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John E. McMurry http://www.cengage.com/chemistry/mcmurry Chapter 8 Reactions of Alkenes and Alkynes Richard Morrison • University of Georgia, Athens Alkene Addition Reactions Alkene addition reactions Preparing Alkenes: A Preview of Elimination Reactions Preparation of alkenes: elimination reactions Precursors to alkenes • • Biological systems – usually alcohols Laboratory – either alcohols or alkyl halides Alkenes and alcohols are chemically related through addition and elimination reactions • • Alkenes add H2O to form alcohols Alcohols eliminate water to form alkenes Preparing Alkenes: A Preview of Elimination Reactions Dehydrohalogenation • Loss of HX from alkyl halide • Usually occurs by reaction of an alkyl halide with a strong base Preparing Alkenes: A Preview of Elimination Reactions Dehydration • Loss of water from an alcohol • Usually occurs by treatment of an alcohol with a strong acid Preparing Alkenes: A Preview of Elimination Reactions In biological pathways dehydrations normally take place on substrates in which –OH is positioned two carbons away from a carbonyl group Halogenation of Alkenes Halogenation • Addition reaction of alkenes • Addition of Br2 and Cl2 to alkenes to yield 1,2-dihalides Halogenation of Alkenes Halogenation of cycloalkenes • Only trans-stereoisomer of dihalide product is formed • Reaction occurs with anti stereochemistry – the two halogen atoms come from opposite faces of doublebond, one from top face and one form bottom face Halogenation of Alkenes Reaction occurs through an intermediate bromonium ion (R2Br+), formed by interaction of the alkene with Br2 and simultaneous loss of Br- Halogenation of Alkenes Bromonium ion shields one side of molecule so that reaction with Br- ion occurs only from opposite side Halohydrins from Alkenes Halohydrin Formation (electrophilic addition) • Reaction of alkenes with hypohalous acids HO-Cl or HO-Br to yield 1,2-halo alcohols called halohydrins Halohydrins from Alkenes • X2 reacts with alkene to give cyclic halonium ion intermediate • Intermediate halonium ion is intercepted by water nucleophile • Oxygen loses proton to give the neutral halohydrin product 8.4 Hydration of Alkenes Alkenes undergo an acid catalyzed addition reaction with water to yield alcohols • Hydration of ethylene is not of much use in the laboratory because of the high temperatures often required • Uncommon in biological pathways Hydration of Alkenes Laboratory hydrations of alkenes • Oxymercuration • Electrophilic addition of Hg2+ to alkene on treatment with mercury(II) acetate [(CH3CO2)2Hg, or Hg(OAc) 2] in aqueous tetrahydrofuran (THF) solvent • Reaction yields an alcohol • Product corresponds to Markovnikov regiochemistry (more highly substituted alcohol formed) Hydration of Alkenes • Hydroboration/oxidation • • • • • • Addition of a B-H bond of borane, BH3, to an alkene Occurs in single step No carbocation intermediate Reaction yields an alcohol Syn stereochemistry • Both C-H and C-B bonds form at the same time and from the same face of the double-bond Product has “anti”-Markovnikov regiochemistry Hydration of Alkenes Alkene Hydroboration Example Predicting the Products of a Hydration Reaction Solution Example Synthesizing an Alcohol How might you prepare the following alcohol? Worked Example 8.2 Synthesizing an Alcohol Strategy • To synthesize a specific target molecule work backwards • Look at target molecule • Identify functional group(s) • Devise a method for preparing functional group Worked Example 8.2 Synthesizing an Alcohol Solution Oxidation of Alkenes: Epoxidation Oxidation • A reaction that results in a loss of electron density by carbon Oxidation • Decreases electron density on carbon by • • Breaking C-H bond Forming C-O, C-N, or C-X bond Note: oxidation often adds oxygen; reduction often adds hydrogen Oxidation of Alkenes: Epoxidation Alkenes on treatment with a