Transcript Slide 1
Organic Chemistry Chapter 7 - Resonance Electron Delocalization and Resonance • Localized electrons = restricted to a particular region • Delocalized electrons do not belong to a single atom or exclusively to a bond between 2 atoms Kekule Structure • Benzene C6H6 – Rapid Equilibrium between 2 structures – Proposed by Fredrich Kekule (1865 German chemist) Rapid Equilibrium Kekule’s Dream •* Benzene Structure • Kekule Structures of Benzene were accepted in the 1930’s when X-ray studies showed ALL SIX C-H bonds equal and ALL SIX C-C bonds equal! H H H H H H Bonding in Benzene • Each C is sp2 hybridized • Each C has an unhybridized p orbital perpendicular to the plane of the ring • The 6 p orbitals overlap to form a cloud Resonance Hybrid • A compound with delocalized e- is said to have resonance – resonance contributor – resonance structure – contributing resonance structure Resonance Hybrid • Benzene – contributing resonance structures Resonance Hybrids Drawing resonance hybrids • 1) Only e- move (not atoms) • 2) Only and non-bonding e move - • 3) Total # e- stays same (as does unpaired e-) Resonance Hybrids e- can be moved only by… e move toward + or toward bond - Resonance Hybrids e- can be moved only by… Nonbonding pair e- toward a bond Resonance Hybrids e- can be moved only by… Nonbonding single e- toward a bond Resonance Hybrids • Drawing resonance hybrids Resonance Hybrids Practice Drawing Resonance Practice Practice Practice • What makes a Resonance Structure Have Decreased Stability? – 1) an atom with an incomplete octet – 2) a negative charge that is not on the most electronegative atom – 3) a positive charge not on the most electropositive atom – 4) charge separation Examples To Examine B is less stable than A Equal Stability Resonance Energy • 1) The greater the predicted stability of a resonance contributor, the more it contributes to the resonance hybrid. • 2) The greater the number of relatively stable resonance contributors, the greater the resonance energy. • 3) The more nearly equivalent the resonance contributors, the greater the resonance energy. Resonance Energy The more nearly equivalent the resonance contributors, the greater the resonance energy Stability of Dienes Stability of butadiene Butadiene – Example H3C O ] [ Br H3C H3C H3C O O O Br Br Br Allylic Cations • Allylic cations have delocalized e• They are more stable due to resonance contributors: Benzylic Cations • Benzylic cations have delocalized e• They are more stable due to resonance contributors: Stability of Cations • * Consequences • Because of the allyl and benzyl cations and radicals are more stable, some products can be formed easier. Mechanisms Mechanisms Mechanisms Look at how conjugation affects reactions Effects on pKa • Carboxylic acids are stronger acids than alcohols due to resonance effects Phenol • Why is this alcohol as acidic as it is? – -OH is on an sp2 hybridized C – In phenol, some resonance contributors have a + charge on O – 3 of phenol’s structures have separated charges Molecular Orbital Theory • Molecular Orbital Theory can also help explain increased stability • READ THROUGH THIS SECTION OF THE BOOK Thermodynamic vs. Kinetic Control • If the kinetic product and thermodynamic product differ, the major product will depend on reaction conditions. • If carried out at mild (low-temp) conditions, the reaction will be irreversible and the kinetic product will be favored. • If carried out at sufficiently vigorous (hi-temp) conditions, the reaction will be reversible and the thermodynamic product will be favored. Thermodynamic vs. Kinetic Control • Kinetic product and thermodynamic product: Formed fastest Most stable Thermodynamic vs. Kinetic Control • Reaction conditions control products! Kinetic product – low temp Thermodynamic product – high temp Diels-Alder Reaction Otto Diels Kiel University Kiel • This reaction makes new carbon carbon bonds • Uses a conjugated diene and a dienophile Kurt Alder Cologne University Cologne Diels-Alder Reaction Otto Diels Kiel University Kiel Kurt Alder Cologne University Cologne • Electron withdrawing groups (O or N) increases the reactivity of the dienophile and increase the reactivity acts as an electrophile Diels-Alder Reaction Otto Diels Kiel University Kiel Kurt Alder Cologne University Cologne • 1,4 addition to a conjugated diene Diels-Alder Stereochemistry Otto Diels Kiel University Kiel Kurt Alder Cologne University Cologne • If a chiral center is created, equal amounts of R, S produced (racemic) Diels-Alder Stereochemistry • Stereospecific: cis dienophile remains cis – trans dienophile remains trans Otto Diels Kiel University Kiel Kurt Alder Cologne University Cologne Diels-Alder Examples Otto Diels Kiel University Kiel • Dienophiles can also be C triple bond C Kurt Alder Cologne University Cologne Bicycloalkanes • Nomenclature – parent is the alkane of the same number of carbons as are in two rings – number from a bridgehead, along longest bridge back to the bridgehead, then along the next longest bridge, etc. – show the lengths of bridges in brackets, from longest to shortest 1 2 6 7 3 5 4 bicyclo[2.2.1]he ptane Bicyclics - Nomenclature • Examples Bicyclics - Nomenclature • Examples