaction with sulfuric acid these do: these H2C=CH2, RCH=CH2, and RCH=CHR' these don't: R2C=CH2, R2C=CHR, and R2C=CR2 (These form polymers, sec 6.21) Dr. Wolf's CHM 201 & 202 6-54 6.9 Acid-Catalyzed Hydration of Alkenes Dr. Wolf's CHM 201 & 202 6-55 Acid-Catalyzed Hydration of Alkenes Acid-Catalyzed Hydration of Alkenes C + C H—OH rreactionis acid eaction reaction is acid reaction ccatalyzed;typical atalyzed; typical hydration medium is hydration medium is 50% H2SO4-50% H2O 50% H SO -50% H O 50% 2 4 50% 2 H Dr. Wolf's CHM 201 & 202 C C OH 6-56 Follows Markovnikov's Rule Follows Markovnikov's Rule H3C H C C H3C CH3 50% H2SO4 50% H2O CH3 CH3 C CH2CH3 OH (90%) Dr. Wolf's CHM 201 & 202 6-57 Follows Markovnikov's Rule Follows Markovnikov's Rule CH2 50% H2SO4 CH3 50% H2O OH (80%) Dr. Wolf's CHM 201 & 202 6-58 Mechanism Mechanism Mechanism involves a carbocation intermediate is the reverse of acid-catalyzed dehydration of alcohols to alkenes H3C C CH2 + H2O H3C Dr. Wolf's CHM 201 & 202 H+ CH3 CH3 C CH3 OH 6-59 Mechanism Mechanism Mechanism Step (1) Protonation of double bond H H3C C CH2 + + O: H H3C H slow H3C H3C Dr. Wolf's CHM 201 & 202 H + C CH3 + :O: H 6-60 Mechanism Mechanism Mechanism Step (2) Capture of carbocation by water H3C H + C H3C fast CH3 CH3 Dr. Wolf's CHM 201 & 202 :O: CH3 + C CH3 H H + O: H 6-61 Mechanism Mechanism Mechanism Step (3) Deprotonation of oxonium ion CH3 CH3 C H O: + CH3 H + :O: H H fast CH3 CH3 C Dr. Wolf's CHM 201 & 202 CH3 .. O: H H + H + O: H 6-62 Relative Rates Relative Rates Acid-catalyzed hydration ethylene CH2=CH2 1.0 propene CH3CH=CH2 1.6 x 106 2-methylpropene 2-methylpropene (CH3)2C=CH2 (CH 2.5 x 1011 The more stable the carbocation, the faster it is formed, and the faster the reaction rate. Dr. Wolf's CHM 201 & 202 6-63 Principle of microscopic reversibility Principle of microscopic reversibility H3C C CH2 + H2O H3C H+ CH3 CH3 C CH3 OH In an equilibrium process, the same intermediates In and transition states are encountered in the forward direction and the reverse, but in the opposite order. Dr. Wolf's CHM 201 & 202 6-64 6.10 Thermodynamics of Addition-Elimination Equilibria Dr. Wolf's CHM 201 & 202 6-65 Hydration-Dehydration Equilibrium H H C H C H+ + H2O H C C OH H How do we control the position of the How equilibrium and maximize the product? equilibrium Dr. Wolf's CHM 201 & 202 6-66 Le Chatelier’s Principle A system at equilibrium adjusts so to system minimize any stress applies to it. minimize For the hydration-dehydration equilibria, the For key stress is water. key Adding water pushes the equilibrium toward Adding more product (alcohol). more Removing water pushes the equilibrium Removing toward more reactant (alkene). toward Dr. Wolf's CHM 201 & 202 6-67 6.11 Hydroboration-Oxidation of Alkenes Dr. Wolf's CHM 201 & 202 6-68 Synthesis Synthesis Suppose you wanted to prepare 1-decanol Suppose from 1-decene? from OH Dr. Wolf's CHM 201 & 202 6-69 Synthesis Synthesis Suppose you wanted to