2c9gaea 14 Important Concepts 1.Allyl Systom-The 2-propenyl system is stabilized 器 2. ChemistryfPropenyl Syatem-Sube 3.a odn的epoe 4. heceeraedby 14 Important Concepts 5 6 Conjugation in 1 7 ck on 1. 8 Particularly stable due to cyclic 1212 Conjugation in cyclopolyenes is governed by a separate set of rules to be discussed later. Benzene is colorless, while azulene is a deep blue color: 14 Important Concepts 1. Allyl System – The 2-propenyl system is stabilized by resonance. • Three π molecular orbitals: one bonding, one non￾bonding and one antibonding • Forms a symmetric structure with charges or odd electrons equally distributed between the two end carbons 2. Chemistry of 2- Propenyl System – Subject to both thermodynamic and kinetic control 3. Allylic Radical Stability – Allows radical halogenations of alkenes at the allylic position 4. Allylic Halides – SN2 reaction is accelerated by orbital overlap in the transition state 14 Important Concepts 5. Allylic Deprotonation – Occurs with a strong base (butyllithium-TMEDA) due to special stability of allylic anions 6. Conjugation in 1,3 Dienes – Revealed by their resonance energy and short internal bond (1.47 Å) 7. Electrophilic Attack on 1,3 Dienes – Leads to preferential formation of allylic cations 8. Extended Conjugated Systems – Are reactive due to many sites of attack and resonance stabilized intermediates 9. Benzene – Particularly stable due to cyclic delocalization