Transcript
Electron Delocalization and Resonance Chapter 7
Chapter 7
1
Contents of Chapter 7
Benzene Delocalized Electrons and Resonance Stability of Allylic and Benzylic Cations and Radicals Effect of Electron Delocalization on p K a Molecular Orbital Description of Stability Relative Stabilities of Dienes Reactivity Considerations Electrophilic Electrophilic Addition Reactions Direct (1,2-) and Conjugate (1,4-) Additions Thermodynamic Thermodynamic vs. Kinetic Control Diels-Alder Reaction Chapter 7
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Delocalized Electrons and Resonance
Chapter 7
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Bonding in Benzene
(a)
Carbo Carbon± n±car carbo bon n and and carbo carbon± n±hy hydro droge gen n
(b)
Carbo Carbon n p-orbi p-orbita tals ls overl overlap ap with with neigh neighbo bors rs
(c )
Cloud of T electrons above and below ring
(d)
Electr Electrost ostati atic c pote potenti ntial al map for for benze benzene ne Chapter 7
W
bonds
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Delocalized Pi Systems
Extend continuously continuously through all connected atoms which lack 4 single bonds (except H) Broken up by saturated atoms with 4 single bonds (C). Two double bonds on same carbon break up system (cumulated dienes) Broken by two saturated connected atoms with same Lewis acid/base properties (ie. 2 O atoms or C+ connected to B) H H ..
:
O:
B
N
..
C +
OH
..
H
Two separate pi systems broken up by saturated tetravalent carbon Chapter 7
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Resonance Tricks Lone-Pair/Double-Bond Lone-Pair/Double-Bond Interconversion + O
..
..
O
H
H
..
+ ..
O:
O:
..
..
+
One atom charge goes down, other goes up.
Atom receiving e¶s has charge go more negative. Chapter 7
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Resonance Tricks Double Bond Flip +
+
..
..
. .
Double bond flips like page in book
Instability (+, -, or ) jumps 1,3
Electrons move diff. diff. but effect same for +Chapter 7
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Resonance Tricks Electron Dominoes .. :
O
..
..
O:
..
:
O
..
O ..
:
Combinations of interconversions and flips
Instability moves thru pi system in 2-atom increments. Chapter 7
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Resonance Structures
Chapter 7
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Resonance Structures
Chapter 7
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Resonance Structures
Chapter 7
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Rules for Drawing Resonance Structures
Chapter 7
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The Resonance Hybrid When there is a choice, electrons tend to move toward the more electronegative atom
Chapter 7
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Resonance Energy
The resonance energy tells us how much more stable the compound with delocalized electrons is due to that delocalization The greater the number of relatively stable resonance contributors, the greater the resonance energy
Chapter 7
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Resonance Energy The more nearly equivalent the resonance structures, the greater the resonance energy
Chapter 7
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Stability of Allylic and Benzylic Cations
Chapter 7
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Stability of Allylic and Benzylic Cations CHR
CHR
CHR
Chapter 7
CHR
CHR
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Stability of Allylic and Benzylic Cations
Chapter 7
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Crude Carbocation Stability Index
Add 1 for each attached carbon. Add 1 for adjacent double bond or phenyl ring. Subtract 1 if C+ on double bond with R on other end. Subtract 2 if C+ on double bond without R on other end.
