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

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 (sp2)

allylic carbon (sp3)


2A.Allylic Chlorination(High Temperature)


  • Mechanism

    • Chain initiation:

  • Chain propagation:


  • Mechanism

    • Chain propagation:

  • Chain termination:


Allylic vs vinyl bond energies:


Allylic vs vinyl activation energies:


Radical stabilities:


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

  • NBS is a solid and nearly insoluble in CCl4.

    • Low concentration of Br•


  • Examples:


3. The Stability of the Allyl Radical

3A.Molecular Orbital Description of the Allyl Radical


Molecular

orbitals:


3B.Resonance Description of the Allyl Radical


The Allyl Cation

  • Relative order of Carbocation stability.


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.


resonance structures

not resonance structures

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


  • All of the structures must be proper Lewis structures.

X

10 electrons!

not a proper

Lewis structure


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

X


  • 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


  • 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.


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


5B.Estimating the Relative Stability of Resonance Structures

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


  • 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


  • Charge separation decreases stability.


Alkadienes and Polyunsaturated Hydrocarbons

  • Alkadienes (“Dienes”):


  • Alkatrienes (“Trienes”):


  • Alkadiynes (“Diynes”):

  • Alkenynes (“Enynes”):


  • Cumulenes:

enantiomers


  • Conjugated dienes:

  • Isolated double bonds:


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


7B.Conformations of 1,3-Butadiene

trans

single

bond

single

bond

cis


7C.Molecular Orbitals of 1,3-Butadiene


The Stability of Conjugated

Dienes

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


Stability due to conjugation:


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.


9A.The Electromagnetic Spectrum


9B.UV–Vis Spectrophotometers


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)


9C.Absorption Maxima for Nonconjugatedand Conjugated Dienes


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.


Electrophilic Attack on ConjugatedDienes: 1,4 Addition


  • Mechanism:

X

(a)

(b)


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


The 1,4-product is thermodynamically more stable.


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


  • e.g.


11A.Factors Favoring the Diels–AlderReaction

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


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


  • Relative rate:


  • Relative rate:


  • Steric effects:


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.


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

X


  • e.g.


  • 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:


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


  • Alder-Endo Rule:

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


  • e.g.


  • Stereospecific reaction:


  • Stereospecific reaction:


  • Examples:


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


  • Examples:


  • 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


 END OF CHAPTER 13 


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