prepare 1-decanol Suppose from 1-decene? from Needed: a method for hydration of Needed: alkenes with a regioselectivity opposite to Markovnikov's rule. Markovnikov's OH Dr. Wolf's CHM 201 & 202 6-70 Synthesis Synthesis Two-step reaction sequence called hydroborationoxidation converts alkenes to alcohols with a oxidation regiochemistry opposite to Markovnikov's rule. regiochemistry 1. hydroboration 2. oxidation OH Dr. Wolf's CHM 201 & 202 6-71 Hydroboration step Hydroboration step C C + H—BH2 H C C BH2 Hydroboration can be viewed as the addition of borane (BH3) to the double bond. But BH3 is not the reagent actually used. not Dr. Wolf's CHM 201 & 202 6-72 Hydroboration step Hydroboration step C + C H—BH2 H C C BH2 Hydroboration reagents: H BH2 H2B H Dr. Wolf's CHM 201 & 202 Diborane (B2H6) normally used in an normally ether- like solvent ethercalled "diglyme" 6-73 Hydroboration step Hydroboration step C + C H—BH2 H C C BH2 Hydroboration reagents: .. +O Borane-tetrahydrofuran complex (H3B-THF) – BH3 Dr. Wolf's CHM 201 & 202 6-74 Oxidation step Oxidation step H2O2, HO– H C C BH2 H C C OH Organoborane formed in the hydroboration step is oxidized with hydrogen peroxide. Dr. Wolf's CHM 201 & 202 6-75 Example Example 1. B2H6, diglyme 2. H2O2, HO– OH (93%) Dr. Wolf's CHM 201 & 202 6-76 Example Example H3C CH3 C C H3C H H 1. H3B-THF 2. H2O2, HO– CH3 OH C C CH3 CH3 H (98%) Dr. Wolf's CHM 201 & 202 6-77 Example Example OH OH 1. B2H6, diglyme 2. H2O2, HO– (no rearrangement) (82%) Dr. Wolf's CHM 201 & 202 6-78 6.12 Stereochemistry of Stereochemistry Hydroboration-Oxidation Hydroboration-Oxidation Dr. Wolf's CHM 201 & 202 6-79 Features of HydroborationOxidation • hydration of alkenes • regioselectivity opposite to Markovnikov's rule • no rearrangement • stereospecific syn addition Dr. Wolf's CHM 201 & 202 6-80 syn-Addition •H and OH become attached to same face of double bond H CH3 CH 1. B2H6 2. H2O2, NaOH H CH3 HO H only product is trans-2-methylcyclopentanol only trans (86%) yield Dr. Wolf's CHM 201 & 202 6-81 6.13 Mechanism of Mechanism Hydroboration-Oxidation Hydroboration-Oxidation Dr. Wolf's CHM 201 & 202 6-82 Hydroboration-oxidation π electrons from alkene flow into 2P orbital of boron electrons Dr. Wolf's CHM 201 & 202 6-83 Hydroboration-oxidation H atom with 2 electrons migrates to C with atom greatest (+) charge (most highly substituted). Syn add. greatest Dr. Wolf's CHM 201 & 202 6-84 Hydroboration-oxidation Anion of hydrogen peroxide attacks boron Dr. Wolf's CHM 201 & 202 6-85 Hydroboration-oxidation Carbon with pair of electrons migrates to oxygen, displacing hydroxide. Oxygen on same side as boron Dr. Wolf's CHM 201 & 202 6-86 Hydroboration-oxidation Hydrolysis cleaves boron-oxygen bond resulting in anti-Markovnikov addition Dr. Wolf's CHM 201 & 202 6-87 1-Methylcyclopentene + BH3 •syn addition of H and B to double bond •B adds to less substituted carbon Dr. Wolf's CHM 201 & 202 6-88 Organoborane intermediate Dr. Wolf's CHM 201 & 202 6-89 Add hydrogen peroxide •OH replaces B on same side Dr. Wolf's CHM 201 & 202 6-90 trans-2Methylcyclopentanol Dr. Wolf's CHM 201 & 202 6-91 6.14 Addition of Halogens to Alkenes Dr. Wolf's CHM 201 & 202 6-92 C C + X2 X C C X electrophilic addition to double bond electrophilic addition to double bond forms a vicinal dihalide forms a vicinal dihalide Dr. Wolf's CHM 201 & 202 6-93 Example Example CH3CH CHCH(CH3)2 Br2 CHCl3 0°C CH3CHCHCH(CH3)2 Br Br (100%) Dr. Wolf's CHM 201 & 202 6-94 Scope Scope Limited to Cl2 and Br2 Limited to Cl and Br Limited 2 2 F2 addition proceeds with explosive violence F2 addition proceeds with explosive violence I2 addition is endothermic: I addition is endothermic: 2 vicinal diiodides dissociate to an alkene and I 2 vvicinal diiodides dissociate to an alkene and I icinal vicinal 2 Dr. Wolf's CHM 201 & 202 6-95 6.15 Stereochemistry of Halogen Addition Anti-addition Anti-addition Dr. Wolf's CHM 201 & 202 6-96 Example Example H H H Br2 Br Br H trans-1,2-Dibromocyclopentane 80% yield; only product Dr. Wolf's CHM 201 & 202 6-97 Example Example H H Cl Cl2 H Cl H trans-1,2-Dichlorocyclooctane 73% yield; only product Dr. Wolf's CHM 201 & 202 6-98 6.16 Mechanism of Halogen Addition to Alkenes: Halonium Ions Dr. Wolf's CHM 201 & 202 6-99 Mechanism is electrophilic addition Mechanism is electrophilic addition Br2 is not polar, but it is polarizable two steps involved (1) formation of bromonium ion (2) nucleophilic attack on (2) bromonium ion by bromide bromonium Dr. Wolf's CHM 201 & 202 6-100 Relative Rates Relative Rates Bromination ethylene H2C=CH2 1 propene CH3CH=CH2 61 2-methylpropene 2-methylpropene (CH3)2C=CH2 (CH 5400 2,3-dimethyl-2-butene (CH3)2C=C(CH3)2 920,000 More highly substituted double bonds react faster. Alkyl groups on the double bond make it more “electron rich.” Dr. Wolf's CHM 201 & 202 6-101 Mechanism? Mechanism? H2C + CH2 Br2 BrCH2CH2Br ? C + C .. – + : Br : .. : Br : .. No obvious explanation for No Dr. Wolf's CHM 201 & 202 anti addition provided by this mechanism. this 6-102 Mechanism Mechanism H2C CH2 + Br2 C C : Br : + Dr. Wolf's CHM 201 & 202 BrCH2CH2Br .. – + : Br : .. Cyclic bromonium ion 6-103 Formation of bromonium ion Formation of bromonium ion Br Mutual polarization of electron distributions of Br2 and alkene Dr. Wolf's CHM 201 & 202 Br 6-104 Formation of bromonium ion Formation of bromonium ion Electrons flow Electrons from alkene toward Br2 from Br δ– Br δ+ Dr. Wolf's CHM 201 & 202 δ+ 6-105 Formation of bromonium ion Formation of bromonium ion – Br π electrons of alkene electrons displace Br– from Br + Br Dr. Wolf's CHM 201 & 202 6-106 Stereochemistry Stereochemistry .. – : Br : .. .. Br + .. .. : Br .. attack of Br– from side opposite C—Br bond of bromonium ion gives anti addition Dr. Wolf's CHM 201 & 202 .. Br : .. 