Chapter 10 Conjugation in Alkadienes and Allylic...
Transcript of Chapter 10 Conjugation in Alkadienes and Allylic...
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©2012 Gregory R Cook
Chapter 10Conjugation in Alkadienes and
Allylic Systemspart 01
CHEM 341: Spring 2012
Prof. Greg Cook
cook.chem.ndsu.nodak.edu/chem341
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©2012 Gregory R Cook
Conjugation
• Conjugation - conjugare (Latin) to link together - when pi bonds are extended beyond 2 carbons
2
conjugated not conjugated
CH
H
β-Carotene - orange colored constituent of carrots
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Conjugation and Color
3
lycopene (red color of tomatos)
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Conjugation and Color
4
O
OHR
R
OROH
HO NaOH O
OR
R
OROH
O
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Allylic Conjugation
5
allylic carbocation allylic free radical allylic carbanion
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Allylic and Vinylic Positions
6
allylic alcohol
OH
allylic bromide
Br
CC
C
H
H
H
H
HH
allylic hydrogens
vinylic hydrogens
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Allylic Carbocations
7
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Allylic Carbocations
8
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Allylic Halide Substitution Reactions
9
3° and allylic bromide
relative ratefor SN1 Subst.
3° bromide
123 1
Cl Cl
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Allylic Halide SN1 Substitution
10
85%
ClClH2O
Na2CO3
H2O
Na2CO3OH
HO+
15%
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Allylic Halide SN2 Substitution
11
Cl
relative ratefor SN1 Subst. 80 1
Cl
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Allylic Free Radicals
12
allylic free radical
spin densities
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Allylic Positions are Activated
13
H
H
+ H
+ H
ΔH° = +101 kcal/mol
ΔH° = +88 kcal/mol
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Free Radical Halogenation
14
R H + X2 R X + HX
HCl2 Cl
NBS
heat or light
heat or light
Br
N
O
O
Br
NBSN-bromosuccinimide
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Allylic Bromination
15
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Allylic Anions
16
allylic carbanion
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Allylic Anions
17
H
H
pKa = 62
base
base
pKa = 43
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Isolated and Conjugated Dienes
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1,4-pentadiene 1,3-pentadiene
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Cumulated Dienes
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H2C C CH2
an alleneC C C
H
HH
H
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Cumulated Dienes
19
H2C C CH2
an alleneC C C
H
HH
H
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Relative Stability of Dienes
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Relative Stability of Dienes
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ΔH° = -70.5 kcal/molH2C C CH2
H2Pd/C
H2Pd/C
ΔH° = -29.9 kcal/mol
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Conformations of Conjugated Dienes
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s-Cis conformation
s-Trans conformation
Friday, April 20, 12