Delocalized π Systems | Organic Chemistry 2
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 halides undergo both SN1 and SN2 reactions
Allylic Organometallic Reagents:
2 double bonds separated by a single bond ⇒ 4 contiguous p orbitals that overlap and allow distribution of π electrons across 4 carbon centers.
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
[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.