Delocalized π Systems | Organic Chemistry 2

The delocalized π systems are studied in this chapter: the overlap of 3 adjacent p orbitals, the allylic reactions, the conjugated dienes and their reactions, the Diels-Alder cycloadditions, the kinetic and thermodynamic control.

Overlap of 3 Adjacent p Orbitals


 

They are stabilized by electron delocalization (resonance).
The activated carbon is called allylic.

 

The stabilization of the allyl system can also be described in terms of molecular orbitals (MO):
 

 
The 3 p orbitals in the allyl group overlap: symmetric structure with delocalized electrons.


The 3 π MO of allyl, obtained by combining the 3 adjacent atomic p orbitals.
​​​​

 

Allylic Reactions

Radical Halogenation:
 


Mechanism:

Initiation step


Propagation steps

 

SN Reactions:

Allylic halides undergo both SN1 and SN2 reactions
 


 

Allylic Organometallic Reagents:

 

Conjugated Dienes

2 double bonds separated by a single bond ⇒ 4 contiguous p orbitals that overlap and allow distribution of π electrons across 4 carbon centers.


 2 conformations:

π-electronic structure: 4 π MO obtained by combining the 4 adjacent atomic p orbitals:
 


 Hydrogenation of Conjugated Dienes:
 

 

Extended π system: more than 2 double bonds in conjugation.
Benzene is unusually stable and unreactive.

Electrophilic Reactions of Dienes

Kinetic vs. Thermodynamic Control

Kinetic Control: Low temperature (irreversible conditions) + Short Time
⇒ Major product = product from the fastest reaction.

Thermodynamic Control: High temperature (reversible conditions) + Long Time
⇒ Major product = the most stable product.

 

Hydrobromination of Dienes:

- Kinetic Product 1,2-addition



Thermodynamic Product = 1,4-addition

Diels-Alder Cycloaddition

 

Mechanism:
 


 

[4+2] cycloaddition + concerted and stereospecific reaction
Diene must be in the s-cis conformation

 

DA reaction is accelerated when the diene contains electron donating group (EDG) ⇒ electronically richer

 

DA reaction is accelerated when the dienophile contains electron withdrawing group (EWG) ⇒ electronically poorer

 

DA reaction forms the endo product.

o = outside; i = inside.