Chapter 13
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Chapter 13. Conjugated Pi Systems. Introduction. A conjugated system involves at least one atom with a p orbital adjacent to at least one p bond. e.g. Allylic Substitution and the Allyl Radical. vinylic carbons (sp 2 ). allylic carbon (sp 3 ).

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Chapter 13

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Chapter 13

Chapter 13

Conjugated Pi

Systems


Chapter 13

Introduction

  • A conjugated system involves at least one atom with a p orbital adjacent to at least one p bond.

    • e.g.


Chapter 13

Allylic Substitution and the Allyl Radical

vinylic carbons (sp2)

allylic carbon (sp3)


Chapter 13

2A.Allylic Chlorination(High Temperature)


Chapter 13

  • Mechanism

    • Chain initiation:

  • Chain propagation:


Chapter 13

  • Mechanism

    • Chain propagation:

  • Chain termination:


Chapter 13

Allylic vs vinyl bond energies:


Chapter 13

Allylic vs vinyl activation energies:


Chapter 13

Radical stabilities:


Chapter 13

2B.Allylic Bromination with N-Bromo-succinimide (Low Concentration of Br2)

  • NBS is a solid and nearly insoluble in CCl4.

    • Low concentration of Br•


Chapter 13

  • Examples:


Chapter 13

3. The Stability of the Allyl Radical

3A.Molecular Orbital Description of the Allyl Radical


Chapter 13

Molecular

orbitals:


Chapter 13

3B.Resonance Description of the Allyl Radical


Chapter 13

The Allyl Cation

  • Relative order of Carbocation stability.


Chapter 13

Resonance Theory Revisited

5A. Rules for Writing Resonance Structures

  • Resonance structures exist only on paper. Although they have no real existence of their own, resonance structures are useful because they allow us to describe molecules, radicals, and ions for which a single Lewis structure is inadequate.

  • We connect these structures by double-headed arrows (), and we say that the hybrid of all of them represents the real molecule, radical, or ion.


Chapter 13

resonance structures

not resonance structures

  • In writing resonance structures, one may only move electrons.


Chapter 13

  • All of the structures must be proper Lewis structures.

X

10 electrons!

not a proper

Lewis structure


Chapter 13

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

X


Chapter 13

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

no delocalization

of p-electrons

delocalization

of p-electrons


Chapter 13

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

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


Chapter 13

  • The more stable a resonance structure is (when taken by itself), the greater is its contribution to the hybrid.


Chapter 13

5B.Estimating the Relative Stability of Resonance Structures

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


Chapter 13

  • Structures in which all of the atoms have a complete valence shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.

this carbon has

6 electrons

this carbon has

8 electrons


Chapter 13

  • Charge separation decreases stability.


Chapter 13

Alkadienes and Polyunsaturated Hydrocarbons

  • Alkadienes (“Dienes”):


Chapter 13

  • Alkatrienes (“Trienes”):


Chapter 13

  • Alkadiynes (“Diynes”):

  • Alkenynes (“Enynes”):


Chapter 13

  • Cumulenes:

enantiomers


Chapter 13

  • Conjugated dienes:

  • Isolated double bonds:


Chapter 13

1,3-Butadiene: Electron Delocalization

7A.Bond Lengths of 1,3-Butadiene

1.47 Å

1.34 Å

sp

sp3

sp3

sp2

sp3

1.46 Å

1.54 Å

1.50 Å


Chapter 13

7B.Conformations of 1,3-Butadiene

trans

single

bond

single

bond

cis


Chapter 13

7C.Molecular Orbitals of 1,3-Butadiene


Chapter 13

The Stability of Conjugated

Dienes

  • Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes.


Chapter 13

Stability due to conjugation:


Chapter 13

Ultraviolet–Visible Spectroscopy

  • The absorption of UV–Vis radiation is caused by transfer of energy from the radiation beam to electrons that can be excited to higher energy orbitals.


Chapter 13

9A.The Electromagnetic Spectrum


Chapter 13

9B.UV–Vis Spectrophotometers


Chapter 13

A

c x ℓ

ore=

  • Beer’s law

A=absorbance

e=molar absorptivity

c=concentration

ℓ=path length

A=e x c x ℓ

  • e.g. 2,5-Dimethyl-2,4-hexadiene

    lmax(methanol) 242.5 nm

    (e = 13,100)


Chapter 13

9C.Absorption Maxima for Nonconjugatedand Conjugated Dienes


Chapter 13

9D. Analytical Uses of UV–Vis Spectroscopy

  • UV–Vis spectroscopy can be used in the structure elucidation of organic molecules to indicate whether conjugation is present in a given sample.

  • A more widespread use of UV–Vis, however, has to do with determining the concentration of an unknown sample.

  • Quantitative analysis using UV–Vis spectroscopy is routinely used in biochemical studies to measure the rates of enzymatic reactions.


Chapter 13

Electrophilic Attack on ConjugatedDienes: 1,4 Addition


Chapter 13

  • Mechanism:

X

(a)

(b)


Chapter 13

10A.Kinetic Control versus Thermodynamic Control of a Chemical Reaction


Chapter 13

The 1,4-product is thermodynamically more stable.


Chapter 13

The Diels–Alder Reaction: A 1,4-Cycloaddition Reaction of Dienes


Chapter 13

  • e.g.


Chapter 13

11A.Factors Favoring the Diels–AlderReaction

  • Type A and Type B are normal Diels-Alder reactions


Chapter 13

  • Type C and Type D are Inverse Demand Diels-Alder reactions


Chapter 13

  • Relative rate:


Chapter 13

  • Relative rate:


Chapter 13

  • Steric effects:


Chapter 13

11B.Stereochemistry of the

Diels–Alder Reaction

The Diels–Alder reaction is stereospecific: The reaction is a syn addition, and the configuration of the dienophile is retained in the product.


Chapter 13

The diene, of necessity, reacts in the s-cis rather than in the s-trans conformation.

X


Chapter 13

  • e.g.


Chapter 13

  • Cyclic dienes in which the double bonds are held in the s-cis conformation are usually highly reactive in the Diels–Alder reaction.

  • Relative rate:


Chapter 13

The Diels–Alder reaction occurs primarily in an endo rather than an exo fashion when the reaction is kinetically controlled.

R is exo

longest bridge

R is endo


Chapter 13

  • Alder-Endo Rule:

    • If a dienophile contains activating groups with p bonds they will prefer an ENDO orientation in the transition state.


Chapter 13

  • e.g.


Chapter 13

  • Stereospecific reaction:


Chapter 13

  • Stereospecific reaction:


Chapter 13

  • Examples:


Chapter 13

  • Diene A reacts 103 times faster than diene B even though diene B has two electron-donating methyl groups.


Chapter 13

  • Examples:


Chapter 13

  • Examples

  • Rate of Diene C > Diene D (27 times), but Diene D >> Diene E

  • In Diene C, t-Bu group  electron donating group  increase rate

  • In Diene E, 2 t-Bu group  steric effect, cannot adopt s-cis conformation


Chapter 13

 END OF CHAPTER 13 


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