Delocalized π Systems | Organic Chemistry 2

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

Overlap of 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 of the allyl group overlap: symmetrical structure with delocalized electrons


The 3 π MO of allyl, obtained by combining the 3 adjacent atomic p orbitals
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Allylic Reactions

Radical Halogenation:
 


​​​​​​​Mechanism:

  1. Initiation step


     
  2. Propagation steps

    ​​​​​​​​​​​​​​

 

SN Reactions:

Allylic halides undergo both SN1 and SN2 reactions

SN1:

SN2:

 

 

Allylic Organometallic Reagents:
 

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Conjugated Dienes

2 double bonds separated by a single bond ⇒ 4 contiguous p orbitals that overlap and allow the 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:
 

 

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

 

Hydrogenation of Conjugated Dienes:
 

Electrophilic Reactions of Dienes

​​​​​​​Addition of hydrogen halide:
 

 

Halogenation:
 

Kinetic versus Thermodynamic Control

Kinetic control:

Irreversible conditions: low temperature + short time
The major product is the product from the fastest reaction
 

Thermodynamic control:

Reversible conditions: high temperature + long time
The major product is the most stable product

 

Hydrobromination of Dienes:
 

The kinetic product is the product of a 1,2-addition

 

The thermodynamic product is the product of a 1,4-addition

Diels-Alder Cycloaddition


​​​​​​​Mechanism:

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

 

Diels-Alder reaction is accelerated when the diene contains an electron donating group (EDG) ⇒ electronically richer

 

Diels-Alder reaction is accelerated when the dienophile contains an electron withdrawing group (EWG) ⇒ electronically poorer

 

Diels-Alder reaction forms the endo product

o = outside; i = inside.