Chapter 13
description
Transcript of Chapter 13
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Created byProfessor William Tam & Dr. Phillis
Chang Ch. 13 - 1
Chapter 13Chapter 13
Conjugated PiConjugated PiSystemsSystems
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Ch. 13 - 2
1. Introduction A conjugated system involves at
least one atom with a p orbital adjacent to at least one bond.● e.g.
O
conjugateddiene
allylicradical
allylic cation
allylicanion
enone enyne
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Ch. 13 - 3
X
H XX2
high temp(and low conc.
of X2)
+
2. Allylic Substitution and the Allyl Radical
vinylic carbons (sp2)
X
X
X2
low tempCCl4
(hi X2 conc.)
allylic carbon (sp3)
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2A.2A. Allylic ChlorinationAllylic Chlorination(High Temperature)(High Temperature)
Cl H Cl+ Cl2 +400oC
gas phase
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Mechanism●Chain initiation:
Cl Cl 2 Cl
●Chain propagation:
H H Cl++ Cl
(allylic radical)
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Mechanism●Chain propagation:
●Chain termination:
Cl Cl Cl+ + Cl
Cl+ Cl
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+ HH
DHo = 369 kJmol-1
DHo = 465 kJmol-1
H + H
Allylic vs vinyl bond energies:
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Ch. 13 - 8
+ HXH + XEact
(low)
H +Eact
(high)HX+X
Relative stabilityof radicals:
allylic > 3o > 2o > 1o > vinylic
Allylic vs vinyl activation energies:
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Radical stabilities:
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2B.2B. Allylic Bromination with N-Bromo-Allylic Bromination with N-Bromo-succinimide (Low Concentration of Brsuccinimide (Low Concentration of Br22))
NBS is a solid and nearly insoluble in CCl4.● Low concentration of Br•
H N
Br
OO
Br N
H
OO
h or ROORheat, CCl4
+
+
(NBS)
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Examples:Br
ROOR, CCl4heat
NBS
BrROOR, CCl4heat
NBS
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3. The Stability of the Allyl Radical
3A.3A. Molecular Orbital Description of Molecular Orbital Description of the Allyl Radicalthe Allyl Radical
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Molecular
orbitals:
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3B.3B. Resonance Description of the Resonance Description of the Allyl RadicalAllyl Radical
12
3 12
3
1
23
1
2
3
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4. The Allyl Cation Relative order of Carbocation
stability.
(3o allylic) (allylic)(3o)
(2o) (1o) (vinylic)
> >
>>>
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5. Resonance Theory Revisited
5A. 5A. Rules for Writing Resonance StructuresRules for Writing Resonance Structures Resonance structures exist only on
paper. Although they have no real existence of their own, resonance structures are useful because they allow us to describe molecules, radicals, and ions for which a single Lewis structure is inadequate.
We connect these structures by double-headed arrows (), and we say that the hybrid of all of them represents the real molecule, radical, or ion.
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In writing resonance structures, one may only move electrons.
H
H
resonance structures
not resonance structures
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All of the structures must be proper Lewis structures.
O O: :10 electrons!X
not a proper Lewis structure
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All resonance structures must have the same number of unpaired electrons.
X
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All atoms that are part of the delocalized -electron system must lie in a plane or be nearly planar.
no delocalizationof -electrons
delocalizationof -electrons
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The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure.
Equivalent resonance structures make equal contributions to the hybrid, and a system described by them has a large resonance stabilization.
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The more stable a resonance structure is (when taken by itself), the greater is its contribution to the hybrid.
(3o allylic cation)
greater contribution
(2o allylic cation)
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5B.5B. Estimating the Relative Stability Estimating the Relative Stability of Resonance Structuresof Resonance Structures
The more covalent bonds a structure has, the more stable it is.
(more stable) (less stable)
O O
(more stable) (less stable)
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Structures in which all of the atoms have a complete valence shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.
O O
this carbon has6 electrons
this carbon has 8 electrons
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Charge separation decreases stability.
