Reactions of Alkenes: Addition Reactions 1

The characteristic reaction of alkenes is addition to the double bond. Reactions of Alkenes The characteristic reaction of alkenes is addition to the double bond. C A C B + A—B 2

Hydrogenation of Alkenes 3

Hydrogenation of ethylene C H H  C    + H—H exothermic H° = –136 kJ/mol catalyzed by finely divided Pt, Pd, Rh, Ni 4

Example H3C CH2 H2, Pt H3C CH3 H (73%) 5

What three alkenes yield 2-methylbutane on catalytic hydrogenation? Problem 6.1 What three alkenes yield 2-methylbutane on catalytic hydrogenation? 6

What three alkenes yield 2-methylbutane on catalytic hydrogenation? Problem 6.1 What three alkenes yield 2-methylbutane on catalytic hydrogenation? H2, Pt 6

Mechanism of catalytic hydrogenation. Figure 6.1 B X Y H 8

Mechanism of catalytic hydrogenation. Figure 6.1 B Y C C A X H H H H 9

Mechanism of catalytic hydrogenation. Figure 6.1 B X Y H H H H 9

Mechanism of catalytic hydrogenation. Figure 6.1 B Y A X H H H C C H 9

Mechanism of catalytic hydrogenation. Figure 6.1 B X Y H H 9

Mechanism of catalytic hydrogenation. Figure 6.1 B X Y H H 9

Heats of Hydrogenation can be used to measure relative stability of isomeric alkenes correlation with structure is same as when heats of combustion are measured 13

126 119 115 CH3CH2CH2CH3 14

Heats of Hydrogenation (kJ/mol) Ethylene 136 Monosubstituted 125-126 cis-Disubstituted 117-119 trans-Disubstituted 114-115 Terminally disubstituted 116-117 Trisubstituted 112 Tetrasubstituted 110 15

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 16

Problem 6.2 Match each alkene of Problem 6.1 with its correct heat of hydrogenation. highest heat of hydrogenation; least stable isomer 126 kJ/mol 118 kJ/mol 112 kJ/mol lowest heat of hydrogenation; most stable isomer 16

Stereochemistry of Alkene Hydrogenation 17

Two spatial (stereochemical) aspects of alkene hydrogenation: syn addition of both H atoms to double bond hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond 18

syn-Addition versus anti-Addition 18

Example of syn-Addition H CO2CH3 CO2CH3 H2, Pt CO2CH3 H (100%) 19

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. 20

Example of stereoselective reaction H3C CH3 H Example of stereoselective reaction H2, cat CH3 H3C H CH3 H3C H Both products correspond to syn addition of H2. 21

Example of stereoselective reaction H3C CH3 H Example of stereoselective reaction H2, cat CH3 H3C H But only this one is formed. 21

Example of stereoselective reaction H3C CH3 H Example of stereoselective reaction H2, cat Top face of double bond blocked by this methyl group CH3 H3C H 21

Example of stereoselective reaction H3C CH3 H Example of stereoselective reaction H2, cat CH3 H3C H H2 adds to bottom face of double bond. 21

Electrophilic Addition of Hydrogen Halides to Alkenes 1

General equation for electrophilic addition  – C E Y C + E—Y 2

When EY is a hydrogen halide  – C H X C + H—X 2

Example CH3CH2 CH2CH3 HBr C CHCl3, -30°C H H CH3CH2CH2CHCH2CH3 Br (76%) 4

Mechanism Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate. Electrons flow from the  system of the alkene (electron rich) toward the positively polarized proton of the hydrogen halide. 5

Mechanism + C C H .. : X – .. : H X C 6

Mechanism + C C H .. : X – .. : H X C H C .. X : 6

Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule 7

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. 8

Markovnikov's Rule HBr CH3CH2CH CH3CH2CHCH3 CH2 acetic acid Br (80%) Example 1 9

Markovnikov's Rule CH3 H HBr C CH3 C acetic acid CH3 H Br (90%) Example 2 10

Markovnikov's Rule CH3 Cl CH3 HCl 0°C (100%) Example 3 11

Mechanistic Basis for Markovnikov's Rule Protonation of double bond occurs in direction that gives more stable of two possible carbocations. 12

Mechanistic Basis for Markovnikov's Rule: Example 1 acetic acid HBr CH2 CH3CH2CH Br CH3CH2CHCH3 9

Mechanistic Basis for Markovnikov's Rule: Example 1 CH3CH2CH—CH3 + Br – + HBr CH2 CH3CH2CH Br CH3CH2CHCH3 9

