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:


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




[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.