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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

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)


  • Chain propagation:


  • Chain termination:





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

  • NBS is a solid and nearly insoluble in CCl4.

    • Low concentration of Br•



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.


X

10 electrons!

not a proper

Lewis structure



no delocalization

of p-electrons

delocalization

of p-electrons




5B. itself), the greater is its contribution to the hybrid.Estimating the Relative Stability of Resonance Structures

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


this carbon has

6 electrons

this carbon has

8 electrons



Alkadienes and Polyunsaturated shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.Hydrocarbons

  • Alkadienes (“Dienes”):


  • Alkatrienes (“Trienes”): shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


  • Alkadiynes (“Diynes”): shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.

  • Alkenynes (“Enynes”):


  • Cumulenes: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.

enantiomers


  • Conjugated dienes: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.

  • Isolated double bonds:


1,3-Butadiene: Electron shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.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. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.Conformations of 1,3-Butadiene

trans

single

bond

single

bond

cis


7C. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.Molecular Orbitals of 1,3-Butadiene


The Stability of Conjugated shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.

Dienes

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


Stability due to conjugation: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


Ultraviolet–Visible shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.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. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.The Electromagnetic Spectrum


9B. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.UV–Vis Spectrophotometers


A shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.

c x ℓ

or e =

  • 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. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.Absorption Maxima for Nonconjugatedand Conjugated Dienes


9D. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.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 Conjugated shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.Dienes: 1,4 Addition


  • Mechanism: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.

X

(a)

(b)


10A. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.Kinetic Control versus Thermodynamic Control of a Chemical Reaction


The 1,4-product is thermodynamically more stable. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


The Diels–Alder Reaction: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.A 1,4-Cycloaddition Reaction of Dienes


  • e.g. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


11A. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.Factors Favoring the Diels–AlderReaction

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



  • Relative rate: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


  • Relative rate: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


  • Steric effects: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


11B. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.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- shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.cis rather than in the s-trans conformation.

X


  • e.g. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.



The Diels–Alder reaction occurs primarily in an shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.endo rather than an exo fashion when the reaction is kinetically controlled.

R is exo

longest bridge

R is endo


  • Alder-Endo Rule: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.

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


  • e.g. shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


  • Stereospecific reaction: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


  • Stereospecific reaction: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


  • Examples: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


  • Diene A reacts 10 shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.3 times faster than diene B even though diene B has two electron-donating methyl groups.


  • Examples: shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


  • Examples shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.

  • 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  shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.


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