Chapter 8
Reactions of Alkenes
08-00-02UN

Labeled
Title
 Reactions of Alkenes
Caption
 Hydrogenation of an alkene is an example of an addition, one of the three major reaction types we have studied: addition, elimination, and substitution. In an addition, two molecules combine to form one product molecule. When an alkene undergoes addition, two groups add to the carbon atoms of the double bond, and the carbons become saturated. In many ways, addition is the reverse of elimination, in which one molecule is split into two fragment molecules. In a substitution, one fragment replaces another fragment in a molecule.
Notes
 The reactions that alkenes undergo fall into one of three types: addition, elimination and substitution.
Keywords
 addition, elimination, substitution
08-01

Labeled
Title
 Bonding in Alkenes
Caption
 Figure 8-1 The electrons in the pi bond are spread farther from the carbon nuclei than the sigma electrons, and they are more loosely held.
Notes
 The sigma bond is formed by overlapping the sp2 hybrid orbitals. The unhybridized p orbital of each carbon has one electron so they overlap forming a pi bonding orbital. The pi orbital has half a lobe above the sigma bond and the other half below the sigma bond.
Keywords
 sp2 hybridized orbitals, pi orbitals, sigma bond
08-01-01UN

Labeled
Title
 Mechanism of Addition to Alkenes
Caption
 In most additions, a nucleophile attacks the carbocation (as in the second step of the SN1 reaction), forming a stable addition product. In the product, both the electrophile and the nucleophile are bonded to the carbon atoms that were in the double bond. The following schematic reaction uses E1 as the electrophile and Nuc:2 as the nucleophile.
Notes
 The pi electrons are not held as strongly as the sigma electrons so they can be attracted by an electrophile. In the first step of the mechanism the pi electrons attack an electrophile forming a carbon-electrophile bond. The formation of the new bond breaks the double bond and creates a carbocation. On the second step of the addition reaction, a nucleophle present in solution will add to the carbocation to give the product.
Keywords
 nucleophile, electrophile, pi electrons, sigma electrons
08-02

Labeled
Title
 Orbital View of Alkene Addition
Caption
 Figure 8-2 A strong electrophile pulls the electrons out of the pi bond to form a new sigma bond. A carbocation results. The (red) curved arrow shows the movement of electrons, from the electron-rich pi bond to the electron-poor electrophile.
Notes
 The pi electrons are not held as strongly as the sigma electrons so they can be attracted by an electrophile. In the first step of the mechanism the pi electrons attack an electrophile forming a carbon-electrophile bond. The formation of the new bond breaks the double bond and creates a carbocation. On the second step of the addition reaction, a nucleophile present in solution will add to the carbocation to give the product.
Keywords
 pi electrons, electrophile, nucleophile
08-02-01UN

Labeled
Title
 Addition of HBr to an Alkene
Caption
 Bromide ion reacts rapidly with the carbocation to give a stable product in which the elements of HBr have added to the ends of the double bond.
Notes
 In the presence of hydrogen halides, the double bond will attack the proton of HBr (the electrophile) forming a carbocation intermediate. The bromide ion will then add to the carbocation in the second step. The net result is the addition of HBr across the double bond.
Keywords
 electrophile, nucleophile, carbocation
08-02-03T01

Labeled
Title
 Types of Additions to Alkenes
Caption
 Table 8-1 summarizes the classes of additions we will cover. Note that the table shows what elements have added across the double bond in the final product, but it says nothing about reagents or mechanisms. As we study these reactions, you should note the regiochemistry of each reaction, also called the orientation of addition, meaning which part of the reagent adds to which end of the double bond. Also note the stereochemistry if the reaction is stereospecific.
Notes
Keywords
 hydration, hydrogenation, hydroxylation, cleavage, epoxidation, halogenation, halohydrin, cyclopropanation.
08-02-05UN