peroxyacid, RCO3H, are oxidized to give epoxides Epoxide (oxiranes) • Cyclic ethers with an oxygen atom in a three-membered ring Oxidation of Alkenes: Epoxidation Synthesis of epoxides from alkenes • Peroxyacid transfers oxygen to alkene • Syn stereochemistry • Both C-O bonds form on the same face of the double • One step mechanism • No intermediates Oxidation of Alkenes: Epoxidation Synthesis of epoxides from halohydrins • Preparation of halohydrin through electrophilic addition of HO-X to alkene • Treatment of halohydrin with base deprotonates OH • O- nucleophile reacts with C-Cl electrophile substituting CO bond for C-Cl bond • Cl- eliminated yielding the epoxide 8.7 Oxidation of Alkenes: Hydroxylation Hydroxylation • • The addition of an –OH group to each of the two double-bond carbons Two step process: 1. 2. Epoxidation Hydration • Epoxides undergo an acid-catalyzed reaction with water to give corresponding 1,2-dialcohol, or diol Oxidation of Alkenes: Hydroxylation Acid catalyzed epoxide-opening takes place by: Protonation of the epoxide increasing the electrophilicity of carbon 2. Nucleophilic addition of water followed by deprotonation • Trans-1,2-diol formed 1. Oxidation of Alkenes: Hydroxylation Hydroxylation in the Laboratory • • • • • Carried out directly by oxidation of an alkene with osmium tetroxide, OsO4 Catalytic amount of OsO4 used in the presence of stoichiometric amount of N-methylmorpholine N-oxide (NMO) Syn stereochemistry No carbocation intermediate Occurs through cyclic osmate intermediate 8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds Ozone (O3) is useful double-bond cleavage reagent • • • Ozone is generated by passing a stream of oxygen through a highvoltage electrical discharge Ozone adds rapidly to C=C bond at low temperature to give molozonide which spontaneously rearranges to ozonide Ozonide is treated with reducing agent to convert it to carbonyl compounds Oxidation of Alkenes: Cleavage to Carbonyl Compounds • • If tetrasubstituted double bond is ozonized, two ketone fragments result If a carbon of the alkene is bonded to hydrogen, ozonolysis will cleave the double bond to yield an aldehyde Oxidation of Alkenes: Cleavage to Carbonyl Compounds Potassium permanganate (KMnO4) in neutral or acidic solution cleaves alkenes to give carbonyl-containing products • • If a carbon of the alkene is bonded to hydrogen a carboxylic acid is produced If a carbon of the alkene is bonded to two hydrogens, CO2 is formed Oxidation of Alkenes: Cleavage to Carbonyl Compounds Alkenes are also cleaved by hydroxylation to a 1,2-diol followed by treatment with periodic acid, HIO4. • • If the two –OH groups of the diol are in an open chain, two carbonyl compounds result If the two –OH groups of the diol are on a ring, a single, open-chain dicarbonyl compound is formed Example Predicting the Reactant in an Ozonolysis Reaction What alkene would yield a mixture of cyclopentanone and propanal on treatment with ozone followed by reduction with zinc? Worked Example 8.3 Predicting the Reactant in an Ozonolysis Reaction Strategy • Reaction alkene with ozone, followed by reduction with zinc, cleaves the carbon-carbon double bond and gives two carbonyl-containing fragments • Working backward, the alkene precursor can be found by removing the oxygen from each product and joining the two carbon atoms to form a double bond Worked Example 8.3 Predicting the Reactant in an Ozonolysis Reaction Solution 8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis A carbene, R2C:, is a neutral molecule containing a divalent carbon with only six electrons in its valence shell • One simple method for generating dichlorocarbene is by treatment of CHCl3 with KOH • Carbenes behave as electrophiles, adding to alkenes to yield cyclopropanes Addition of Carbenes to Alkenes: Cyclopropane Synthesis Mechanism of the formation of dichlorocarbene Addition of Carbenes to Alkenes: Cyclopropane Synthesis • Dichlorocarbene