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II.C. ResonanceII.C. Resonance1. Definition

2. Curved arrow notation

a. The arrow

b. Mechanics of e- movement

3. Patterns to recognize

a. Lone pair next to a pi bond

b. Lone pair next to an electron deficient atom

c. Pi bond next to an electron deficient atom

d. Pi bond between two atoms with an electronegativity difference

e. Alternating pi bonds in a ring

4. Examples

a. Single positive charge

b. Single negative charge

c. No charge (polarized pi bond)

5. Rules for resonance structures

6. Predicting relative energies of resonance contributors

11. . DefinitionDefinition

C NH

HH O

OC NH

HH O

O

Lewis structure for CH3NO2 = 24 valence e-

Resonance contributorResonance contributor

Resonance HybridC NH

HH O

O

d-

d-

Electrons are DELOCALIZED

• A molecule can’t always be accurately represented by one Lewis structure• These molecules are weighted average, or hybrids, of two or more Lewis structures

(electrons do not move to either one of the oxygen atoms or the other)

When one Lewis structure just isn’t enough . . .

2

A Resonance MetaphorA Resonance Metaphor

resonance contributorimaginary resonance contributor

imaginary

Resonance HybridReal!

Why is Resonance So Why is Resonance So VeryVery Important? Important?>95% of 5.12 reactions occur because one molecule containing a region of

high e- density is attracted to a molecule containing a region of low e- density

+

To predict how and when two molecules will react, need to be able topredict the regions of low and high electron density

O N O O N OONO

Resonance hybrid(lone pairs not depicted in

resonance hybrids)

O N Od- d-d-

C4H6NO2-

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2. Tracking2. Tracking Changes in e Changes in e-- Configuration Configuration

O N O O N OONO

Need a way to keep track of the changes in electronNeed a way to keep track of the changes in electronconfiguration between resonance contributorsconfiguration between resonance contributors

22.a. The Arrow.a. The Arrow

Electron redistribution (change in configuration)

is the origin of chemical change (reactivity)

e- pairdouble-headed arrow

single e-

fishhook arrow

shows “movement” of electrons (electron flow)

Electrons move from a “source” to a “sink”

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Curved Arrow NotationCurved Arrow Notation1. Graphical way to depict changes in electron configuration during a reaction.

2. Show how electron configurations can be rearranged to generate analternative bonding representation of the same structure (resonance)

NH H

H F BFF+

BF F

F

NHH

HCO

HH

OC

H H

Electron SourceElectron Source• Pi e-

• Nonbonded e- (lone pairs)

• Electronegative atom

• Atom with an open shell

• Atom with a positive charge

Electron SinkElectron Sink

Arrows must be very precise on problem sets and exams!Electrons must move from “source” to “sink”

22.b. Mechanics of e.b. Mechanics of e-- Movement Movement

A

A

A

Mechanics1. Nonbonding pair to adjacent bond (vertex-to-edge transfer)

A

2. Bonding pair to an adjacent atom (edge-to-vertex transfer)

3. Bonding pair to an adjacent bond (vertex-edge-vertex transfer)

O N OONO O N O

Notice that the sigma network does not change and the placement of atoms remains the same.

A

A

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55. Rules for Resonance Structures. Rules for Resonance Structures1. Resonance forms are imaginary

2. They differ only in the placement of pi or nonbonding electrons, atomplacement is the same(Electron movement takes place in the pi system, not sigma systemElectron movement takes place in the pi system, not sigma system)

3.3. Must be valid Lewis structures and obey the rules ofMust be valid Lewis structures and obey the rules of valency valency

4. Different resonance forms don’t have to be energetically equivalent

(Lower energy resonance structures contribute most to overall structureof molecule)

5. The resonance hybrid (weighted average) is more stable than anyindividual resonance form

6. Use a double-headed arrow between structures and brackets aroundthem, keep track of lone pairs and formal charges

66. Energies of Resonance Contributors. Energies of Resonance Contributors

Resonance hybrid: weighted average of resonance contributors

Which structure contributes more in resonance hybrid?

In other words: which structure is more STABLESTABLE

(has lower potential ENERGYENERGY))

O N OONO O N O

O N Od- d-d-

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H2CNCH3

CH3 H2CNCH3

CH3

Predicting Energies of Resonance StructuresPredicting Energies of Resonance Structures

i. Filled octets for second row elements (C, N, O, F)possible for C only to be electron deficient (6 e-)

ii. Minimum # of formal charges and maximum number of bonds

iii. Negative charge on most electronegative atom (C<N<O)

iv. Minimize charge separation, keep formal charges close together

H2CNCH3

CH3

MajorFollows guidelines

MinorViolates i

MinorViolates i

A Second Example . . .A Second Example . . .

A: follows all guidelines

B: violates ii (2 formal charges)

C: violates ii (2 formal charges) and iii (negative charge on C)

D: violates i (6 e- on C) and ii (2 formal charges)

E: violates i (6 e- on C), ii (2 formal charges), and iv (more chargeseparation than D

H3CO

CN H3C

OC

N H3CO

CN

H3CO

CN H3C

OC

N

A

ED

CB

Relative energy: A << B < C << D < E

Relative contribution to resonance hybrid: A > B > C > D > E