Transcript Reactions of Alkenes and Alkynes

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
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•
Biological systems – usually alcohols
Laboratory – either alcohols or alkyl halides
Alkenes and alcohols are chemically related through addition and
elimination reactions
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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Combination of two growing chains
2-R–CH2CH2
→
R–CH2CH2CH2CH2–R
Radical Addition to Alkenes: Alkene Polymers
Vinyl monomers
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Substituted ethylene
Undergo polymerization to yield polymer with substituted
groups regularly spaced in alternating carbon atom long chain
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Polypropylene
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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:
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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
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Reaction occurs once
Intermediate is then quenched and reaction stops.
Additions to Alkenes, Radicals
Radical vs. Electrophilic Addition Reactions
Radical addition
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Difficult to control
Limited use in the laboratory
Reaction intermediate is not quenched so reaction continues
Conjugated Dienes
Sites of unsaturation
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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
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Stability due to orbital hybridization
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Alkanes
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C-C single bonds
σ overlap of sp3 orbitals on both carbons
Conjugated dienes
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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
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Allylic means next to a double bond
When buta-1,3-diene reacts with H+ electrophile two
carbocation intermediates are possible:
1.
2.
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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
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A substance that donates H+
• Acidity order:
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Established by measuring acid dissociation constants and
expressing the results as pKa values
Low pKa = strong acid
High pKa = weak acid
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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
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Acetylide ions are more stable than vinylic (alkenyl) or alkyl
ions
• Difference in acidities due to hybridization of negatively
charged carbon atom
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Acetylide anion has sp-hybridized carbon
Reactions of Alkynes
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Presence of negative charge and an unshared electron pair on
carbon makes acetylide anions strongly nucleophilic
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Nucleophilic substitutions not limited to acetylene