(more stable) (less stable)
OMe OMe
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6. Alkadienes and Polyunsaturated Hydrocarbons
1,3-Butadiene
(2E,4E)-2,4-Hexadiene
1,3-Cyclohexadiene
12
3
4
1
2
3
4
5
6
1
2 3
4
56
Alkadienes (“Dienes”):
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Alkatrienes (“Trienes”):
1
2
3
4
5
6
7
8
(2E,4E,6E)-Octa-2,4,6-triene
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Alkadiynes (“Diynes”):
1 2 3 4 5 6
2,4-Hexadiynes
1
23
456 1
2
3
4
5 6 7 8
Hex-1-en-5-yne (2E)-Oct-2-en-6-yne
Alkenynes (“Enynes”):
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Cumulenes:
(Allene)(a 1,2-diene)
C C C
H
HH
H
C C C
H
HH
H
enantiomers
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Conjugated dienes:
Isolated double bonds:
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7. 1,3-Butadiene: Electron Delocalization
1
2
3
4
7A.7A. Bond Lengths of 1,3-Butadiene Bond Lengths of 1,3-Butadiene
1.34 Å
1.47 Å
1.54 Å 1.50 Å 1.46 Å
sp3 sp3spsp3sp2
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7B.7B. Conformations of 1,3-ButadieneConformations of 1,3-Butadiene
(s-cis) (s-trans)
H H
(less stable)
cis
transsinglebond
singlebond
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7C.7C. Molecular Orbitals of 1,3-ButadieneMolecular Orbitals of 1,3-Butadiene
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8. The Stability of Conjugated Dienes
Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes.
2 + 2 H2 2 2 x (-127)=-254
H o (kJmol-1)
=-239
Difference 15
+ 2 H2
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Stability due to conjugation:
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9. Ultraviolet–Visible Spectroscopy
The absorption of UV–Vis radiation is caused by transfer of energy from the radiation beam to electrons that can be excited to higher energy orbitals.
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9A.9A. The Electromagnetic SpectrumThe Electromagnetic Spectrum
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9B.9B. UVUV––Vis SpectrophotometersVis Spectrophotometers
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Beer’s law
A = absorbance= molar absorptivityc = concentrationℓ = path length
A = x c x ℓ A
c x ℓor =
●e.g. 2,5-Dimethyl-2,4-hexadienemax(methanol) 242.5 nm( = 13,100)
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9C.9C. Absorption Maxima for NonconjugatedAbsorption Maxima for Nonconjugatedand Conjugated Dienesand Conjugated Dienes
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O OAcetone
Ground state
n
max = 280 nmmax = 15
* Excited state
O
n
max = 324 nm,max = 24
max = 219 nm,max = 3600
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9D. 9D. Analytical Uses of UVAnalytical Uses of UV––Vis SpectroscopyVis Spectroscopy
UV–Vis spectroscopy can be used in the structure elucidation of organic molecules to indicate whether conjugation is present in a given sample.
A more widespread use of UV–Vis, however, has to do with determining the concentration of an unknown sample.
Quantitative analysis using UV–Vis spectroscopy is routinely used in biochemical studies to measure the rates of enzymatic reactions.
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10. Electrophilic Attack on ConjugatedDienes: 1,4 Addition
Cl
HCl
H
1
2
3
4 H Cl
25oC
+
(78%)(1,2-Addition)
(22%)(1,4-Addition)
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(a)
Cl
H
Mechanism:
Cl H + H
(a)
H
(b)
H
X
H+ +
Cl
(b)
ClH
(a)
(b)
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10A.10A. Kinetic Control versus Kinetic Control versus Thermodynamic Control of a Thermodynamic Control of a Chemical ReactionChemical Reaction
+
HBr
Br
Br+
(80%)
-80oC
(20%)
(80%)40oC
Br
Br+
(20%)
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Br
Br
40oC, HBr
1,2-Additionproduct
1,4-Additionproduct
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The 1,4-product is thermodynamically more stable.
