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1 CHAPTER 12 Reactions of Alkenes Why Addition Reactions Proceed: Thermodynamic Feasibility 12-1 Because the C-C π bond is relatively weak, alkene chemistry is dominated by its reactions. The addition of a reagent, A-B, to give a saturated compound is the most common transformation of an alkene. ΔHo for the above reaction can be estimated from the relevant bond energies: ΔHo = (DHo π bond + DHo A-B) – (DHo C-A + DHo C-B) Most additions to alkenes should proceed to products with the release of energy. 12-2 Catalytic Hydrogenation Hydrogenation takes place on the surface of a heterogeneous catalyst. In the absence of a catalyst, hydrogenations of alkenes, although exothermic, do not spontaneously occur, even at high temperatures. In the presence of a catalyst, the same hydrogenations proceed at a steady rate, even at room temperature. The most frequently used catalysts for hydrogenation reactions are: •Palladium dispersed on carbon (Pd-C) •Collodial platinum (Adam’s catalyst, PtO2) •Nickel (Raney nickel, Ra-Ni) The primary function of a catalyst in hydrogenation reactions is to provide metal-bound hydrogen atoms on the catalyst surface. Common solvents used for hydrogenations include methanol, ethanol, acetic acid, and ethyl acetate. Hydrogenation is stereospecific. During a hydrogenation reaction, both atoms of hydrogen are added to the same face of the double bond (syn addition). In the absence of steric hindrance, addition to either face of the double bond can occur with equal probability which results in a racemic mixture of products
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2 Nucleophilic Character of the Pi Bond: Electrophilic Addition of Hydrogen Halides 12-3 The π electrons of a double bond are more loosely held than those of the σ bond. As a result, the π electrons, which extend above and below the molecular plane of the alkene, can act as a nucleophile in a manner similar to that of more typical Lewis bases. 2,3-dimethylbutene The electrophilic addition reactions of alkenes can be both regioselective and stereospecific. Electrophilic attack by protons gives carbocations. A strong acid may add a proton to a double bond to give a carbocation. This reaction is simply the reverse of the last step in an E1 elimination reaction and has the same transition state. At low temperatures and with a good nucleophile, an electrophilic addition product is formed. Typically, the gaseous HX (HCl, HBr, or HI) is bubbled through the pure or dissolved alkene. The reaction can also be carried out in a solvent such as acetic acid. The Markovnikov rule predicts regioselectivity in electrophilic additions. The only product formed during the reaction of propene with HCl is 2-chloropropane: Other addition reactions show similar results: If the carbon atoms participating in the double bond are not equally substituted, the proton from the hydrogen halide attaches itself to the less substituted carbon. As a result, the halogen attaches to the more substituted carbon. This result is known as “Markovnikov’s rule” and is based on the stability of the carbocation formed by the addition of the proton. Markovnikov’s rule can also be stated: HX adds to unsymmetric alkenes in a way that the initial protonation gives the more stable carbocation. Product mixtures will be formed from alkenes that are similarly substituted at both sp2 carbon atoms. If addition to an achiral alkene generates a chiral product, a racemic mixture will be obtained. Carbocation rearrangements may follow electrophilic addition. In the absence of a good nucleophile, a rearrangement of the carbocation may occur prior to the addition of the nucleophile. An example of such a rearrangement is the addition of trifluoroacetic acid to 3-methyl-1-butene, where a hydride shift converts a secondary carbocation into a more stable tertiary carbocation:
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3 The extent of carbocation rearrangement depends upon: •alkene structure •solvent •strength and concentration of nucleophile •temperature Rearrangements are generally favored under strongly acidic, nucleophile-deficient conditions. Alcohol Synthesis by Electrophilic Hydration: Thermodynamic Control 12-4 When other nucleophiles are present, they may also attack the intermediate carbocation. Electrophilic hydration results when an alkene is exposed to an aqueous solution of sulfuric acid (HSO4 - is a poor nucleophile). The addition of water by electrophilic hydration follows Markovnikov’s rule, however carbocation rearrangements can occur because water is a poor nucleophile. The electrophilic hydration process is the reverse of the acidinduced elimination of water (dehydration) of alcohols previously discussed. Alkene hydration and alcohol dehydration are equilibrium processes. In the absence of protons, alkenes are stable in water. The position of the equilibrium in the hydration reaction can be changed by adjusting the reaction conditions. All steps are reversible in the hydration of alkenes. The proton serves as a catalyst only: it is regenerated in the reaction. The reversibility of alkene protonation leads to alkene equilibration. Protonation-deprotonation reactions may interconvert related alkenes and produce an equilibrium mixture of isomers. Under these conditions, a reaction is said to be under thermodynamic control. This mechanism can convert less stable alkenes into their more stable isomers:
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4 12-5 Electrophilic Addition of Halogens to Alkenes Halogen molecules also act as electrophiles with alkenes giving vicinal dihalides. The reaction with bromine results in a color change from red to colorless, which is sometimes used as a test for unsaturation. Halogenations are best carried out at or below room temperature and in inert halogenated solvents (i.e. halomethanes) 12-5 Electrophilic Addition of Halogens to Alkenes Bromination takes place through anti addition. Consider the bromination of cyclohexene. No cis-1,2- dibromocyclohexane is formed. Only anti addition is observed. The product is racemic since the initial attack of bromine can occur with equal probability at either face of the cyclohexene. With acyclic alkenes the reaction is cleanly stereospecific: Cyclic bromonium ions explain the stereochemistry. The polarizability of the Br-Br bond allows heterolytic cleavage when attacked by a nucleophile, forming a cyclic bromonium ion: The bridging bromine atoms serves as the leaving group as the bromonium ion is attacked from the bottom by a Br- ion. In symmetric bromonium ions, attack is equally probable at either carbon atom leading to racemic or meso products. 12-6 The Generality of Electrophilic Addition The bromonium ion can be trapped by other nucleophiles. Bromonation of cyclopentene using water as the solvent gives the vicinal bromoalcohol (bromohydrin). The water molecule is added anti to the bromine atom and the other product is HBr. Vicinal haloalcohols are useful synthetic intermediates
ced if an alcohol is used as the elec ive. - suic i27OmeCtomcemraionAspecal y h 合Letoana9eweeb2Cdaaaaiemdemercura5om Q 5
5 Vicinal haloethers can be produced if an alcohol is used as the solvent, rather than water. Halonium ion opening can be regioselective. Mixed additions to double bonds can be regioselective: The nucleophile attacks the more highly substituted carbon of the bromonium ion, because it is more positively polarized. Electrophilic additions of unsymmetric reagents add in a Markovnikov-like fashion: The electrophilic unit becomes attached to the less substituted carbon of the double bond. Mixtures of products are formed only when the two carbons are not sufficiently differentiated. Reagents of the type A-B, in which A acts as the electrophile, A+, and B the nucleophile, B- , can undergo stereo- and regiospecific addition reactions to alkenes: Oxymercuration-Demercuration: A Special Electrophilic Addition 12-7 The electrophilic addition of a mercuric salt to an alkene is called mercuration. The product formed is known as an alkylmercury derivative. A reaction sequence known as “oxymercuration-demercuration” is a useful alternative to acid-catalyzed hydration: Oxymercuration is anti stereospecific and regioselective. The alcohol obtained from oxymercuration-demercuration is the same as that obtained from Markovnikov hydration, however, since no carbocation is involved in the reaction mechanism, rearrangements of the transition state do not occur
