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Reposted for PSHS WV III Beryllium, 2011
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© 2006 Thomson Higher Education
Chapter 7 Reactions of Alkenes and Alkynes
Alkene Addition Reactions
• Addition of a halogen to give 1,2-dihalide
• Addition of a hypohalous acid to give halohydrin
• Addition of water to give alcohol
• Addition of hydrogen to give alkane
• Addition of single oxygen to give three-membered cyclic ether: epoxide
• Addition of two hydroxyl groups to give 1,2-diol
Alkene addition reactions
7.1 Preparations of 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
Preparations of 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
Preparations of Alkenes: A Preview of Elimination Reactions
Dehydration • Loss of water from an alcohol• Usually occurs by treatment of an alcohol with a strong
acid
Preparations of 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
7.2 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 double-bond, 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
Alkene halogenation reaction• Common laboratory reaction• Limited primarily to marine organisms in nature
• Carried out by enzymes called haloperoxidases that
oxidize Br- or Cl- ions to a biological equivalent of Br+ or Cl+
7.3 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
• In marine organisms halohydrin formation is carried
out by haloperoxidases that oxidize Br- or Cl- ions to corresponding HOBr or HOCl bonded to a metal atom in the enzyme for subsequent addition to the double bond of substrate
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
7.4 Hydration of Alkenes
Alkenes undergo an acid catalyzed addition reaction with water to yield alcohols
• Not of much use in the laboratory because of the high temperatures often required
Hydration of Alkenes
Acid-catalyzed hydration of isolated double bonds• Uncommon in biological pathways
Acid-catalyzed hydration of double bond adjacent to carbonyl group
• More common in biological pathways• Adjacent carbonyl group required for elimination of
water• Not an electrophilic addition mechanism
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 non-Markovnikov regiochemistry
Hydration of Alkenes
Alkene Hydroboration
Worked Example 7.1Predicting the Products of a Hydration Reaction
What products would you obtain from reaction of 2-methylpent-2-ene with:
(a) BH3, followed by H2O2,OH-
(b) Hg(OAc)2, followed by NaBH4
Worked Example 7.1Predicting the Products of a Hydration Reaction
Strategy• Determine type of reaction being carried out
• Two methods of hydration• Hydroboration/oxidation
• Occurs with syn stereochemistry• Gives non-Markovnikov alcohol
• Oxymercuration • Gives the Markovnikov alcohol
Worked Example 7.1Predicting the Products of a Hydration Reaction
Solution
Worked Example 7.2Synthesizing an Alcohol
How might you prepare the following alcohol?
Worked Example 7.2Synthesizing 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 7.2Synthesizing an Alcohol
Solution
Note: 4-methylhex-2-ene has a disubstituted double and would probably give a mixture of two alcohol products with either hydration method
7.5 Reduction of Alkenes
Hydrogenation • Addition reaction process by which alkenes are
reduced to alkanes
Reduction• Increases electron density on carbon by
• Forming C-H • Breaking C-O, C-N, or C-X bond
Common catalysts for alkene hydrogenation:• Platinum – PtO2 (Adams’ Catalyst)• Palladium – very fine powder supported on inert
material such as charcoal (Pd/C)
Reduction of Alkenes
Catalytic hydrogenation• A heterogeneous process that takes place on the
surface of insoluble catalyst particles• Occurs with syn stereochemistry
• Both hydrogens add to the double bond from the same side
Reduction of Alkenes
Steps of Catalytic hydrogenation:
1. Adsorption of H2 onto catalyst surface
2. Complexation between catalyst and alkene occurs as a vacant orbital on metal interacts with filled p orbitals
3. Hydrogen added to double bond
4. Saturated product diffuses away from catalyst
Reduction of Alkenes
Hydrogenation• Unsaturated vegetable oils reduced to
produce saturated fats used in margarine and cooking products• Vegetable oils
• Triesters of glycerol, HOCH2CH(OH)CH2OH, with three long-chain carboxylic acids called fatty acids
• Fatty acids• Polyunsaturated carboxylic acids containing
long hydrocarbon chains• Double bonds have cis stereochemistry
Reduction of Alkenes
Catalytic hydrogenation of polyunsaturated fats
Reduction of Alkenes
Catalytic hydrogenation of polyunsaturated fats• Complete hydrogenation leads to saturated fatty acids• Incomplete hydrogenation results in isomerized trans fats that
release trans fatty acids upon digestion, increasing blood cholesterol levels
Biological hydrogenation (reduction) of isolated double bonds• Double bond must be adjacent to a carbonyl group
• The reduction of isolated double bonds is rare in biological pathways
• Process occurs in two steps1. NADPH (coenzyme reduced nicotinamide adenine
dinucleotide phosphate) adds hydride ion (H:-) to double bond to produce an ion
2. Protonation of an anion by acid HA leading to an overall addition of H2
Reduction of Alkenes
Biological reduction of double bond in trans-Crotonyl ACP leads to the formation of Butyryl ACP
7.6 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 C-O bond for C-Cl bond
• Cl- eliminated yielding the epoxide
Oxidation of Alkenes: Epoxidation
Epoxides in biological pathways:
• Epoxides prepared from alkenes as intermediates
• Peroxyacids are not involved
• Conversion of squalene into 2,3- oxidosqualene; a key step in the biosynthesis of steroids
