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Chapter 7 1
Electron Delocalization and Electron Delocalization and ResonanceResonance
Chapter 7Chapter 7
Chapter 7 2
Contents of Chapter 7 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 3
Delocalized Electrons and Resonance
Chapter 7 4
Bonding in Benzene
(a) Carbon–carbon and carbon–hydrogen bonds
(b) Carbon p-orbitals overlap with neighbors
(c) Cloud of electrons above and below ring
(d) Electrostatic potential map for benzene
Chapter 7 5
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)
O
B
N
C
O
H
H
H
H
Two separate pi systems broken up by saturated tetravalent carbon
+: : ..
..
..
Chapter 7 6
Resonance TricksLone-Pair/Double-Bond Interconversion
One atom charge goes down, other goes up. Atom receiving e’s has charge go more
negative.
O
H
−
+
+
+
:..
..
..
.. ..
..:
O
H
O
O
Chapter 7 7
Resonance TricksDouble Bond Flip
Double bond flips like page in book Instability (+, -, or •) jumps 1,3 Electrons move diff. but effect same for +-•
−
+ +
. .
−.. ..
Chapter 7 8
Resonance TricksElectron Dominoes
Combinations of interconversions and flips Instability moves thru pi system in 2-atom
increments.
O
O
O
O
− −: : : :..
.. .. .. ....
Chapter 7 9
Resonance Structures
Chapter 7 10
Resonance Structures
Chapter 7 11
Resonance Structures
Chapter 7 12
Rules for Drawing Resonance Structures
Chapter 7 13
The Resonance Hybrid
When there is a choice, electrons tend to move toward the more electronegative atom
Chapter 7 14
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 15
Resonance Energy
The more nearly equivalent the resonance structures, the greater the resonance energy
Chapter 7 16
Stability of Allylic and Benzylic Cations
Chapter 7 17
Stability of Allylic and Benzylic Cations
CHR CHR CHR CHR CHR
Chapter 7 18
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. Alkene C doesn’t count but C attached to either alkene C does.
Chapter 7 19
Relative Stabilities of Carbocations
Vinyl cations are one level less stable than alkyl cations
Chapter 7 20
Effect of Delocalization on pKa
Carboxylic acids (RCOOH) are much more acidic than and alcohols (ROH)
pKa = 4.76 pKa = 15.9
CH3COH CH3CH2OH
O
Chapter 7 21
Effect of Delocalization on pKa
Chapter 7 22
Effect of Delocalization on pKa
A protonated aniline is more acidic than a protonated cyclohexylamine
pKa = 4.6
pKa = 11.2
Chapter 7 23
Types of Dienes
When double bonds are separated by at least one sp3 carbon, isolated diene
Chapter 7 24
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 25
Types of Dienes
When both sets of double bonds emanate from the same carbon, cumulated diene
Chapter 7 26
Relative Stabilities of Dienes
Chapter 7 27
Relative Stabilities of Dienes Doubly-bonded carbons in isolated and
conjugated dienes all are sp2 hybridized The central carbon in a cumulated diene
is sp hybridized
Chapter 7 28
Electrophilic Addition Reactions of Isolated Dienes
Reaction of 1,5-hexadiene with excess HBr adds HBr independently to each double bond
Markovnikov’s Rule is followed
Chapter 7 29
Electrophilic Addition Reactions of Conjugated Dienes
Conjugated dienes can give both 1,2- and 1,4- addition products
Chapter 7 30
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 31
Electrophilic Additions to Conjugated Dienes
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 32
Electrophilic Addition to Conjugated Dienes Example