Labeled
Title
 Markovnikov's Rule
Caption
 The first step is protonation of the double bond. If the proton adds to the secondary carbon, the product will be different from the one formed if the proton adds to the tertiary carbon.
Notes
 Markovnikov's rule states that the proton will add to the least subsituted carbon, i.e., the carbon with the most hydrogens. This type of addition will create the most substituted more stable carbocation.
Keywords
 addition, Markovnikov's rule
08-03

Labeled
Title
 Addition Follows Markovnikov's Rule
Caption
 Figure 8-3 An electrophile adds to the less substituted end of the double bond to give the more highly substituted (and therefore more stable) carbocation.
Notes
 Markovnikov's rule ensures the formation of the most stable carbocation on protonation of the double bond.
Keywords
 addition, Markovnikov's rule, carbocation
08-03-03

Labeled
Title
 Free Radical Addition of HBr to Alkenes
Caption
 Alkoxy radicals catalyze the anti-Markovnikov addition of HBr.
Notes
 In the radical mechanism the addition of the free radical of Br occurs first, adding to the less substituted carbon to produce a stable radical. The radical formed in the first step will react with a second molecule of HBr and abstract a hydrogen radical to give the anti-Markovnikov product. This reaction only works for HBr in the presence of peroxides. HCl and HI will add according to Markovnikov's rule in the presence of peroxides.
Keywords
 Markovnikov, free radical, initiation, propagation, peroxides
08-03-16UN1-2

Labeled
Title
 Hydration of Alkenes and Dehydration of Alcohols
Caption
 An alkene may react with water in the presence of a strongly acidic catalyst to form an alcohol.
Notes
 Water can be added across the double bond in a reaction known as hydration. Hydration reactions produce alcohols. This is the reverse reaction of the dehydration of alcohols. A dehydration is the elimination of water from an alcohol to produce an alkene.
Keywords
 hydration, dehydration, pathway, reversible
08-03-17UN

Labeled
Title
 Mechanism of Acid-Catalyzed Hydration of Alkenes
Caption
 According to the principle of microscopic reversibility, we can write the hydration mechanism by reversing the steps of the dehydration.
Notes
 The hydration of alkenes occurs in three steps. The first step is the protonation of the double bond to form a carbocation. This step follows Markovnikov's rule so the most stable carbocation will be formed. The carbocation is attacked by a water molecule in the second step of the reaction. The final step is the deprotonation of the alcohol to produce a neutral alcohol. Since there is a carbocation intermediate, rearrangements such as hydride and methyl shifts should be expected to occur.
Keywords
 protonation, Markovnikov's rule, carbocation, deprotonation, rearrangements, hydride shift, methyl shift
08-03-21UN

Labeled
Title
 Oxymercuration-Demercuration Reaction
Caption
Notes
 Oxymercuration-demercuration is a different way of hydrating an alkene and obtain the Markovnikov product. The reaction is milder than normal hydration conditions and since there are no carbocation intermediates no rearrangements are possible.
Keywords
 oxymercuration, demercuration, rearrangments
08-03-23UN

Labeled
Title
 Mechanism of Oxymercuration-Demercuration Reaction
Caption
Notes
 The mechanism of oxymercuration has two steps. The first step is the addition of the positively charged mercury species to the double bond to form a mercurinium ion. In the second step of the reaction, the water opens the mercurinium ion and loses a proton to give an organomercurial alcohol.
Keywords
 mercuric acetate, mercurinium ion, oxymercuration, organomercurial alcohol
08-03-25UN

Labeled
Title
 Demercuration Reaction
Caption
Notes
 In the demercuration reaction a hydride furnished by the sodium borohydride (NaBH4) replaces the mercuric acetate. The oxymercuration-demercuration reaction gives the Markovnikov product with the hydroxy group on the most substituted carbon.
Keywords
 demercuration, Markovnikov, hydride
08-03-26UN

