Delocalized π Systems | Organic Chemistry 2
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:
Mechanism:
- Initiation step
- Propagation steps
SN Reactions:
Allylic halides undergo both SN1 and SN2 reactions
SN1:
SN2:
Allylic Organometallic Reagents:
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.