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11.The Diels–Alder Reaction: A 1,4-Cycloaddition Reaction of Dienes
[4+2]+
(diene) (dienophile) (adduct)
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O
O
O
O
O
O
1,3-Butadiene(diene)
Maleicanhydride
(dienophile)
Adduct(100%)
+benzene
100oC
e.g.
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11A.11A. Factors Favoring the DielsFactors Favoring the Diels––AlderAlderReactionReaction
EDG
EWG
EDG
EWG
+
Type A
● Type A and Type B are normal Diels-Alder reactions
+
Type B
EDG
EWG EWG
EDG
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EWG
EDG
EWG
EDG
+
Type C
● Type C and Type D are Inverse Demand Diels-Alder reactions
+
Type D
EWG
EDG EDG
EWG
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Relative rate:
Diene D.A. cycloadduct+30oC
O
O
O
OMe
> >Diene
t1/2 20 min. 70 min. 4 h.
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Relative rate:
Dienophile D.A. cycloadduct+20oC
> >Dienophile
t1/2 0.002 sec. 20 min. 28 h.
NC CN
NC CN
CN
CN
CN
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Steric effects:
> >Dienophile:
Relative rate: 1 0.14 0.007
COOEt COOEt COOEt
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11B.11B. Stereochemistry of the Stereochemistry of the DielsDiels––Alder ReactionAlder Reaction
O
O
OMe
OMeH
H
OMe
O
OMe
OH
H
+
Dimethyl maleate(a cis-dienophile)
Dimethyl cyclohex-4-ene-cis-1,2-dicarboxylate
1. The Diels–Alder reaction is stereospecific: The reaction is a syn addition, and the configuration of the dienophile is retained in the product.
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O
OMeH
OMe
O
OMe
OH
H
+
Dimethyl fumarate(a trans -dienophile)
Dimethyl cyclohex-4-ene-trans -1,2-
dicarboxylate
HMeO
O
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2. The diene, of necessity, reacts in the s-cis rather than in the s-trans conformation.
s-cis Configuration s-trans Configuration
R
O
+
O
R
Highly strained
X
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e.g.COOMe COOMe
heat+
(diene lockedin s-cis
conformation)
COOMe
+ No Reaction
(diene lockedin s-trans
conformation)
heat
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Cyclic dienes in which the double bonds are held in the s-cis conformation are usually highly reactive in the Diels–Alder reaction.
Relative rate:
Diene D.A. cycloadduct+30oC
O
O
O
> >Diene
t1/2 11 sec. 130 sec. 4 h.
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3. The Diels–Alder reaction occurs primarily in an endo rather than an exo fashion when the reaction is kinetically controlled.
H H
H H
R
H
H
Rlongest bridge R is exo
R is endo
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Alder-Endo Rule:●If a dienophile contains
activating groups with bonds they will prefer an ENDO orientation in the transition state.
X
XX
X
HH
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e.g.
OO O
O
O
O
HH
+
100% endo
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Stereospecific reaction:
X
X
X
X
+
X X
X
+
X
(i)
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Stereospecific reaction:
+
+
(ii) Y
Y
Y
Y
Y
Y
Y
Y
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Examples:
CN
CN
+
Me
NC
NC
CN
CNCN
CNMe(A)
D.A.
CN
+
NC
Me
Me
NC
CN
CN
CN
CN
CN
MeMe(B)
D.A.
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Diene A reacts 103 times faster than diene B even though diene B has two electron-donating methyl groups.
Me
Me
H
Me
Me
(s-cis) (s-trans)
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Examples:
+
(C)
O
O
O
O
H
H
O
O
D.A.
+
(D)
O
O
O
O
H
H
O
O
D.A.
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Examples
+
(E)
O
O
O
D.A.No Reaction
● Rate of Diene C > Diene D (27 times), but Diene D >> Diene E
● In Diene C, t-Bu group electron donating group increase rate
● In Diene E, 2 t-Bu group steric effect, cannot adopt s-cis conformation
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END OF CHAPTER 13