Labeled
Title
 Oxymercuration-Demercuration of 2-Methyl-2-butene
Caption
Notes
 Oxymercuration-demercuration of 2-methyl-2-butene produces 2-methyl-2-butanol in 90% yield. Notice that the product is the Markovnikov addition of water across the double bond.
Keywords
 oxymercuration-demercuration, Markovnikov product
08-03-28UN

Labeled
Title
 Alkoxymercuration-Demercuration Reaction
Caption
Notes
 If the oxymercuration reaction is carried out in the presence of alcohols, it will be the alcohol that will open the mercurinium ion formed in step one. The final product after demercuration is an ether with Markovnikov orientation.
Keywords
 oxymercuration, demercuration, ether
08-03-34UN

Labeled
Title
 Structure of Diborane
Caption
Notes
 Borane (BH3) is usually found in its dimeric form in which two molecules of borane are joined by two bridging hydrogens.
Keywords
 borane, diborane
08-03-36UN

Labeled
Title
 Hydroboration-Oxydation Reaction
Caption
Notes
 The net result of the hydroboration-oxidation reaction is the addition of water across the double bond in an anti-Markovnikov fashion. The reaction has a syn stereochemistry where the OH and the hydrogen will be added to the same side of the double bond.
Keywords
 hydroboration, oxidation, syn
08-04

Labeled
Title
 Mechanism of Hydroboration
Caption
 Borane adds to the double bond in a single step, with boron adding to the less highly substituted carbon and hydrogen adding to the more highly substituted carbon. This orientation places the partial positive charge in the transition state on the more highly substituted carbon atom.
Notes
 Addition of the borane is Markovnikov and it goes through a four-membered transition state where the two of new bonds are forming at the same time that two bonds are breaking.
Keywords
 hydroboration
08-04-01UN

Labeled
Title
 Oxidation of the Boron Atom
Caption
Notes
 The boron atom can be removed by oxidizing it with hydrogen peroxide (H2O2) in the presence of aqueous base. A hydroxide will take the place of the boron. The hydroboration-oxidation reaction is anti-Markovnikov since the alcohol ends up in the less substituted carbon of the double bond.
Keywords
 oxidation, hydrogen peroxide
08-04-11UN

Labeled
Title
 Hydrogenation of Alkenes
Caption
Notes
 Hydrogen (H2) can be added across the double bond in a process known as catalytic hydrogenation. The reaction only takes place if a catalyst is used. The most commonly used catalysts are palladium (Pd), platinum (Pt), and nickel (Ni), but there are other metals that work just as well. Hydrogenation reduces the double bond.
Keywords
 catalytic hydrogenation, catalyst, reduction
08-05

Labeled
Title
 Mechanism of Catalytic Hydrogenation
Caption
Notes
 In a catalytic hydrogenation both substrates, the hydrogen and the alkene, need to be adsorbed on the metal surface. Once adsorbed the hydrogens insert across the same face of the double bond and the reduced product is released from the metal. The reaction has a syn stereochemistry since both hydrogens will add to the same side of the double bond.
Keywords
 catalytic hydrogenation, adsorption, syn stereochemistry
08-06-04UN

Labeled
Title
 Cyclopropanation Reaction
Caption
Notes
 A carbene is an intermediate in which the carbon has two bonds and two non-bonded electrons. Carbenes can add to double bonds to form cyclopropane rings.
Keywords
 carbene, cyclopropane
08-06-08UN

Labeled
Title
 Simmons-Smith Reaction
Caption
Notes
 The Simmons-Smith reaction is is considered the best way to carry out a cyclopropanation reaction. The reaction uses iodomethyl zinc iodide (ICH2ZnI) as the source of the third carbon. This reagent, known as the Simmons-Smith reagent, is not a carbene but it reacts like it so it is called a carbenoid.
Keywords
 carbene, carbenoid, cyclopropanation, Simmons-Smith reagent
08-06-10UN

Labeled
Title
 Synthesis of Carbenes by Alpha Elimination
Caption
Notes
 Bromoform can be dehalogenated once to form dibromocarbene by using strong bases such as potassium hydroxide.
Keywords
 dibromocarbene, carbene
08-06-17UN

