Section 2: Conjugated Dienes – Chapter 13

beta-carotene

Practice Problems:

13.1 – 13.513.9 – 13.1413.15 – 13.2013.24 – 13.2513.29 – 13.3013.34 – 13.4213.47 – 13.4813.50

Omitted Sections:

13.9

Rules for Writing Resonance Structures

1. The double-headed arrow indicates resonance structures.

2. Resonance structures only exist on paper.

3. In writing resonance structures, we are only allowed to move electrons.

4. All of the structures must be proper Lewis structures.

5. All resonance structures must have the same number of unpaired electrons.

6. All atoms that are part of the delocalized p-electron system must lie in a plane or be nearly planar.

7. The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure.

8. Equivalent resonance structures make equal contibution to the hybrid, and a system described by them has a large resonance stabilization.

9. The more stable a resonance structure is (by itself) the greater its contribution to the hybrid.

What Is A Resonance Hybrid?

=( )

Rules for Estimating Resonance Structure Stability

1. The more covalent bonds a structure has, the more stable it is.

2. Structures in which all of the atoms have a complete valence shell of electrons are especially stable and make large contributions to the hybrid.

3. Charge separation decreases stability.

Rules for Writing Resonance Structures

1. The double-headed arrow indicates resonance structures.

2. Resonance structures only exist on paper.

3. In writing resonance structures, we are only allowed to move electrons.

4. All of the structures must be proper Lewis structures.

5. All resonance structures must have the same number of unpaired electrons.

6. All atoms that are part of the delocalized p-electron system must lie in a plane or be nearly planar.

7. The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure.

8. Equivalent resonance structures make equal contibution to the hybrid, and a system described by them has a large resonance stabilization.

9. The more stable a resonance structure is (by itself) the greater its contribution to the hybrid.

9a.The more covalent bonds a structure has, the more stable it is.

9b.Structures where all of the atoms have a complete valence shell of electrons are especially stable and make large contributions to the hybrid.

9c.Charge separation decreases stability.

The Allyl Radical

Conjugated Alkenes

Progesterone an enone

Retinal (Vitamin A) an enal

Two alkenes that are isolated from one another by sp3 centres react (pretty much) like the parent alkenes. Conjugated and cumulated dienes exhibit special reactivity. The reactions of conjugated systems will be the focus of the rest of this section.

Importance of the Diels-Alder Reaction in Synthesis

plus many thousands more!

Some Facts About Dienes:1. A “normal-demand” Diels-Alder reaction is favoured by:

● electron-withdrawing groups in the dienophile● electron-donating groups on the diene

2. In a Diels-Alder reaction, the endo product dominates at low temperatures.

3. The “single bond” in a conjugated system is short.

4. Conjugated dienes are more stable than isolated dienes.

Why?

Why?

Why?

Why?

A Molecular Orbital Interaction Diagram for the DA Reaction

A Molecular Orbital Interaction Diagram for the DA Reaction

Kinetic Preference for the Endo Diels Alder Product

DieneHOMO

DienophileLUMO

CH3

H

H

O

OMe

H

H

H3C

O

MeO

OCH3

H

CH3

H

OCH3

H

CH3

HDieneHOMO

DienophileLUMO

Endo TS - favoured kinetically due to

2o orbital overlap

Exo TS - may (or may not) lead to a more stable product, but disfavoured kinetically