6-107 Example Example H H H Br2 Br Br H trans-1,2-Dibromocyclopentane 80% yield; only product Dr. Wolf's CHM 201 & 202 6-108 Cyclopentene +Br2 Dr. Wolf's CHM 201 & 202 6-109 Bromonium ion – Dr. Wolf's CHM 201 & 202 6-110 – – Dr. Wolf's CHM 201 & 202 Bromide ion attacks the bromonium ion from side opposite carbon-bromine bond 6-111 trans-Stereochemistry in vicinal dibromide Dr. Wolf's CHM 201 & 202 6-112 6.17 Conversion of Alkenes to Vicinal Halohydrins Dr. Wolf's CHM 201 & 202 6-113 C C + X2 X C C X alkenes react with X2 to form vicinal dihalides Dr. Wolf's CHM 201 & 202 6-114 C C + X2 X C C X alkenes react with X2 to form vicinal dihalides alkenes react with X2 in water to give vicinal halohydrins halohydrins C C + X2 + H2O Dr. Wolf's CHM 201 & 202 X C C OH + H—X 6-115 Examples Examples H2C CH2 + Br2 H2O BrCH2CH2OH (70%) H H Cl2 OH H2O H Cl H anti addition: only product anti addition: Dr. Wolf's CHM 201 & 202 6-116 Mechanism Mechanism O: .. .. Br + .. • bromonium ion is bromonium intermediate intermediate • water is nucleophile that water attacks bromonium ion attacks Dr. Wolf's CHM 201 & 202 O .. + .. Br : .. 6-117 Examples Examples H2C CH2 + Br2 H2O BrCH2CH2OH (70%) H H Cl2 OH H2O H Cl H anti addition: only product anti addition: Dr. Wolf's CHM 201 & 202 6-118 Cyclopentene + Cl2 Dr. Wolf's CHM 201 & 202 6-119 Chloronium ion Dr. Wolf's CHM 201 & 202 6-120 Water attacks chloronium ion from side opposite carbon-chlorine bond Dr. Wolf's CHM 201 & 202 6-121 transStereochemistry in oxonium ion Dr. Wolf's CHM 201 & 202 6-122 trans-2-Chlorocyclopentanol Dr. Wolf's CHM 201 & 202 6-123 Regioselectivity Regioselectivity H3C C H3C CH2 Br2 H2O CH3 CH3 C CH2Br OH (77%) Markovnikov's rule applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens. greater Dr. Wolf's CHM 201 & 202 6-124 Explanation Explanation H H H .. δ+ O .. O δ+ H3C H3C δ+ C CH2 : Br : δ+ H3C H3C H δ+ CH2 CH C : Br : δ+ transition state for attack of water on bromonium ion has transition carbocation character; more stable transition state (left) has positive charge on more highly substituted carbon has Dr. Wolf's CHM 201 & 202 6-125 6.18 Free-radical Addition Free-radical of HBr to Alkenes of The "peroxide effect" Dr. Wolf's CHM 201 & 202 6-126 Markovnikov's Rule CH3CH2CH CH2 HBr acetic acid CH3CH2CHCH3 Br (80%) Example 1 Example 1 Dr. Wolf's CHM 201 & 202 6-127 Addition of HBr to 1-Butene Addition of HBr to 1-Butene CH3CH2CH CH2 HBr CH3CH2CHCH3 Br only product in absence of peroxides Dr. Wolf's CHM 201 & 202 CH3CH2CH2CH2Br only product when only peroxides added to reaction mixture reaction 6-128 Addition of HBr to 1-Butene Addition of HBr to 1-Butene CH3CH2CH CH2 HBr addition opposite to addition opposite to addition addition Markovnikov's rule Markovnikov's rule Markovnikov's Markovnikov's occurs with HBr (not occurs with HBr (not occurs occurs HCl or HI) HCl or HI) HCl HCl Dr. Wolf's CHM 201 & 202 CH3CH2CH2CH2Br only product when only peroxides added to reaction mixture reaction 6-129 CH2 + HBr hν CH2Br H (60%) Addition of HBr with a regiochemistry opposite to Markovnikov's rule can also occur when initiated with light