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Two Step Mechanisms

Diels-Alder Reaction

  • A Diels-Alder reaction is an addition reaction between a 1,3 diene and an alkene (dienophile) to form a new six membered ring
  • The rate of the reaction is determined by its energy of activation and the amount of product present at equilibrium is determined by its stability
  • At a low temperature, the lower energy of activation pathway is followed and the kinetic product is formed
  • At a high temperature, the higher energy of activation pathway is followed and the thermodynamic product is formed

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Isolated Dienes

Conjugated Dienes

  • The pi bonds are too far apart to be considered conjugated
  • Isolated dienes are less stable than the similar conjugated dienes

Three Atom "Allyl" System

  • Conjugated dienes are compounds with two double bonds joined with one sigma bond
  • They are also called 1,3 dienes
  • The C,C single bond between the double bonds is short
  • When three atoms have one double bond and the third atom contains a p orbital with zero, one, or two electrons
  • Two resonance structures are possible

Rules of Diels-Alder Reactions

Products of Electrophilic Addition

1,4-pentadiene

  • Electrophilic additon of HBr to an isolated diene yields one product and Markovnikov's rule is followed
  • Electrophilic addition of HBr to a conjugated diene gives a mixture of two products called the 1,4 product (thermodynamic) and the 1,2 product (kinetic)
  • The diene can react only when it adopts s-cis conformation
  • Electron-withdrawing substituents in the dienophile increase the reaction rate
  • The stereochemistry of the dienophile is retained
  • When a diene reacts with a substituted alkene as the dienophile, the substituent can be oriented in exo or endo in the product

1,4 product

1,2 product

Properties of Conjugated Dienes

Conjugated Double Bond

Conjugated Dienes

  • Cyclic, completely conjugated rings have two resonance structures
  • Conjugated dienes have three resonance structures

By JoEllen Holland and Kenneth Kuemeta

  • Conjugation - occurs when p orbitals overlap on three or more adjacent atoms
  • Delocalization - having three or more p orbitals on adjacent atoms which allows p orbitals to overlap and electrons to delocalize

1,3-butadiene

Formation of Ring Systems

Stereoisomers of Conjugated Dienes

  • a bicyclic product is formed from a cyclic dienophile
  • a fused ring system is when two rings share a common C,C bond
  • a bridged ring system is when two rings share nonadjacent carbon atoms
  • Stereoisomers are the different orientations of the molecule
  • The number of stereoisomers depends on the number of different, possible orientations of the molecule

trans, trans isomer

cis, cis isomer

fused ring system

bridged ring system

Resonance Hybrids

Cations with a Positive Charge Adjacent to Lone Pair

  • The resonance hybrid is a combination of the resonance structures
  • Resonance structures with more bonds and less charges are more stable
  • Structures with atoms having full octets are more stable
  • Structures with that have a negative charge on the more electronegative atom are more stable
  • The delocalization of the pi bond causes the hybrid to be more stable than the resonance structures

Double Bonds with a More Electronegative Atom

  • When a lone pair and a positive charge are located on adjacent atoms, two resonance structures can be drawn
  • The overall charge is the same in both resonance structures
  • For a double bond in which the electronegativity is greater in one atom, a second resonance structure can be drawn by moving the pi electrons onto the more electronegative atom

Conformers of Conjugated Dienes

Retro Diels-Alder Reactions

  • When heated, dicyclopentadiene undergoes a retro Diels-Alder reaction and two molecules of cyclopentadiene are re-formed
  • Two possible conformations result from rotation around the C,C double bond that join the two double bonds
  • These conformations are either eclipsed or staggered. The staggered conformation is the more stable

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s-cis conformation

s-trans conformation

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