Mechanistic Basis for Markovnikov's Rule: Example 1 + CH3CH2CH2—CH2 primary carbocation is less stable: not formed CH3CH2CH—CH3 + Br – + HBr CH2 CH3CH2CH Br CH3CH2CHCH3 9

Mechanistic Basis for Markovnikov's Rule: Example 3 Cl CH3 0°C HCl 14

Mechanistic Basis for Markovnikov's Rule: Example 3 + CH3 Cl – HCl CH3 H Cl CH3 0°C HCl 14

Mechanistic Basis for Markovnikov's Rule: Example 3 secondary carbocation is less stable: not formed H H H + CH3 H + CH3 Cl – HCl CH3 H Cl CH3 0°C HCl 14

Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes 15

Rearrangements sometimes occur H2C CHCH(CH3)2 HCl, 0°C H + CH3CHCH(CH3)2 + CH3CHC(CH3)2 CH3CHCH(CH3)2 Cl CH3CH2C(CH3)2 Cl (40%) (60%) 16

Addition of Sulfuric Acid to Alkenes 22

Isopropyl hydrogen sulfate Addition of H2SO4 HOSO2OH CH3CH CH2 CH3CHCH3 OSO2OH Isopropyl hydrogen sulfate follows Markovnikov's rule: yields an alkyl hydrogen sulfate 23

.. Mechanism .. CH3CH CH2 + H O SO2OH .. slow – + CH3CH CH3 .. + : O fast CH3CHCH3 OSO2OH : .. 24

Alkyl hydrogen sulfates undergo hydrolysis in hot water CH3CHCH3 O SO2OH + H—OH heat CH3CHCH3 O H + HO—SO2OH 25

Application: Coversion of alkenes to alcohols 1. H2SO4 2. H2O, heat (75%) 26

H2C=CH2, RCH=CH2, and RCH=CHR' these don't: But... not all alkenes yield alkyl hydrogen sulfates on reaction with sulfuric acid these do: H2C=CH2, RCH=CH2, and RCH=CHR' these don't: R2C=CH2, R2C=CHR, and R2C=CR2 (These form polymers, sec 6.21) 27

Acid-Catalyzed Hydration of Alkenes 19

Acid-Catalyzed Hydration of Alkenes + H—OH reaction is acid catalyzed; typical hydration medium is 50% H2SO4-50% H2O OH C H 2

Follows Markovnikov's Rule OH C CH2CH3 CH3 H3C CH3 H C 50% H2SO4 50% H2O (90%) 3

Follows Markovnikov's Rule CH3 OH CH2 50% H2SO4 50% H2O (80%) 4

involves a carbocation intermediate Mechanism involves a carbocation intermediate is the reverse of acid-catalyzed dehydration of alcohols to alkenes OH C CH3 H3C C CH2 H+ + H2O 5

Step (1) Protonation of double bond Mechanism Step (1) Protonation of double bond H3C C CH2 O + H : + slow H3C O H : + C CH3 + H3C H 6

Step (2) Capture of carbocation by water Mechanism Step (2) Capture of carbocation by water + H3C C CH3 H O : fast + H O : C CH3 6

Step (3) Deprotonation of oxonium ion Mechanism Step (3) Deprotonation of oxonium ion H O : C CH3 O H : + + fast H O : C CH3 .. O + H : + 6

Acid-catalyzed hydration Relative Rates Acid-catalyzed hydration ethylene CH2=CH2 1.0 propene CH3CH=CH2 1.6 x 106 2-methylpropene (CH3)2C=CH2 2.5 x 1011 The more stable the carbocation, the faster it is formed, and the faster the reaction rate. 9

Principle of microscopic reversibility OH C CH3 H3C C CH2 H+ + H2O In an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in the opposite order. 11

Thermodynamics of Addition-Elimination Equilibria 19

Hydration-Dehydration Equilibrium How do we control the position of the equilibrium and maximize the product? 13

Le Chatelier’s Principle A system at equilibrium adjusts so to minimize any stress applies to it. For the hydration-dehydration equilibria, the key stress is water. Adding water pushes the equilibrium toward more product (alcohol). Removing water pushes the equilibrium toward more reactant (alkene). 13

Hydroboration-Oxidation of Alkenes 19

Suppose you wanted to prepare 1-decanol from 1-decene? Synthesis Suppose you wanted to prepare 1-decanol from 1-decene? OH 13

Suppose you wanted to prepare 1-decanol from 1-decene? Synthesis Suppose you wanted to prepare 1-decanol from 1-decene? Needed: a method for hydration of alkenes with a regioselectivity opposite to Markovnikov's rule. OH 13