7.7 Oxidation of Alkenes: Hydroxylation
Hydroxylation• The addition of an –OH group to each of the two double-
bond carbons• Two step process:
1. Epoxidation 2. 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:1. Protonation of the epoxide increasing the electrophilicity of
carbon
2. Nucleophilic addition of water followed by deprotonation• Trans-1,2-diol formed
Oxidation of Alkenes: Hydroxylation
Epoxides in biological pathways
• Epoxide hydrolyses are common • Pathways animals use to
detoxify harmful substances
• Benzo[a]pyrene • Carcinogenic substance
found in cigarette smoke, chimney soot, and barbecued meat
• Detoxified by conversion to a diol epoxide
Oxidation of Alkenes: HydroxylationHydroxylation in the Laboratory • Carried out directly by oxidation of an alkene with
osmium tetroxide, OsO4
• Syn stereochemistry• No carbocation intermediate• Occurs through cyclic osmate intermediate
7.8 Radical Addition to Alkenes: Alkene Polymers
Radicals add to alkene double bonds• Radicals remove one electron from double bond• One electron left behind yielding a new radical
Polymer • A large molecule built up by repetitive bonding
together of many smaller molecules called monomers• Cellulose (glucose polymer)
Radical Addition to Alkenes: Alkene Polymers• Proteins (amino acid polymers)
• Nucleic acid (nucleotide polymer)
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 PolymersInitiation 1. Small amount of benzoyl peroxide catalyst is heated
breaking weak O-O bonds and yielding radicals2. Benzoyloxy radical adds to C=C bond of ethylene
forming a carbon radical3. a) One electron from C=C bond pairs up with electron of
benzoyloxy radical to form C-O bondb) 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 PolymersVinyl 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
Radical Addition to Alkenes: Alkene Polymers
Radical addition• Difficult to control• Limited use in the laboratory• Reaction intermediate is not
quenched so reaction continues
Electrophilic addition• Reaction occurs once• Intermediate is then quenched
and reaction stops.
Radical vs. Electrophilic Addition Reactions
Worked Example 7.3 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 7.3 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 7.3 Predicting the Structure of a Polymer
Solution
The general structure of poly(vinyl chloride) is
7.9 Biological Additions of Radicals to Alkenes
Biological Reactions• Only one substrate molecule at a time is present
in the active site of the enzyme where the reaction occurs (necessary reactant groups nearby)
• More controlled • More common than laboratory radical reactions
Biological Additions of Radicals to Alkenes
Step 1 – formation of a carbon radical at C13
Step 2 – C13 radical reacts with O2 at C11 through resonance form
Step 3 – Oxygen radical reacts with C8-C9 double bond forming carbon radical at C8
Step 4 – C8 radical adds to C12-C13 double bond forming carbon radical at C13
Step 5 – resonance form of C13 carbon radical adds at C15 to a second O2 molecule
Step 6 – Reduction of O-O bond gives prostaglandin H2
7.10 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 HydrogenationConjugated 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 stability1) 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)
Conjugated Dienes
2. Molecular Orbital Theory • Interaction between the p orbitals of the two
double bonds• Two p orbitals combine to form two p molecular
orbitals• Both electrons occupy the low-energy bonding
orbital leading to a net lowering of energy and formation of a stable bond
Conjugated Dienes
• Four adjacent p atomic orbitals of a conjugated diene
Four molecular orbitals of buta-1,3-diene
7.11 Reaction 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
Reaction of Conjugated Dienes
Allylic carbocation is an intermediate• When buta-1,3-diene reacts with H+ electrophile two
carbocation intermediates are possible:
1. A primary carbocation
2. A secondary allylic carbocation (stabilized by resonance between two forms)
• Secondary allylic carbocation is more stable and forms faster than the nonallylic carbocation
Reaction 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
Worked Example 7.4Predicting the Products of Electrophilic Addition to a Conjugated Diene
Give the structures of the likely products from reaction of 1 equivalent of HCl with 2-methylcyclohexa-1,3-diene. Show both 1,2- and 1,4- adducts.
Worked Example 7.4Predicting the Products of Electrophilic Addition to a Conjugated Diene
Strategy Electrophilic addition of HCl to a conjugated
diene involves the formation of allylic carbocation intermediates
First – • Protonate the two ends of the diene • Draw resonance forms of the two allylic
carbocations that resultSecond – • Allow each resonance form to react with Cl- to
generate four possible products
Worked Example 7.4Predicting the Products of Electrophilic Addition to a Conjugated Diene
Solution
7.12 Reaction of Alkynes
Alkyne Addition Reactions • Alkynes behave similarly to alkenes• Alkynes are less reactive than alkenes• Various reactions can often be stopped at the
monoaddition stage if one molar equivalent of reagent is used
Reaction of Alkynes
Reaction of Alkynes
Reaction 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
Reaction of Alkynes
Bronsted-Lowry Acid• A substance that donates H+ Acidity order:• Established by measuring acid dissociation constants and
expressing the results as pKa valuesLow pKa = strong acidHigh pKa = weak acid
• Amide ion (NH2-), the conjugated base of ammonia (pKa = 35),
is often used to deprotonate terminal alkynes
Reaction 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