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8.12 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
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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)
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
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Conjugated Dienes
In a conjugated diene, the lowest p MO (y1) has a
favorable bonding interaction between C2 and C3
that is absent in the nonconjugated diene
•
Certain amount of double-bond character to C2-C3 bond,
making that bond stronger and shorter that a typical single
bond
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
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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
Worked Example 8.5
Predicting the Products of Electrophilic Addition to
a Conjugated Diene
Give the structures of the likely products from
reaction of 1 equivalent of HCl with 2methylcyclohexa-1,3-diene. Show both 1,2- and
1,4- adducts.
8.14 The Diels-Alder Cycloaddition Reaction
Conjugated dienes undergo reactions with alkenes
to yield substituted cyclohexene products
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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
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The Diels-Alder Cycloaddition Reaction
• The double or triple bond of the dienophile is adjacent to the
positively polarized carbon of an electron-withdrawing
substituent
• The double-bond carbons in these substances are
substantially less electron-rich than the carbons in ethylene
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
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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
Worked Example 8.6
Predicting the Product of a Diels-Alder Reaction
Predict the product of the following Diels-Alder reaction
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8.15 Reactions 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
Reactions of Alkynes
Reactions of Alkynes
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Reactions of Alkynes
Alkyne acidity
• Terminal alkynes (RC≡CH) are relatively acidic
• RC≡CH treated with a strong base NaNH 2
• 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
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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
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