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

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:

Allylic Reactions

Radical Halogenation:


  1. Initiation step

  2. Propagation steps



SN Reactions:

Allylic halides undergo both SN1 and SN2 reactions





Allylic Organometallic Reagents:



Electrophilic Reactions of Dienes

​​​​​​​Addition of hydrogen halide:



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


[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.