Labeled
Title
 Addition of Halogens to Alkenes
Caption
Notes
 Halogens can be added to the double bond to form vicinal dihalides. The stereochemistry of this addition is anti, i.e., the halogen atoms are added to opposite sides of the double bond.
Keywords
 vicinal dihalides, anti addition
08-06-20UN

Labeled
Title
 Mechanism of Halogen Addition to Alkenes
Caption
Notes
 The double bond acts as a nucleophile and attacks the bromine molecule, displacing a bromide ion. A three membered ring bromonium ion is formed. The second step of the reaction is the opening of the bromonium ion ring by a bromide ion. The bromide ion attacks the ring from the back side of the leaving group.
Keywords
 bromonium ion, bromide ion
08-06-22UN

Labeled
Title
 Stereochemistry of the Halogen Addition to Alkenes
Caption
Notes
 Once the bromonium ion has been formed, a bromide ion attacks it from the back side and opens the ring to give a stable vicinal dibromide. The bromines are anti to each other so the reaction is stereospecific.
Keywords
 bromonium ion, stereospecific reaction, anti addition
08-07-04UN

Labeled
Title
 Halohydrin Formation
Caption
Notes
 An alcohol with a halogen on the vicinal carbon is called a halohydrin. Halohydrins are obtained when halogenations are carried out in aqueous solution. The first step of the reaction is analogous to the first step of halogenation, the double bond attacks a molecule of the halide to form a three membered intermediate halonium ion. In the second step of the reaction a water molecule nucleophilically attacks the halonium ring opening it. This attack occurs from the back side of the halonium ion ring so the halogen and the alcohol groups will be anti to each other in the product.
Keywords
 halohydrins, vicinal, halonium ion, anti
08-07-05UN

Labeled
Title
 Reaction of Cyclopentene to Form a Halohydrin
Caption
Notes
 Cyclopentene reacts with bromine in aqueous solution to give trans-2-bromocyclopentanol (a halohydrin), and its corresponding enantiomer.
Keywords
 cyclopentene, halohydrin, enantiomer
08-08-03UN

Labeled
Title
 Epoxidation of Alkenes
Caption
Notes
 Peroxyacids will react with double bonds to form epoxides. Epoxides, also known as oxiranes, are three-membered cyclic ethers. This is an oxidation reaction because the number of C-O bonds is increased.
Keywords
 epoxide, oxirane
08-08-05UN

Labeled
Title
 Mechanism of Epoxidation of Alkenes
Caption
Notes
 The peroxyacid and the alkene react with each other in a one-step reaction that produces the epoxide and a molecule of acid. The most common peroxyacid used is meta-chloroperoxybenzioc acid (MCPBA).
Keywords
 peroxyacid, epoxide, MCPBA
08-08-06UN

Labeled
Title
 Stereochemistry of Epoxidation
Caption
Notes
 The addition is syn, so the stereochemistry of the starting material will be preserved in the product, i.e., a cis alkene will produce a cis epoxide, and a trans alkene a trans epoxide.
Keywords
 cis, trans, stereochemistry, epoxide
08-08-07UN

Labeled
Title
 Acid-Catalyzed Opening of Epoxides
Caption
Notes
 An epoxide ring can be opened by a water molecule. The water molecule will attack on the back side of the ring. The final product after deprotonation is a glycol or vicinal diol with an anti orientation.
Keywords
 epoxide, glycol, vicinal diol
08-09-02UN

Labeled
Title
 Syn Hydroxylation of Alkenes
Caption
Notes
 Osmium tetroxide and potassium pemanganate can be used to oxidize a double bond to the vicinal diol. This is a syn addition across the double bond.
Keywords
 osmium tetroxide, potassium permanganate, vicinal diol, syn addition
08-09-04UN

