Conjugated Unsaturated Systems
13.1 Introduction
Allyl radical C 2 C C 2 C C C Allyl cation C 2 C C 2 C C C 1,3-Butadiene C 2 C C C 2 C C C C Molecules with delocalized π bonds are called conjugated unsaturated systems.
Conjugated unsaturated systems have a p orbital on a carbon adjacent to a double bond The p orbital can come from another double or triple bond The p orbital may be the empty p orbital of a carbocation or a p orbital with a single electron in it (a radical) Conjugation affords special stability to the molecule Conjugated molecules can be detected using UV spectroscopy
13.2 Allylic Substitution and the Allyl Radical
Reaction of propene with bromine varies depending on reaction conditions At low temperature the halogen adds across the double bond At high temperature or at very low concentration of halogen an allylic substitution occurs
C 2 C C 3 + X 2 hv or C 2 C C 2 X + X Allylic substitution Allylic hydrogen atoms C 2 C C 2 Allylic radical C C C
13.2A Allylic Chlorination (igh Temperature) Allylic chlorination can be performed at high temperature in the gas phase C 2 C C 3 + Cl 2 400 C C2 C C 2 Cl + Cl Relative stability of free radicals: Allylic or allyl > 3 > 2 >1 > vinylic or vinyl
13.2B Allylic Bromination with N-Bromosuccinimide (Low Concentration of Br 2 ) O C 2 C C 3 + N Br O N-Bromosuccinimide (NBS) light or ROOR C 2 C C 2 Br CCl 4 O NBS provides a continuous low concentration of bromine for the radical reaction N A low bromine concentration favors allylic substitution over alkene addition + O
The radical reaction is initiated by a small amount of bromine radical formed by exposure of NBS to light or peroxides O O N Br + Br N + Br 2 O O
13.3 The Stability of the Allyl Radical
13.3A Molecular Orbital Description of the Allyl Radical C C C sp 2 hybridized Conjugated unsaturated system antibonding orbital nonbonding orbital Bonding orbital
The three p orbitals of the allylic system combine to form three molecular orbitals
13.3B Resonance Description of the Allyl Radical C 2 C C 2 C 2 C C 2 A B Resonance structures 1 2 1 2 C 2 C C 2 C A and B are equivalent resonance structures. The allyl radical is even more stable than a tertiary radical.
13.4 The Allyl Cation
The allyl cation is intermediate in stability between a tertiary and secondary carbocation C C 2 C C 2 C sp 2 hybridized C Conjugated unsaturated system antibonding orbital nonbonding orbital Bonding orbital
Stability arises from the delocalization of the positive charge over C1 and C3
Resonance theory predicts that the allyl cation is a hybrid of equivalent structures D and E C 2 C C 2 C 2 C C 2 D E Resonance structures 1 2 1 2 C 2 C C 2 F Both molecular orbital theory and resonance theory suggest that structure F is the best representation for the allyl cation
13.5 Summary of Rules for Resonance
13.5A Rules for Writing Resonance Structures 1. Resonance structures exist only on paper. 2. In writing resonance structures we are only allowed to move electrons. 3. All of the structures must be proper Lewis structures. 4. All resonance structures must have the same number of unpaired electrons. 5. All atoms that are a part of the delocalized system must lie in a plane or be nearly planar
6. The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure. 7. Equivalent resonance structures make equal contributions to the hybrid, and a system described by them has a large resonance stabilization. 8. The more stable a structure is (when taken by itself), the greater is its contribution to the hybrid.
13.5B Estimating the Relative Stability of Resonance Structures a) The more covalent bonds a structure has, the more stable it is. b) Structures in which all of the atoms have a complete valence shell of electrons(i.e., the noble gas structure) are especially stable and make large contributions to the hybrid. c) Charge separation decreases stability.
C 2 C C C 2 C 2 C C C 2 1 2 C 2 C C C 2 4 C 2 C C C 2 3 C 2 C C C 2 C 2 C C 5 6 C 2 C 2 C C C 2 7 Relative stability: 1 > 4, 5, 6, 7 > 2, 3