Synthesis Two-step reaction sequence called hydroboration- oxidation converts alkenes to alcohols with a regiochemistry opposite to Markovnikov's rule. 1. hydroboration 2. oxidation OH 13

Hydroboration step C H BH2 C + H—BH2 Hydroboration can be viewed as the addition of borane (BH3) to the double bond. But BH3 is not the reagent actually used. 14

Hydroboration reagents: Hydroboration step C H BH2 C + H—BH2 Hydroboration reagents: H Diborane (B2H6) normally used in an ether- like solvent called "diglyme" H2B BH2 H 14

Hydroboration reagents: Hydroboration step C H BH2 C + H—BH2 Hydroboration reagents: Borane-tetrahydrofuran complex (H3B-THF) .. + O – BH3 14

Oxidation step H2O2, HO– H BH2 C H OH C Organoborane formed in the hydroboration step is oxidized with hydrogen peroxide. 17

Example 1. B2H6, diglyme 2. H2O2, HO– OH (93%) 13

Example H C CH3 OH H3C CH3 1. H3B-THF 2. H2O2, HO– C H3C H (98%) 13

Example OH 1. B2H6, diglyme 2. H2O2, HO– (no rearrangement) (82%) 13

Stereochemistry of Hydroboration-Oxidation 19

Features of Hydroboration-Oxidation hydration of alkenes regioselectivity opposite to Markovnikov's rule no rearrangement stereospecific syn addition 18

only product is trans-2-methylcyclopentanol (86%) yield syn-Addition H and OH become attached to same face of double bond H CH3 HO CH3 H 1. B2H6 2. H2O2, NaOH only product is trans-2-methylcyclopentanol (86%) yield 20

Mechanism of Hydroboration-Oxidation 19

Hydroboration-oxidation  electrons from alkene flow into 2P orbital of boron 22

Hydroboration-oxidation H atom with 2 electrons migrates to C with greatest (+) charge (most highly substituted). Syn add. 22

Hydroboration-oxidation Anion of hydrogen peroxide attacks boron 22

Hydroboration-oxidation Carbon with pair of electrons migrates to oxygen, displacing hydroxide. Oxygen on same side as boron 22

Hydroboration-oxidation Hydrolysis cleaves boron-oxygen bond resulting in anti-Markovnikov addition 22

1-Methylcyclopentene + BH3 syn addition of H and B to double bond B adds to less substituted carbon 22

Organoborane intermediate 23

Add hydrogen peroxide OH replaces B on same side 23

trans-2-Methylcyclopentanol 24

Addition of Halogens to Alkenes 1

electrophilic addition to double bond forms a vicinal dihalide + X2 X X electrophilic addition to double bond forms a vicinal dihalide 2

Example Br2 CH3CH CH3CHCHCH(CH3)2 CHCH(CH3)2 CHCl3 0°C Br Br (100%) 4

F2 addition proceeds with explosive violence Scope Limited to Cl2 and Br2 F2 addition proceeds with explosive violence I2 addition is endothermic: vicinal diiodides dissociate to an alkene and I2 3

Stereochemistry of Halogen Addition Anti-addition 5

trans-1,2-Dibromocyclopentane 80% yield; only product Example Br H H Br2 H Br trans-1,2-Dibromocyclopentane 80% yield; only product 6

trans-1,2-Dichlorocyclooctane 73% yield; only product Example H H Cl2 Cl H Cl trans-1,2-Dichlorocyclooctane 73% yield; only product 6

Mechanism of Halogen Addition to Alkenes: Halonium Ions 7

Mechanism is electrophilic addition Br2 is not polar, but it is polarizable two steps involved (1) formation of bromonium ion (2) nucleophilic attack on bromonium ion by bromide 8

2-methylpropene (CH3)2C=CH2 5400 Relative Rates Bromination ethylene H2C=CH2 1 propene CH3CH=CH2 61 2-methylpropene (CH3)2C=CH2 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.” 6

No obvious explanation for anti addition provided by this mechanism. H2C + CH2 Br2 BrCH2CH2Br ? + C Br : .. – No obvious explanation for anti addition provided by this mechanism. 10

Cyclic bromonium ion Mechanism H2C + CH2 Br2 BrCH2CH2Br .. – + C C : 10

Formation of bromonium ion Mutual polarization of electron distributions of Br2 and alkene Br 11

Formation of bromonium ion – Electrons flow from alkene toward Br2 Br Br + + 11

Formation of bromonium ion – Br  electrons of alkene displace Br– from Br + Br 11

Stereochemistry – : Br .. + .. : Br .. attack of Br– from side opposite C—Br bond of bromonium ion gives anti addition .. Br : .. 12

trans-1,2-Dibromocyclopentane 80% yield; only product Example Br H H Br2 H Br trans-1,2-Dibromocyclopentane 80% yield; only product 6