Labeled
Title
 Mechanism of Hydroxylation with OsO4
Caption
Notes
 The osmium tetroxide adds to the double bond of an alkene in a concerted mechanism forming an osmate ester. The osmate ester can be hydrolized to produce a cis-glycol and regenerate the osmium tetroxide.
Keywords
 osmium tetroxide, concerted mechanism, osmate ester, glycol
08-10

Labeled
Title
 Oxidative Cleavage of Alkenes with KMnO4
Caption
Notes
 The reaction of KMnO4 with alkenes will depend on the condition under which the reaction is carried out. Keeping the reaction cold will produce the cis-glycol. Heat or acid will promote the oxidative cleavage (breaking) of the double bond to form ketones and/or aldehydes. The aldehydes will react further with KMnO4 to produce carboxylic acids.
Keywords
 oxidative cleavage
08-10-01UN

Labeled
Title
 Ozonolysis of Alkenes
Caption
Notes
 Ozone will oxidatively cleave (break) the double bond to produce aldehydes and ketones. Ozonolysis is milder than KMnO4 and will not oxidize aldehydes further. A second step of the ozonolysis is the reduction of the intermediate by zinc or dimethyl sulfide.
Keywords
 ozone, ozonolysis, oxidative cleavage, reduction
08-10-03UN

Labeled
Title
 Mechanism of Ozone Addition
Caption
Notes
 The ozone adds to the double bond forming a 5-membered ring intermediate called molozonide, which rearranges to form the ozonide.
Keywords
 ozonolysis, molozonide, ozonide
08-10-04UN

Labeled
Title
 Reduction of the Ozonide
Caption
Notes
 The ozonide is not isolated but immediately reduced by a mild reducing agent such as zinc or dimethyl sulfide to give the aldehydes and ketones as the main products. When dimethyl sulfide is used the sulfur atom gets oxidized, forming dimethyl sulfoxide (DMSO).
Keywords
 ozonide, reducing agent, dimethyl sulfoxide
08-10-16UN

Labeled
Title
 Cationic Polymerization of Isobutylene
Caption
Notes
 The polymerization of isobutylene starts with the protonation of the alkene with an acid forming a carbocation. A second molecule of isobutylene attacks the carbocation forming a dimer. A third molecule of alkene attacks the carbocation forming a trimer. The subsequent addition of isobutylene molecules to the cationic polymer lengthens the chain.
Keywords
 polymerization, cationic polymerization,
08-10-17UN

Labeled
Title
 Termination Step of Cationic Polymerization
Caption
Notes
 The chain growth ends when a proton is abstracted by the weak base of the acid used to initiate the reaction. The loss of a hydrogen forms an alkene and ends the chain growth so this is a termination step.
Keywords
 termination, chain growth
08-10-18UN

Labeled
Title
 Cationic Polymerization Using BF3 as Catalyst
Caption
Notes
 Boron trifluoride (BF3) can be used to initiate a polymerization. Boron will add to the double bond to give the more stable carbocation. Each additional molecule of alkene adds to the previous one in the same orientation lengthening the chain.
Keywords
 boron trifluoride, polymerization
08-10-20UN

Labeled
Title
 Free Radical Polymerization
Caption
Notes
 Peroxides are used to initiate radical polymerization reactions. Upon heating, an alkyl peroxide will break homolytically to form 2 molecules of radicals. The radical adds to the double bond of the alkene creating a carbon free radical. The chain grows as more molecules of alkene adds to the radical end of the chain. The reaction stops either by coupling with another chain, by impurities or by running out of monomer.
Keywords
 peroxides, radical polymerization
08-10-23UN

Labeled
Title
 Anionic Polymerization
Caption
Notes
 The hydroxide ion attacks the double bond of an alkene creating a carbanion. The intermediate carbanion is stabilized by electron-withdrawing groups on the carbon bearing the negative charge. The propagation of the chain occurs as the anion formed attacks a second molecule of alkene forming a dimer. The chain grows as more and more alkene molecules are added to the chain.
Keywords
 carbanion, anion
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