Organic Chemistry Chapter 7 - Resonance. Electron Delocalization and Resonance Localized electrons =...

Organic Chemistry

Chapter 7 - Resonance

Electron Delocalization and Resonance

• Localized electrons = restricted to a particular region

• Delocalized electrons do not belong to a single atom or exclusively to a bond between 2 atoms

• BenzeneC6H6

– Rapid Equilibrium between 2 structures

– Proposed by Fredrich Kekule (1865 German chemist)

Kekule Structure

Rapid Equilibrium

• *

Kekule’s Dream

• Kekule Structures of Benzene were accepted in the 1930’s when X-ray studies showed ALL SIX C-H bonds equal and ALL SIX C-C bonds equal!

Benzene Structure

H

H

H

H

H

H

• Each C is sp2 hybridized

• Each C has an unhybridized p orbital perpendicular to the plane of the ring

• The 6 p orbitals overlap to form a cloud

Bonding in Benzene

• A compound with delocalized e- is said to have resonance– resonance contributor– resonance structure– contributing resonance structure

Resonance Hybrid

• Benzene– contributing resonance structures

Resonance Hybrid

Drawing resonance hybrids• 1) Only e- move (not atoms)

• 2) Only and non-bonding e- move

• 3) Total # e- stays same (as does unpaired e-)

Resonance Hybrids

e- can be moved only by…

e- move toward + or toward bond

Resonance Hybrids


Nonbonding pair e- toward a bond

Resonance Hybrids


Nonbonding single e- toward a bond

Resonance Hybrids

• Drawing resonance hybrids

Resonance Hybrids

Resonance Hybrids

Practice Drawing Resonance

Practice

Practice

Practice

• What makes a Resonance Structure Have Decreased Stability?

– 1) an atom with an incomplete octet– 2) a negative charge that is not on the most

electronegative atom– 3) a positive charge not on the most

electropositive atom– 4) charge separation

Examples To Examine

B is less stable than AEqual Stability

• 1) The greater the predicted stability of a resonance contributor, the more it contributes to the resonance hybrid.

• 2) The greater the number of relatively stable resonance contributors, the greater the resonance energy.

• 3) The more nearly equivalent the resonance contributors, the greater the resonance energy.

Resonance Energy

Resonance Energy

The more nearly equivalent the resonance contributors,

the greater the resonance energy

Stability of Dienes

Stability of butadiene

Butadiene – Example

Br

OCH3

[

]

Br

OCH3

Br

OCH3

Br

OCH3

• Allylic cations have delocalized e-

• They are more stable due to resonance contributors:

Allylic Cations

• Benzylic cations have delocalized e-

• They are more stable due to resonance contributors:

Benzylic Cations

• *

Stability of Cations

• Because of the allyl and benzyl cations and radicals are more stable, some products can be formed easier.

Consequences

Mechanisms Mechanisms MechanismsLook at how conjugation affects reactions

• Carboxylic acids are stronger acids than alcohols due to resonance effects

Effects on pKa

• Why is this alcohol as acidic as it is?– -OH is on an sp2 hybridized C– In phenol, some resonance contributors have a +

charge on O– 3 of phenol’s structures have separated charges

Phenol

• Molecular Orbital Theory can also help explain increased stability

• READ THROUGH THIS SECTION OF THE BOOK

Molecular Orbital Theory

Thermodynamic vs. Kinetic Control

• If the kinetic product and thermodynamic product differ, the major product will depend on reaction conditions.

• If carried out at mild (low-temp) conditions, the reaction will be irreversible and the kinetic product will be favored.

• If carried out at sufficiently vigorous (hi-temp) conditions, the reaction will be reversible and the thermodynamic product will be favored.


• Kinetic product and thermodynamic product:

Formed fastest

Most stable


• Reaction conditions control products!

Kinetic product – low temp

Thermodynamic product – high temp

Diels-Alder Reaction

• This reaction makes new carbon carbon bonds

• Uses a conjugated diene and a dienophile

Otto DielsKiel University Kiel

Kurt AlderCologne University Cologne


• Electron withdrawing groups (O or N) increases the reactivity of the dienophile and increase the reactivity

acts as an electrophile




• 1,4 addition to a conjugated diene



Diels-Alder Stereochemistry

• If a chiral center is created, equal amounts of R, S produced (racemic)



Diels-Alder Stereochemistry

• Stereospecific: cis dienophile remains cis– trans dienophile remains trans



Diels-Alder Examples

• Dienophiles can also be C triple bond C



Bicycloalkanes

• Nomenclature– parent is the alkane of the same number of carbons as are in two rings– number from a bridgehead, along longest bridge back to the

bridgehead, then along the next longest bridge, etc.– show the lengths of bridges in brackets, from longest to shortest

bicyclo[2.2.1]heptane

1

2

3

4

5

6

7

Bicyclics - Nomenclature

• Examples

Organic Chemistry Chapter 7 - Resonance. Electron Delocalization and Resonance Localized electrons =...

Documents

Transcript of Organic Chemistry Chapter 7 - Resonance. Electron Delocalization and Resonance Localized electrons =...