with or without added peroxides. Dr. Wolf's CHM 201 & 202 6-130 Mechanism Mechanism Addition of HBr opposite to Markovnikov's rule Addition proceeds by a free-radical chain mechanism. proceeds Initiation steps: .. RO .. Dr. Wolf's CHM 201 & 202 .. O .. R .. .. R O . + .O .. .. R 6-131 Mechanism Mechanism Mechanism Addition of HBr opposite to Markovnikov's rule Addition proceeds by a free-radical chain mechanism. proceeds Initiation steps: .. RO .. .. O .. .. R O. + H .. Dr. Wolf's CHM 201 & 202 R .. Br : .. .. .. R O . + .O .. .. R .. .. R O H + . Br : .. .. 6-132 Propagation steps: .. CH2 + . Br : .. CH3CH2CH . CH3CH2CH CH2 .. Br : .. Gives most stable free radical CH3CH2CH . H Dr. Wolf's CHM 201 & 202 CH2 .. Br : .. .. Br : .. .. CH3CH2CH2CH2 Br : .. + .. . Br : .. 6-133 6.19 Epoxidation of Alkenes Dr. Wolf's CHM 201 & 202 6-134 Epoxides Epoxides •are examples of heterocyclic compounds •three-membered rings that contain oxygen ethylene oxide ethylene CH2 H2C O Dr. Wolf's CHM 201 & 202 propylene oxide propylene CHCH3 H2C O 6-135 Epoxide Nomenclature Epoxide Nomenclature Substitutive nomenclature: Substitutive •named as epoxy-substituted named alkanes. alkanes. •“epoxy” precedes name of alkane 1,2-epoxypropane H2C CHCH3 O Dr. Wolf's CHM 201 & 202 H3C 1 4 2 3 2-methyl-2,3-epoxybutane C CHCH3 H3C O 6-136 Problem 6.17 Give the IUPAC name, including Problem 6.17 Give the IUPAC name, including Problem Problem stereochemistry, for disparlure. sstereochemistry, for disparlure. tereochemistry, stereochemistry, HOH cis-2-Methyl-7,8epoxyoctadecane Dr. Wolf's CHM 201 & 202 6-137 Epoxidation of Alkenes Epoxidation of Alkenes O C C + RCOOH peroxy acid O C Dr. Wolf's CHM 201 & 202 C O + RCOH 6-138 Example Example O + CH3COOH O + CH3COH O (52%) Dr. Wolf's CHM 201 & 202 6-139 Epoxidation of Alkenes Epoxidation of Alkenes O C C + RCOOH syn addition O C Dr. Wolf's CHM 201 & 202 C O + RCOH 6-140 Relative Rates Relative Rates Epoxidation ethylene H2C=CH2 1 propene CH3CH=CH2 22 2-methylpropene 2-methylpropene (CH3)2C=CH2 (CH 484 2-methyl-2-butene 2-methyl-2-butene (CH3)2C=CHCH3 (CH 6526 More highly substituted double bonds react More faster. faster. Alkyl groups on the double bond make it Dr. Wolf's CHM 201 & 202 6-141 Mechanism of Epoxidation Mechanism of Epoxidation Dr. Wolf's CHM 201 & 202 6-142 Mechanism of Epoxidation Mechanism of Epoxidation Dr. Wolf's CHM 201 & 202 6-143 Mechanism of Epoxidation Mechanism of Epoxidation Dr. Wolf's CHM 201 & 202 6-144 Mechanism of Epoxidation Mechanism of Epoxidation Dr. Wolf's CHM 201 & 202 6-145 Mechanism of Epoxidation Mechanism of Epoxidation Dr. Wolf's CHM 201 & 202 6-146 6.20 Ozonolysis of Alkenes Ozonolyis has both synthetic and analytical Ozonolyis applications. applications. • synthesis of aldehydes and ketones • iidentification of substituents on the dentification double bond of an alkene double Dr. Wolf's CHM 201 & 202 6-147 Ozonolysis of Alkenes Ozonolysis of Alkenes First step is the reaction