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
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
- 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
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
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
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