Transcript Electron Delocalization, Resonance and Aromaticity

Electron Delocalization and
Resonance
Chapter 7
Chapter 7
1
Contents of Chapter 7
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Benzene
Delocalized Electrons and Resonance
Stability of Allylic and Benzylic Cations and Radicals
Effect of Electron Delocalization on pKa
Relative Stabilities of Dienes
Reactivity Considerations
Electrophilic Addition Reactions
Direct (1,2-) and Conjugate (1,4-) Additions
Chapter 7
2
Delocalized Electrons and
Resonance
Chapter 7
3
Bonding in Benzene
(a)
(b)
(c)
(d)
Carbon–carbon and carbon–hydrogen s bonds
Carbon p-orbitals overlap with neighbors
Cloud of p electrons above and below ring
Electrostatic potential map for benzene
Chapter 7
4
Delocalized Pi Systems
 Extend 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
..
N
B
..OH
..
: O:
C
+ H
Two separate pi systems broken up
by saturated tetravalent carbon
Chapter 7
5
Resonance Tricks
Lone-Pair/Double-Bond Interconversion
..
..O H
+
..O 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. 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
8
Resonance Structures
Chapter 7
9
Resonance Structures
Chapter 7
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Resonance Structures
Chapter 7
11
Rules for Drawing Resonance
Structures
Chapter 7
12
The Resonance Hybrid
When there is a choice, electrons tend
to move toward the more
electronegative atom
Chapter 7
13
Resonance Energy
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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
14
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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Crude Carbocation Stability
Index
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Add 1 for each attached carbon.
Add 1 for adjacent double bond or phenyl ring.
Subtract 1 if C+ on double bond.
Alkene C doesn’t count but C attached to either alkene C does.
Chapter 7
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Relative Stabilities of
Carbocations
Vinyl cations are one level less
stable than alkyl cations
Chapter 7
19
Effect of Delocalization on pKa
Carboxylic acids (RCOOH) are much
more acidic than and alcohols (ROH)
O
CH3COH
CH3CH2OH
pKa = 4.76
pKa = 15.9
Chapter 7
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Effect of Delocalization on pKa
Chapter 7
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Effect of Delocalization on pKa
A protonated aniline is more acidic than a
protonated cyclohexylamine
pKa = 4.6
pKa = 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
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Doubly-bonded carbons in isolated and
conjugated dienes all are sp2 hybridized
The central carbon in a cumulated diene
is sp hybridized
Chapter 7
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Electrophilic Addition
Reactions of Isolated Dienes
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Reaction of 1,5-hexadiene with excess
HBr adds HBr independently to each
double bond
Markovnikov’s Rule is followed
Chapter 7
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Electrophilic Addition
Reactions of Conjugated
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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Electrophilic Additions to
Conjugated Dienes
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Electrophile adds to one end of pi system
Draw all resonance structures for C+ for adding electrophile to each
end of pi system
Evaluate which set of structures is more stable (stability index)
Number most stable C+ positions with even numbers, end of pi system
#1
Electrophile (H) adds to position #1
Add nucleophile to each C+ (even-numbered C atom) in most stable
set of resonance structures to get observed products
Chapter 7
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Electrophilic Addition to
Conjugated Dienes Example
Chapter 7
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