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