chapter 6 resonance and electron delocalization. chapter 6 topics for test u sections 6.1 through...
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Chapter 6
Resonance and Electron
Delocalization
Chapter 6 Topics for Test
Sections 6.1 through 6.13 I will emphasize drawing resonance
structures-- Sections 6.1-6.3 Be sure to look at the supplement and
problems on Hückel’s rule available from my website (Not covered Winter 2007)
Problem Assignments
In Text Problems:
1, 2, 3, 4, 5, 6, 7 10, 11, 12a, c, d End of Chapter Problems:
17, 18, 19, 20, 21, 22, 23, 24
28 30, 31 Problems on Hückel’s rule
Chapter 6 Topics Section 6.1- -Failure of Lewis Dot Notation
a) Get more than one Lewis structure when have non-bonded electrons next to a double bond - - always shift electrons!!
b) Individual structures are called resonance structures
c) The “real structure” is a hybrid of individual resonance structures-- “summary structure”
Chap 6 Topics, Continued
We draw resonance structures because we are interested in figuring out what the “real molecule or ion“ looks like!
When we delocalize electrons (push electrons) this leads to resonance stabilization
resonance = delocalization --> stabilization
Resonance
The real molecule cannot be represented by any single Lewis (line-bond) formula.
The real molecule is a hybrid of the contributing Lewis (line-bond) structures.
RHINOCEROS
DRAGON UNICORN
real
mythical
Wheland’s Analogy
..
:
_ ..
O C
O
O
..
..
..:
:
: :
_
_
..
O
: :
O CO
..
..
..
:
:
_
_
Resonance in the Carbonate IonResonance in the Carbonate Ion
O C
O
O
..
..
: :
: :_
CO32-
THREE EQUIVALENT STRUCTURES
RESONANCE HYBRID
:
O C
O
O
..
..
..:
: :
_
_
..
Typical bond lengths
C=O 1.22 AC O 1.43 A
o
oshortlong
All bond lengths are equalin carbonate ion, at about1.30 A
o
but …..
Charge is evenly distributedto the three oxygen atoms
The behavior of areal carbonate iondoes not match theLewis diagram.
determined by x-ray crystallographyof CaCO3 crystals
O
O
O _
C
_2/3_
2/3
2/3
Summary Structure for Carbonate IonSummary Structure for Carbonate Ion
2_
CarbonateCarbonate IonIon
van der Waal’s(isodensity)surface
electrostaticpotentialmapped onisodensitysurface
(ab initio 3-21g calculation)
the same …showing location of atoms
red = negblue = pos
Sect. 6.2 and 6.3: Resonance Structures (electron pushing)
a) Anions: move an electron pair away from the negative charge towards double bond! Two “flips” necessary.
b) Cations: move electron pair towards positive charge!
c) Neutral compound: move towards electronegative atom
d) Assign formal charges after “moving e-”
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O O O
OO
_
_
_
_
_
Phenolate Ion ResonancePhenolate Ion Resonance
O-
notice the alternate pattern
(-) (-)
(-)
Not all contributingstructures are equal.Why?
SUMMARY
CH2 CH2
CH2CH2CH2+
+
+
+
+
Benzyl CationBenzyl Cation
+
(+) (+)
(+)
Sect. 6.2 and 6.3 Rules for Evaluating Resonance
The octet rule should be obeyed, if possible. There should be a minimum of charge separation. The formal charges may correspond to the
relative electronegativities of the atoms, but don’t be surprised if an electronegative atom has a positive charge!
Avoid formal charges of the same sign on adjacent atoms.
Rules for Evaluating Resonance Forms - Part 2
Keep the magnitude of the formal charges as low as possible
Avoid multiple formal charges on any one atom.
Section 6.4
Molecular Orbital Picture of Resonance
Allyl AnionAllyl Anion
..
..
CH2 CH CH2 CH2 CH CH2
Energy levels
nodes
0
1
2
Three atomic p orbitalscombine to make a pisystem of three molecular orbitals.
nodes
0
1
2
CH2 CH CH2 CH2 CH CH2+ +
..
+
Allyl CationAllyl Cation
nodes
0
1
2
RESONANCE IS FOUND IN CONJUGATED SYSTEMS
Sect. 6.5 Conjugated SystemsSect. 6.5 Conjugated Systems
A conjugated system has alternating double and single bonds.
These systems are not conjugated.
Unconjugated Unconjugated SystemsSystems
CH2 CH CH2 CH CH2
two singlebonds
three singlebonds
Unconjugated
1,3-Butadiene
CH CH
CH2
CH2
Resonance in Butadiene
Resonance provides single bond character betweencarbons 1 and 2 and also between carbons 3 and 4. These bonds become LONGER
CH2 CH CH CH2
CH2 CH CH CH2 CH2 CH CH CH2
+ + __ 1 2 3 4
Resonance in Butadiene
CH2 CH CH CH2
CH2 CH CH CH2 CH2 CH CH CH2
+ + __
Resonance provides double bond character betweencarbons 2 and 3. This bond become SHORTER
1 2 3 4
Bond Lengths in Butadiene
H
HC
HH
H
H
Longer 1.338
Longer 1.338Shorter 1.354
Full double bond 1.330
Full single bond 1.370
3
Sect. 6.6 M.O. Description ofSect. 6.6 M.O. Description of ButadieneButadiene
. .. .
2
1
0
nodes
Four atomic p-orbitals = Four molecular orbitals
There are four electrons!
four p orbitalscombine to makea system of fourpi molecular orbitals
( a system ) node
node
nodes
3 nodesPictures of Molecular orbitals!
Section 6.7 Bond Order
Double bonds = 2
Single bonds = 1
Resonance = bond orders between 1 and 2
Sect. 6.8-6.13 Benzene
Kekule Structures
Molecular Orbital Picture of Benzene
Friedrich Auguste Kekulé
Source: Michigan State University, Department of Chemistryhttp://www.chemistry.msu.edu/Portraits/PortraitsHH_collection.shtml
Benzene Pi Molecular OrbitalsBenzene Pi Molecular Orbitalssix p’ssix mo’s
topview
0 node
1 node
2 node
3 node
Resonance Energy
H(kcal/mole)
calc.
expt.
-85.8(calc)
-36 kcal/mol(resonance energy)
-49.8 kcal/mol (expt.)
-28.6 kcal/mol
" "
3x 28.6
Resonance in other Systems
etc
Cyclopentadienyl AnionCyclopentadienyl Anion
0 node
1 node
2 node
HH
HH
.. .
.
H
..
Orbitals in Cyclopentadienyl AnionOrbitals in Cyclopentadienyl Anion
Cyclopentadienyl Anion Cyclopentadienyl Anion