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Chapter 13

Conjugated Unsaturated

Systems

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  1. Introduction
  • A conjugated system involves at least one atom with a p orbital adjacent to at least one π bond
    • e.g.

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  1. Allylic Substitution and the �Allyl Radical

vinylic carbons (sp2)

allylic carbon (sp3)

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2A. Allylic Chlorination�(High Temperature)

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  • Mechanism
    • Chain initiation
    • Chain propagation

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  • Mechanism
    • Chain propagation
    • Chain termination

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

  • NBS is a solid and nearly insoluble in CCl4
    • Low concentration of Br•

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  • Examples

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3B. Resonance Description of the �Allyl Radical

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  1. The Allyl Cation
  • Relative order of Carbocation stability

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  1. Resonance Theory Revised

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

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

resonance structures

not resonance structures

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  • All of the structures must be proper Lewis structures

10 electrons!

X

not a proper

Lewis structure

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

X

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

no delocalization

of π-electrons

delocalization

of π-electrons

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  • 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

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

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5B. Estimating the Relative Stability �of Resonance Structures

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

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  • 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

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  • Charge separation decreases stability

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  1. Alkadienes and Polyunsaturated �Hydrocarbons
  • Alkadienes (“Dienes”)

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  • Alkatrienes (“Trienes”)

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  • Alkadiynes (“Diynes”)
  • Alkenynes (“Enynes”)

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  • Cumulenes

enantiomers

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  • Conjugated dienes
  • Isolated double bonds

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  1. 1,3-Butadiene: Electron �Delocalization

7A. Bond Lengths of 1,3-Butadiene

1.34 Å

1.47 Å

1.54 Å

1.50 Å

1.46 Å

sp3

sp3

sp

sp3

sp2

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7B. Conformations of 1,3-Butadiene

cis

trans

single

bond

single

bond

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  1. Electrophilic Attack on Conjugated�Dienes: 1,4 Addition

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  • Mechanism

X

(a)

(b)

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

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

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  • e.g.

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11A. Factors Favoring the Diels–Alder�Reaction

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

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

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  • Relative rate

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  • Relative rate

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  • Steric effects

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11B. Stereochemistry of the

Diels–Alder Reaction

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

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

X

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  • e.g.

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  • 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

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

longest bridge

R is exo

R is endo

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  • Stereospecific reaction

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  • Stereospecific reaction

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  • Examples

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

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  • Examples

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  • Examples
    • Rate of Diene C > Diene D (27 times), but Diene D >> Diene E
    • In Diene C, tBu group 🡪 electron donating group 🡪 increase rate
    • In Diene E, 2 tBu group 🡪 steric effect, cannot adopt s-cis conformation