carbon atom is sp2-hybridized with a vacant p orbital extending above and below the plane of the three atoms with an unshared pair of electrons occupying the third sp2 lobe Addition of Carbenes to Alkenes: Cyclopropane Synthesis • Reaction of dichlorocarbene with an alkene results in a dichlorocyclopropane • Addition is stereospecific, meaning that only a single stereoisomer is formed as product Radical Addition to Alkenes: Alkene Polymers Simplest polymerization • Result when an alkene is treated with a small amount of a radical as an initiator Radical Addition to Alkenes: Alkene Polymers Initiation 1. 2. 3. Small amount of benzoyl peroxide catalyst is heated breaking weak O-O bonds and yielding radicals Benzoyloxy radical adds to C=C bond of ethylene forming a carbon radical a) One electron from C=C bond pairs up with electron of benzoyloxy radical to form C-O bond b) Other electron remains on carbon (a carbon-centered radical) Radical Addition to Alkenes: Alkene Polymers Propagation • Polymerization occurs when the carbon radical adds to another ethylene molecule to yield another radical Termination • Chain process ends by a reaction that consumes a radical • Combination of two growing chains 2-R–CH2CH2 → R–CH2CH2CH2CH2–R Radical Addition to Alkenes: Alkene Polymers Vinyl monomers • Substituted ethylene Undergo polymerization to yield polymer with substituted groups regularly spaced in alternating carbon atom long chain • • Polypropylene • Styrene Radical Addition to Alkenes: Alkene Polymers Polymerization of unsymmetrically substituted vinyl monomers Propylene or Styrene • Radical addition steps can take place at either end of the double bond to yield: • • • • A primary radical intermediate (RCH2.) A secondary radical (R2CH.) Similar to electrophilic addition reaction More highly substituted, secondary radical is formed Worked Example 8.4 Predicting the Structure of a Polymer Show the structure of poly(vinyl chloride), a polymer made from H2C=CHCl, by drawing several repeating units Worked Example 8.4 Predicting the Structure of a Polymer Strategy • Mentally break the carbon-carbon double bond in the monomer unit, and form single bonds by connecting numerous units together Worked Example 8.4 Predicting the Structure of a Polymer Solution The general structure of poly(vinyl chloride) is Additions to Alkenes Polar Radical vs. Electrophilic Addition Reactions Electrophilic addition • • Reaction occurs once Intermediate is then quenched and reaction stops. Additions to Alkenes, Radicals Radical vs. Electrophilic Addition Reactions Radical addition • • • Difficult to control Limited use in the laboratory Reaction intermediate is not quenched so reaction continues Conjugated Dienes Sites of unsaturation • Many compounds have numerous sites of unsaturation • If sites are well separated in molecule they react independently • If sites are close together they may interact with one another Conjugated double bonds • Double bonds that alternate with single bonds Conjugated Dienes Heats of Hydrogenation Conjugated dienes are more stable than nonconjugated dienes Conjugated Dienes Buta-1,3-diene is approximately 16 kJ/mol (3.8 kcal/mol) more stable than expected Conjugated Dienes Explanations for conjugated diene stability 1) Valence Bond Theory • Stability due to orbital hybridization • Alkanes • • • C-C single bonds σ overlap of sp3 orbitals on both carbons Conjugated dienes • σ overlap of sp2 orbitals (shorter and stronger) 8.13 Reactions of Conjugated Dienes Conjugated dienes • Undergo electrophilic addition reactions readily • Mixture of products obtained • Addition of HBr to buta-1,3-diene yields mixture of two addition products Reactions of Conjugated Dienes Allylic carbocation is an intermediate • • Allylic means next to a double bond When buta-1,3-diene reacts with H+ electrophile two carbocation intermediates are possible: 1. 