of the alkene with First ozone. The product is an ozonide. ozonide. C C + O3 C O Dr. Wolf's CHM 201 & 202 O C O 6-148 Ozonolysis of Alkenes Ozonolysis of Alkenes Second step is hydrolysis of the ozonide. Second Two aldehydes, two ketones, or an aldehyde and a ketone are formed. and C C + O3 C O O C O H2O, Zn C O Dr. Wolf's CHM 201 & 202 + O C 6-149 Ozonolysis of Alkenes Ozonolysis of Alkenes As an alternative to hydrolysis, the As ozonide ozonide can be treated with dimethyl sulfide. can C C + O3 C O O C O (CH3)2S C O Dr. Wolf's CHM 201 & 202 + O C 6-150 Example Example CH2CH3 CH3 C C CH2CH3 H 1. O3 2. H2O, Zn CH2CH3 CH3 C H Dr. Wolf's CHM 201 & 202 O (38%) + O C (57%) CH2CH3 6-151 Introduction to Organic Chemical Synthesis Dr. Wolf's CHM 201 & 202 6-152 Prepare cyclohexane from cyclohexanol Prepare cyclohexane from cyclohexanol OH devise a synthetic plan reason backward from the target molecule always use reactions that you are sure will always work work Dr. Wolf's CHM 201 & 202 6-153 Prepare cyclohexane from cyclohexanol Prepare cyclohexane from cyclohexanol OH ask yourself the key question "Starting with anything, how can I make "Starting cyclohexane in a single step by a reaction I am sure will work?" am Dr. Wolf's CHM 201 & 202 6-154 Prepare cyclohexane from cyclohexanol Prepare cyclohexane from cyclohexanol OH H2 Pt The only reaction covered so far for preparing The alkanes is catalytic hydrogenation of alkenes. alkanes This leads to a new question. "Starting with This anything, how can I prepare cyclohexene in a cyclohexene single step by a reaction I am sure will work?" single Dr. Wolf's CHM 201 & 202 6-155 Prepare cyclohexane from cyclohexanol Prepare cyclohexane from cyclohexanol OH H2SO4 H2 heat Pt Alkenes can be prepared by dehydration of Alkenes alcohols. alcohols. The synthesis is complete. Dr. Wolf's CHM 201 & 202 6-156 Prepare 1-bromo-2-methyl-2-propanol Prepare 1-bromo-2-methyl-2-propanol from tert-butyl alcohol from ttert-butyl alcohol from ert from tert (CH3)3COH (CH3)2CCH2Br OH "Starting with anything, how can I make the "Starting desired compound in a single step by a reaction I am sure will work?" am The desired compound is a vicinal bromohydrin. The How are vicinal bromohydrins prepared? How Dr. Wolf's CHM 201 & 202 6-157 Prepare 1-bromo-2-methyl-2-propanol Prepare 1-bromo-2-methyl-2-propanol from tert-butyl alcohol from ttert-butyl alcohol from ert from tert (CH3)2C CH2 Br2 H2O (CH3)2CCH2Br OH Vicinal bromohydrins are prepared by treatment Vicinal of alkenes with Br2 in water. of How is the necessary alkene prepared? Dr. Wolf's CHM 201 & 202 6-158 Prepare 1-bromo-2-methyl-2-propanol Prepare 1-bromo-2-methyl-2-propanol from tert-butyl alcohol from ttert-butyl alcohol from ert from tert (CH3)3COH H2SO4 heat (CH3)2C CH2 Br2 H2O (CH3)2CCH2Br OH 2-Methylpropene is prepared from tert-butyl 2-Methylpropene tert-butyl alcohol by acid-catalyzed dehydration. alcohol The synthesis is complete. Dr. Wolf's CHM 201 & 202 6-159 6.21 Reactions of Alkenes with Reactions Alkenes: Alkenes: Polymerization Dr. Wolf's CHM 201 & 202 6-160 Polymerization of alkenes Polymerization of alkenes cationic polymerization free-radical polymerization coordination polymerization Dr. Wolf's CHM 201 & 202 6-161 Cationic Polymerization Cationic Polymerization Dimerization of 2-methylpropene monomer (C4H8) (CH3)2C CH2 H2SO4 two dimers (C8H16) CH3 CH3CCH Dr. Wolf's CHM 201 & 202 CH3 CH3 C(CH3)2 + CH3CCH2C CH3 CH3 CH2 6-162 Mechanism of Cationic Polymerization Mechanism of Cationic Polymerization CH3 H2C CH3C + C CH3 Dr. Wolf's CHM 201 & 202 H+ CH3 CH3 6-163 Mechanism of Cationic Polymerization Mechanism of Cationic Polymerization CH3 CH3C + CH3 + H2C C CH3 CH3 CH3 + CH3CCH2C Dr. Wolf's CHM 201 & 202 CH3 CH3 CH3 6-164 Mechanism of Cationic Polymerization Mechanism of Cationic Polymerization CH3 CH3CCH CH3 C(CH3)2 + CH3CCH2C CH3 CH2 CH3 CH3 CH3 + CH3CCH2C Dr. Wolf's CHM 201 & 202 CH3 CH3 CH3 6-165 Free-Radical Polymerization of Ethylene Free-Radical Polymerization of Ethylene H2C CH2 200 °C 2000 atm CH2 CH2 CH2 CH2 O2 peroxides CH2 CH2 CH2 polyethylene Dr. Wolf's CHM 201 & 202 6-166 .. RO • .. H2C Dr. Wolf's CHM 201 & 202 Mechanism Mechanism CH2 6-167 .. RO: H2C Dr. Wolf's CHM 201 & 202 Mechanism Mechanism Mechanism CH2 • 6-168 .. RO: Mechanism Mechanism Mechanism H2C CH2 • H2C Dr. Wolf's CHM 201 & 202 CH2 6-169 .. RO: H2C Mechanism Mechanism Mechanism CH2 H2C Dr. Wolf's CHM 201 & 202 CH2 • 6-170 .. RO: H2C Mechanism Mechanism Mechanism CH2 H2C CH2 • H2C Dr. Wolf's CHM 201 & 202 CH2 6-171 .. RO: H2C Mechanism Mechanism CH2 H2C CH2 H2C Dr. Wolf's CHM 201 & 202 CH2 • 6-172 .. RO: H2C Mechanism Mechanism Mechanism CH2 H2C CH2 H2C CH2 • H2C Dr. Wolf's CHM 201 & 202 CH2 6-173 Free-Radical Polymerization of Propene Free-Radical Polymerization of Propene H2C CHCH3 CH CH CH CH CH CH CH H CH3 H CH3 H CH3 H polypropylene Dr. Wolf's CHM 201 & 202 6-174 .. RO • .. H2C Dr. Wolf's CHM 201 & 202 Mechanism Mechanism CHCH3 6-175 .. RO: H2C Dr. Wolf's CHM 201 & 202 Mechanism Mechanism Mechanism CHCH3 • 6-176 .. RO: Mechanism Mechanism Mechanism H2C CHCH3 • H2C Dr. Wolf's CHM 201 & 202 CHCH3 6-177 .. RO: H2C Mechanism Mechanism Mechanism CHCH3 H2C Dr. Wolf's CHM 201 & 202 CHCH3 • 6-178 .. RO: H2C Mechanism Mechanism Mechanism CHCH3 H2C CHCH3 • H2C Dr. Wolf's CHM 201 & 202 CHCH3 6-179 .. RO: H2C Mechanism Mechanism CHCH3 H2C CHCH3 H2C Dr. Wolf's CHM 201 & 202 CHCH3 • 6-180 .. RO: H2C Mechanism Mechanism Mechanism CHCH3 H2C CHCH3 H2C CHCH3 • H2C Dr. Wolf's CHM 201 & 202 CHCH3 6-181 H2C=CHCl C=CHCl H2C=CHCl C=CHCl H2C=CHC6H5 H2C=CHC6H5 F2C=CF2 F C=CF 2 Dr. Wolf's CHM 201 & 202 2 → → → → → → polyvinyl chloride polyvinyl chloride polyvinyl polyvinyl polystyrene polystyrene polystyrene polystyrene Teflon Teflon Teflon Teflon 6-182 End of Chapter 6 Dr. Wolf's CHM 201 & 202 6-183 ...
View Full Document

This document was uploaded on 01/03/2012.

Ask a homework question - tutors are online