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6 Oxymercuration-demercuration in an alcohol solvent yields an ether: Hydroboration-Oxidation: A Stereospecific AntiMarkovnikov Hydration 12-8 The boron-hydrogen bond adds across double bonds. Borane, BH3, adds to double bonds without catalytic activation: The borane is commercially available in an ethertetrahydrofuran solvent. Because the borane is electron poor, and the alkene is electron rich, an initial Lewis acid-base complex similar to the bromonium ion can form: Because of the four center transition state, the addition reaction is syn. All three B-H bonds can react. Hydroboration is regioselective as well as stereospecific (syn addition). Here, steric factors are more important than electronic factors. The boron binds to the less hindered (substituted) carbon. The oxidation of alkylboranes gives alcohols. The oxidation of a trialkylborane by hydrogen peroxide produces an alcohol in which the hydroxyl group has replaced the boron atom. In this reaction, the hydroxyl group ends up at the less substituted carbon: an anti-Markovnikov addition. During the oxidation, an alkyl group migrates with its electron pair (with retention of configuration) to the neighboring oxygen atom. After all three alkyl groups have migrated to oxygen atoms, the trialkyl borate is hydrolyzed by base to the alcohol and sodium borate
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7 Hydroboration-oxidation of alkenes allows stereospecific and regioselective synthesis of alcohols. The reaction sequence exhibits anti-Markovnikov regioselectivity which complements acid-catalyzed hydration and oxymercuration-demercuration. The reaction mechanism does not involve a carbocation and thus rearrangements are not observed. Diazomethane, Carbenes and Cyclopropane Synthesis 12-9 Cyclopropanes can be readily prepared by the addition of a carbene to the double bond of an alkene. A carbene has the general structure, R2C:, in which the central carbon is surrounded by six electrons (sextet), and is thus electron deficient. The electron-deficient carbene readily adds to an electron rich alkene. Diazomethane forms methylene, which converts alkenes into cyclopropanes. The highly reactive species methylene, H2C: (the simplest carbene) can be produced from the decomposition of diazomethane: When methylene is generated in the presence of an alkene, an addition reaction occurs producing a cyclopropane. This reaction is usually stereospecific, with retention of the original double bond configuration. Halogenated carbenes and carbenoids also give cyclepropanes. Halogenated carbenes, prepared from halomethanes, can also be used to synthesize cyclopropanes. Treatment of trichloromethane (chloroform) with strong base causes an elimination reaction in which both a proton and a chlorine atom are removed from the same carbon. The resulting product is a dichlorocarbene which reacts with alkenes to produce cyclopropanes. To avoid the hazards associated with diazomethane preparation, an alternate route using diiodomethane and zinc (Simmons-Smith reagent) to produce ICH2ZnI is used. This substance is an example of a carbenoid, a carbenelike substance that converts alkenes into cyclopropanes stereospecifically. Oxacyclopropane (Epoxide) Synthesis: Epoxidation by Peroxycarboxylic Acids 12-10 Oxacyclopropanes contain a single oxygen atom connected to two carbons to form a three-membered ring. Oxacyclopropanes may be converted into vicinal anti diols
dellver Otrehenm5gn8de,5aog8etanpaate acids have the general formula of the xacYeproaneandacartomyti Oxacycleprepane Fermatien:Epecidation of a Double Bond 一· e9oYaYkcoearoaeeu8abaotnceasc CH. wn-3-Bam 2 da-m, 12-11 Vicinal Syn Dihydroxylaion with Osmium peentenehtiheno3teeeaedadtonofthe Viinal Sm Dhydrosstation with Osmium Tetroxid
8 Peroxycarboxylic acids deliver oxygen atoms to double bonds. Peroxycarboxylic acids have the general formula: These compounds react with double bonds because one of the oxygen atoms is electrophilic. The resulting products are an oxacyclopropane and a carboxylic acid. This reaction is referred to as an “epoxidation.” The older common name of an oxacyclopropane was an “epoxide.” Commonly used peroxycaraboxylic acids for this reaction are meta-chloroperoxybenzoic acid (MCPBA) which is somewhat shock sensitive, and magnesium monoperoxyphthalate (MMPP). The mechanism of this epoxidation reaction involves a cyclic transition state: The peroxycarboxylic acid reactivity with double bonds increases with alkyl substitution, allowing for selective oxidations: Hydrolysis of oxacyclopropanes furnishes the products of anti dihydroxylation of an alkene. Ring opening of oxacyclopropanes with water produces anti vicinal diols. Vicinal Syn Dihydroxylation with Osmium Tetroxide 12-11 The reaction of osmium tetroxide with alkenes yields syn vicinal diols in a two step process: The reaction mechanism involves the concerted addition of the osmium tetroxide to the π bond of the alkene: Catalytic amounts of osmium tetroxide in the presence of an oxidizing agent (H2O2) to regenerate the spent osmium tetroxide are often used, due to the expense and toxicity of OsO4