"
}
} 2
" 2
3
3
4
4
"
}
" 1
} CH 3 0
Chapter 7
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" 1 19
Chemical Consequences of Electron Delocalization Rearrangements also also are driven by electron delocalization
The driving force for the rearrangement is the formation of a more stable secondary benzylic cation Chapter 7
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Effect of Delocalization on pK a Carboxylic Carboxylic acids (RCOOH) are much more acidic than and alcohols (ROH) O CH3COH
pK a = 4.76
CH3CH2OH
pK a = 15.9
Chapter 7
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Effect of Delocalization on pK a
Chapter 7
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Effect of Delocalization on pK a A protonated protonated aniline is more acidic than a protonated cyclohexylamine
pK a = 4.6
pK a = 11.2 Chapter 7
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Types of Dienes When double bonds are separated by at least one sp3 carbon, isolated diene
Chapter 7
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Types of Dienes When double bonds are separated by only one single bond (i.e. four sp2 carbons in a row), conjugated diene
Chapter 7
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Types of Dienes When both sets of double bonds emanate from the same carbon, cumulated diene
Chapter 7
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Relative Stabilities of Dienes
Chapter 7
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Relative Stabilities of Dienes
Doubly-bonded car bons bons in isolated and 2 conjugated dienes all ar e sp hybr idized ized The central car bon bon in a cumulated diene is sp hybr idized ized
Chapter 7
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Electrophilic Addition Reactions of Isolated Dienes
1,5-hex 1,5-hexadiene Reaction of
with exc excess HBr adds independently to each double bond HBr Markovni ovnikov¶s Rule is follow followed
Chapter 7
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Electrophilic Addition Reactions of Conjuga Conjugated ted Dienes Conjugated dienes can give both 1,2and 1,4- addition products
Chapter 7
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Mechanism
of Addition of HBr to Conjugated Dienes
The
positive charge on the allylic cation is not localized on C-2, but is shared between C-2 and C-4
Chapter 7
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Thermodynamic vs. Kinetic Thermodynamic Control
The product that is formed most rapidly is the kinetic product The most stable product is the thermodynami c product
Chapter 7
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Electrophilic Additions to Conjugated Dienes
Electrophile adds to one end of pi system Draw all resonance structures for C+ for f or adding electrophile to each end of pi system Evaluate which set of structures is more stable (stability index) Add nucleophile to each C+ in this set of structures to get observed products Product with most highly substituted double bonds is thermo prod (count total numbr of R¶s directly attached to alkene carbons). Kinetic product is 1,2 addition product.
Chapter 7
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Electrophilic Addition to Conjugated Dienes Example Br 2 3O
add'n of to lef t end of pi system Br gives r eson esonance str uctur es es ith index values of 3, 3, and 2 (less (less stable)
Br
index 2
3
Br
Br O
1,2 addi ddition kinetic pr oduc oduct
Br
index 3
stability index 4
O
Br
3
4 R gr ou oups on alkenes
Chapter 7
Br O
3
5 R gr ou oups on alkenes thermodynamic pr oduc oduct
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Retrosynthetic Analysis of 1,4 Addition Products 2
2 1
3 HO
3
1
4
4
Find the group that added as nucleophile in product If it¶s attached to an allylic carbon you can make mak e product from 1,4 addition Number carbon with nucleophile 4 and number downards through double bond to next C (no. 1) Remove added groups and put double bonds between C¶s 1&2, 3&4.
Chapter 7
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Diels±Alder Reaction Regioselectivity
Diene is nucleophilic; make it reactive with e donor Dienophile electrophilic; activate by e withdrawing Nucleophile has more pi e¶s (4) than electrophile (2) Donor and withdrawing grp wind up on adjacent C¶s
Chapter 7
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Diels±Alder Reaction Stereoselectivity The Diels±Alder reaction is stereospecific because different stereoisomeric reactants give different stereoisomeric stereoisomeric products
Chapter 7
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Diels±Alder Reaction Stereoselectivity When the diene is cyclic, the endo product is preferred
Chapter 7
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Diels±Alder Product Analysis 8 8
10
7
9 5
6
CN
O C H3
4
1
3
2
(
7
9
CN 6
5
10
4
O CH 3 1 2
3
Number diene and dienophile atoms to facilitate analysis Diene gets numbers 1-4 with end e donor given No. 1 Dienophile Dienophile gets numbers 5 & 6 with EWG given No. 6 Numbers on other C¶s will show what¶s connected in prod Draw 6-membered ring of prod first and number 1-6 Extra double bond goes between C¶s numbered 2 & 3 Chapter 7
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Diels-Alder Retrosynthetic Analysis d
/ d /
C
CH
CH
+ C
CH C
CH
D-A prod is 6-membered 6-m embered ring with double bond (db)
First project db to opposite side of ring
Break 2 single bonds adjacent to new db pos¶n
Double 2 single bonds adjacent to old db pos¶n
Do analysis with scripting notation first, then use eraser Chapter 7
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