2. • A primary carbocation A secondary allylic carbocation (stabilized by resonance between two forms) Secondary allylic carbocation is more stable and forms faster than the nonallylic carbocation Reactions of Conjugated Dienes Allylic carbocation reacts with Br- to complete the electrophilic addition • Reaction can occur at C1 or C3 • Both carbons share positive charge • Mixture of 1,2- and 1,4-addition products results Example Predicting the Products of Electrophilic Addition to a Conjugated Diene The Diels-Alder Cycloaddition Reaction Conjugated dienes undergo reactions with alkenes to yield substituted cyclohexene products The Diels-Alder Cycloaddition Reaction Diels-Alder cycloaddition reaction is a Pericyclic reaction • Pericyclic reactions take place in a single step by a cyclic redistribution of bonding electrons The Diels-Alder Cycloaddition Reaction • In the Diels-Alder transition state, the two alkene carbons and carbons 1 and 4 of the diene rehybridize from sp2 to sp3 to form two new single bonds, while carbons 2 and 3 of the diene remain sp2 hybridized to from the new double bond in the cyclohexene product • Diels-Alder cycloaddition reaction occurs most rapidly if the alkene component, or dienophile (“diene lover”), has an electron-withdrawing substituent group The Diels-Alder Cycloaddition Reaction The Diels-Alder Cycloaddition Reaction • Diels-Alder reaction is stereospecific • Reactant stereochemistry is also maintained The Diels-Alder Cycloaddition Reaction • Diene must adopt an s-cis conformation, meaning “cis-like” about the single bond The Diels-Alder Cycloaddition Reaction • Some dienes cannot adopt the s-cis conformation and cannot undergo Diels-Alder cycloaddition reactions The Diels-Alder Cycloaddition Reaction • Some dienes are fixed in the s-cis conformation and are highly reactive in Diels-Alder cycloaddition reactions The Diels-Alder Cycloaddition Reaction • A few biological Diels-Alder reactions are known • Biosynthesis of lovastatin involves an intramolecular Diels- Alder reaction in the key step Worked Example 8.6 Predicting the Product of a Diels-Alder Reaction Predict the product of the following Diels-Alder reaction Worked Example 8.6 Predicting the Product of a Diels-Alder Reaction Strategy Draw the diene so that the ends of the two double bonds are near the dienophile double bond. Then form two single bonds between the partners, convert the three double bonds into single bonds, and convert the former single bond of the diene into a double bond. Because the dienophile double bond is cis to begin with, the two attached hydrogens must remain cis in the product Worked Example 8.6 Predicting the Product of a Diels-Alder Reaction Solution 8.15 Reactions of Alkynes Alkyne Addition Reactions • Alkynes behave similarly to alkenes • Alkynes are more reactive than alkenes • Various reactions can often be stopped at the monoaddition stage if one molar equivalent of reagent is used Reactions of Alkynes Reactions of Alkynes Reactions of Alkynes Alkyne acidity • Terminal alkynes (RC≡CH) are relatively acidic • RC≡CH treated with a strong base NaNH2 • Terminal hydrogen is removed forming and acetylide anion Reactions of Alkynes Alkyne acidity • BrØnsted-Lowry Acid • A substance that donates H+ • Acidity order: • Established by measuring acid dissociation constants and expressing the results as pKa values Low pKa = strong acid High pKa = weak acid • Amide ion (NH2-), the conjugated base of ammonia (pKa = 35), is often used to deprotonate terminal alkynes Reactions of Alkynes Reactions of Alkynes Terminal alkynes more acidic than alkenes or alkanes • Acetylide ions are more stable than vinylic (alkenyl) or alkyl ions • Difference in acidities due to hybridization of negatively charged carbon atom • Acetylide anion has sp-hybridized carbon Reactions of Alkynes • Presence of negative charge and an unshared electron pair on carbon makes acetylide anions strongly nucleophilic • Nucleophilic substitutions not limited to acetylene