12-12 Oxidative Cleavage:Ozonolysis anate Test for Alkene Double Bonds og6eegao <·=一· HO: 28tya8t地o 6segh6 <一台· 12-13 Radical Additions:Anti-Markovnikov Product rkg9enk8Coaieonaatokenmeehnantt g品a6g CHCHCH-CH CHCHCH-CH :品： CH. CH.CH.CHCH. hain squence mechanis amesorfeapm%n0m00fomdnakome n or by some other rem nd我al lyndrendeefa 4·年” 一 9
9 An older reagent for vicinal syn dihydroxylation of alkenes is KMnO4. This reagent is less useful than OsO4 because of its tendency towards overoxidation. The deep purple KMnO4 is converted into a brown precipitate, (MnO2) during the reaction, which can serve as a useful test for the presence of alkenes. 12-12 Oxidative Cleavage: Ozonolysis The mildest reagent capable of breaking both the σ and π bonds in a double bond is ozone, O3. This process is known as “ozonolysis.” Ozone is produced by an electrical discharge in dry oxygen in a instrument called an ozonator. The initial product of the reaction of ozone with an alkene is an ozonide which is then directly reduced to two carbonyl products. The mechanism of ozonolysis proceeds through a molozonide, which breaks apart into two fragments, which then recombine to form the ozonide: Radical Additions: Anti-Markovnikov Product Formation 12-13 Hydrogen bromide can add to alkenes in antiMarkovnikov fashion: a change in mechanism. The reaction products from the treatment of 1-butene with HBr depend upon the presence or absence of molecular oxygen in the reaction mixture: In the presence of oxygen, a radical chain sequence mechanism leads to the anti-Markovnikov product. Small amounts of peroxides (RO-OR) are formed in alkene samples stored in the presence of air (O2). The peroxides initiate the radical chain sequence mechanism, which is much faster than the ionic mechanism operating in the absence of peroxides. The halogen’s attack is regioselective, generating the more stable secondary radical rather than the primary one. The alkyl radical subsequently abstracts a hydrogen from HBr which regenerates the chain-carrying bromine atom. Both propagation steps are exothermic. Termination is by radical recombination or by some other removal of the chain carriers. Commonly used peroxides for initiating radical additions include:
Are radical additions general? 12-14er ee 8en, mportance: CHCH-CH. Carbocations attack pi bonds. CH.-CCH % to the Markov ruet sso8ammcgeeuctiomeheradjacantcart veatnatokanleadtooigomeratonand C-0cc 。 10
10 Are radical additions general? HCl and HI do not give anti-Markovnikov addition products with alkenes. The chain propagation steps involving these hydrogen halides are endothermic, which leads to very slow reactions and chain termination. HCl and HI give Markovnikov products by ionic mechanisms regardless of the presence of radicals. Other reagents, such as thiols, do undergo successful radical additions to alkenes: Dimerization, Oligomerization, and Polymerization of Alkenes 12-14 Alkenes can react with one another in the presence of an appropriate catalyst: an acid, a radical, a base, or a transition metal. Polymer synthesis is of great industrial importance: Carbocations attack pi bonds. Protonation of 2-methylpropene by hot aqueous sulfuric acid leads to the formation of two dimers: The initial protonation produces a 1,1-dimethylethyl (tert-butyl) cation which then attacks the double bond of a second 2- methylpropene molecule. The cation addition proceeds according to the Markovnikov rule to generate the more stable carbocation. Deprotonation of the addition product from either adjacent carbon leads to a mixture of two products. Repeated attack can lead to oligomerization and polymerization. When 2-methylpropene is treated with mineral acid under more stringent conditions, higher oligiomers can be obtained through repeated addition reactions: At higher temperatures, polymers containing many subunits are formed