Cyclopentene +Br2 15

Bromonium ion – 17

– Bromide ion attacks the bromonium ion from side opposite carbon-bromine bond – 17

trans-Stereochemistry in vicinal dibromide 19

Conversion of Alkenes to Vicinal Halohydrins 20

alkenes react with X2 to form vicinal dihalides

alkenes react with X2 to form vicinal dihalides alkenes react with X2 in water to give vicinal halohydrins C C + X2 + H2O X OH + H—X 2

anti addition: only product Examples H2O + H2C CH2 Br2 BrCH2CH2OH (70%) OH H H Cl2 H2O H Cl anti addition: only product 6

bromonium ion is intermediate Mechanism .. O : Br .. + .. O + bromonium ion is intermediate water is nucleophile that attacks bromonium ion .. Br : .. 12

anti addition: only product Examples H2O + H2C CH2 Br2 BrCH2CH2OH (70%) OH H H Cl2 H2O H Cl anti addition: only product 6

Cyclopentene + Cl2 26

Chloronium ion 28

Water attacks chloronium ion from side opposite carbon-chlorine bond 29

trans-Stereochemistry in oxonium ion 30

trans-2-Chlorocyclopentanol 31

Regioselectivity CH3 OH C CH2Br H3C C CH2 Br2 H2O (77%) Markovnikov's rule applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens. 32

Explanation H O .. H O .. H3C H3C H3C H3C C CH2 CH2 C : Br : : Br :  transition state for attack of water on bromonium ion has carbocation character; more stable transition state (left) has positive charge on more highly substituted carbon 34

Free-radical Addition of HBr to Alkenes The "peroxide effect" 17

Markovnikov's Rule HBr CH3CH2CH CH3CH2CHCH3 CH2 acetic acid Br (80%) Example 1 9

Addition of HBr to 1-Butene CH2 CH3CH2CH HBr Br CH3CH2CHCH3 CH3CH2CH2CH2Br only product when peroxides added to reaction mixture only product in absence of peroxides 18

Addition of HBr to 1-Butene CH2 CH3CH2CH HBr addition opposite to Markovnikov's rule occurs with HBr (not HCl or HI) CH3CH2CH2CH2Br only product when peroxides added to reaction mixture 18

CH2 + HBr h 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. 19

Mechanism Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism. Initiation steps: .. O R O . R .. .. O R . + 20

Mechanism Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism. Initiation steps: .. O R O . R .. .. O R . + H .. : O . R .. .. . Br .. : + Br R O H + .. 20

. Br .. .. .. .. .. .. H Br .. . Br .. Propagation steps: : CH3CH2CH + . CH3CH2CH CH2 Br : .. Gives most stable free radical CH3CH2CH CH2 Br : .. .. CH3CH2CH2CH2 Br : .. . .. + H Br : .. . Br .. : 21

Epoxidation of Alkenes 1

are examples of heterocyclic compounds Epoxides are examples of heterocyclic compounds three-membered rings that contain oxygen ethylene oxide propylene oxide H2C CH2 O H2C CHCH3 O 2

Substitutive nomenclature: named as epoxy-substituted alkanes. Epoxide Nomenclature Substitutive nomenclature: named as epoxy-substituted alkanes. “epoxy” precedes name of alkane 1,2-epoxypropane 2-methyl-2,3-epoxybutane 1 CHCH3 O C H3C 2 3 4 H2C CHCH3 O 2

cis-2-Methyl-7,8-epoxyoctadecane Problem 6.17 Give the IUPAC name, including stereochemistry, for disparlure. O H cis-2-Methyl-7,8-epoxyoctadecane 5

Epoxidation of Alkenes RCOOH C + peroxy acid O RCOH C O + 6

Example + CH3COOH O + CH3COH O O (52%) 7

Epoxidation of Alkenes RCOOH C + syn addition O RCOH C O + 6

ethylene H2C=CH2 1 Relative Rates Epoxidation propene CH3CH=CH2 22 2-methylpropene (CH3)2C=CH2 484 2-methyl-2-butene (CH3)2C=CHCH3 6526 More highly substituted double bonds react faster. Alkyl groups on the double bond make it more “electron rich.” 6

Mechanism of Epoxidation 6

Mechanism of Epoxidation 6

Mechanism of Epoxidation 6

Mechanism of Epoxidation 6

Mechanism of Epoxidation 6

Ozonolyis has both synthetic and analytical applications. Ozonolysis of Alkenes Ozonolyis has both synthetic and analytical applications. synthesis of aldehydes and ketones identification of substituents on the double bond of an alkene 15

Ozonolysis of Alkenes First step is the reaction of the alkene with ozone. The product is an ozonide. C C O + O3 17

Ozonolysis of Alkenes Second step is hydrolysis of the ozonide. Two aldehydes, two ketones, or an aldehyde and a ketone are formed. C C O + O3 H2O, Zn C O + C O 17

Ozonolysis of Alkenes As an alternative to hydrolysis, the ozonide can be treated with dimethyl sulfide. C C O + O3 (CH3)2S C O C O + 17

Example CH3 CH2CH3 H C 1. O3 2. H2O, Zn CH2CH3 C O C O CH3 H + (57%) (38%) 19

Introduction to Organic Chemical Synthesis 1

Prepare cyclohexane from cyclohexanol devise a synthetic plan reason backward from the target molecule always use reactions that you are sure will work 6

Prepare cyclohexane from cyclohexanol ask yourself the key question "Starting with anything, how can I make cyclohexane in a single step by a reaction I am sure will work?" 6

Prepare cyclohexane from cyclohexanol Pt The only reaction covered so far for preparing alkanes is catalytic hydrogenation of alkenes. This leads to a new question. "Starting with anything, how can I prepare cyclohexene in a single step by a reaction I am sure will work?" 6

Prepare cyclohexane from cyclohexanol H2SO4 heat H2 OH Pt Alkenes can be prepared by dehydration of alcohols. The synthesis is complete. 6

Prepare 1-bromo-2-methyl-2-propanol from tert-butyl alcohol (CH3)3COH (CH3)2CCH2Br OH "Starting with anything, how can I make the desired compound in a single step by a reaction I am sure will work?" The desired compound is a vicinal bromohydrin. How are vicinal bromohydrins prepared? 6

Prepare 1-bromo-2-methyl-2-propanol from tert-butyl alcohol (CH3)2C CH2 (CH3)2CCH2Br OH H2O Vicinal bromohydrins are prepared by treatment of alkenes with Br2 in water. How is the necessary alkene prepared? 6

Prepare 1-bromo-2-methyl-2-propanol from tert-butyl alcohol (CH3)3COH H2SO4 heat (CH3)2CCH2Br OH Br2 H2O (CH3)2C CH2 2-Methylpropene is prepared from tert-butyl alcohol by acid-catalyzed dehydration. The synthesis is complete. 6

Reactions of Alkenes with Alkenes: Polymerization 1

Polymerization of alkenes cationic polymerization free-radical polymerization coordination polymerization 2

Cationic Polymerization Dimerization of 2-methylpropene (CH3)2C CH2 monomer (C4H8) H2SO4 two dimers (C8H16) CH3CCH CH3 C(CH3)2 CH3CCH2C CH3 CH2 + 3

Mechanism of Cationic Polymerization CH3C CH3 + CH3 H2C C + H 3

Mechanism of Cationic Polymerization + CH3 H2C C CH3C CH3CCH2C CH3 + 3

Mechanism of Cationic Polymerization CH3CCH CH3 C(CH3)2 CH3CCH2C CH3 CH2 + CH3CCH2C CH3 + 3

Free-Radical Polymerization of Ethylene H2C CH2 200 °C 2000 atm O2 peroxides CH2 polyethylene 6

.. RO Mechanism • H2C CH2 7

.. RO: Mechanism H2C CH2 • 9

.. RO: Mechanism H2C CH2 • H2C CH2 10

.. RO: Mechanism H2C CH2 H2C CH2 • 10

.. RO: Mechanism H2C CH2 H2C CH2 • H2C CH2 10

.. RO: Mechanism H2C CH2 H2C CH2 H2C CH2 • 10

.. RO: Mechanism H2C CH2 H2C CH2 H2C CH2 • H2C CH2 10

Free-Radical Polymerization of Propene H2C CHCH3 CH H CH3 polypropylene 6

.. RO Mechanism • H2C CHCH3 7

.. RO: Mechanism H2C CHCH3 • 9

.. RO: Mechanism H2C CHCH3 • H2C CHCH3 10

.. RO: Mechanism H2C CHCH3 H2C CHCH3 • 10

.. RO: Mechanism H2C CHCH3 H2C CHCH3 • H2C CHCH3 10

.. RO: Mechanism H2C CHCH3 H2C CHCH3 H2C CHCH3 • 10

.. RO: Mechanism H2C CHCH3 H2C CHCH3 H2C CHCH3 • H2C CHCH3 10

H2C=CHCl  polyvinyl chloride H2C=CHC6H5  polystyrene F2C=CF2  Teflon 19