Created by Professor William Tam & Dr. Phillis Chang Ch Chapter 13 Conjugated Pi Systems

Ch Introduction  A conjugated system involves at least one atom with a p orbital adjacent to at least one  bond. ●e.g.

Ch Allylic Substitution and the Allyl Radical vinylic carbons (sp 2 ) allylic carbon (sp 3 )

Ch A.Allylic Chlorination (High Temperature)

Ch  Mechanism ●Chain initiation: ●Chain propagation:

Ch  Mechanism ●Chain propagation: ●Chain termination:

Ch Allylic vs vinyl bond energies:

Ch Allylic vs vinyl activation energies:

Ch Radical stabilities:

Ch B.Allylic Bromination with N-Bromo- succinimide (Low Concentration of Br 2 )  NBS is a solid and nearly insoluble in CCl 4. ●Low concentration of Br

Ch  Examples:

Ch The Stability of the Allyl Radical 3A.Molecular Orbital Description of the Allyl Radical

Ch Molecular orbitals:

Ch B.Resonance Description of the Allyl Radical

Ch The Allyl Cation  Relative order of Carbocation stability.

Ch Resonance Theory Revisited 5A. Rules for Writing Resonance Structures  Resonance structures exist only on paper. Although they have no real existence of their own, resonance structures are useful because they allow us to describe molecules, radicals, and ions for which a single Lewis structure is inadequate.  We connect these structures by double- headed arrows (  ), and we say that the hybrid of all of them represents the real molecule, radical, or ion.

Ch  In writing resonance structures, one may only move electrons. resonance structures not resonance structures

Ch  All of the structures must be proper Lewis structures. 10 electrons! X not a proper Lewis structure

Ch  All resonance structures must have the same number of unpaired electrons. X

Ch  All atoms that are part of the delocalized  -electron system must lie in a plane or be nearly planar. no delocalization of  -electrons delocalization of  -electrons

Ch  The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure.  Equivalent resonance structures make equal contributions to the hybrid, and a system described by them has a large resonance stabilization.

Ch  The more stable a resonance structure is (when taken by itself), the greater is its contribution to the hybrid.

Ch B.Estimating the Relative Stability of Resonance Structures  The more covalent bonds a structure has, the more stable it is.

Ch  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. this carbon has 6 electrons this carbon has 8 electrons

Ch  Charge separation decreases stability.

Ch Alkadienes and Polyunsaturated Hydrocarbons  Alkadienes (“Dienes”):

Ch  Alkatrienes (“Trienes”):

Ch  Alkadiynes (“Diynes”):  Alkenynes (“Enynes”):

Ch  Cumulenes: enantiomers

Ch  Conjugated dienes:  Isolated double bonds:

Ch ,3-Butadiene: Electron Delocalization 7A.Bond Lengths of 1,3-Butadiene 1.34 Å 1.47 Å 1.54 Å1.50 Å 1.46 Å sp 3 sp sp 3 sp 2

Ch B.Conformations of 1,3-Butadiene cis trans single bond single bond

Ch C.Molecular Orbitals of 1,3-Butadiene

Ch The Stability of Conjugated Dienes  Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes.

Ch Stability due to conjugation:

Ch Ultraviolet–Visible Spectroscopy  The absorption of UV–Vis radiation is caused by transfer of energy from the radiation beam to electrons that can be excited to higher energy orbitals.

Ch A.The Electromagnetic Spectrum

Ch B.UV – Vis Spectrophotometers

Ch

Ch  Beer’s law A=absorbance   =molar absorptivity c=concentration ℓ =path length A=  x c x ℓ A c x ℓ or  = ●e.g. 2,5-Dimethyl-2,4-hexadiene max (methanol) nm (  = 13,100)

Ch C.Absorption Maxima for Nonconjugated and Conjugated Dienes

Ch

Ch D. Analytical Uses of UV – Vis Spectroscopy  UV–Vis spectroscopy can be used in the structure elucidation of organic molecules to indicate whether conjugation is present in a given sample.  A more widespread use of UV–Vis, however, has to do with determining the concentration of an unknown sample.  Quantitative analysis using UV–Vis spectroscopy is routinely used in biochemical studies to measure the rates of enzymatic reactions.

Ch Electrophilic Attack on Conjugated Dienes: 1,4 Addition

Ch  Mechanism: X (a) (b)

Ch A.Kinetic Control versus Thermodynamic Control of a Chemical Reaction

Ch

Ch The 1,4-product is thermodynamically more stable.

Ch The Diels–Alder Reaction: A 1,4-Cycloaddition Reaction of Dienes

Ch  e.g.

Ch A.Factors Favoring the Diels – Alder Reaction ●Type A and Type B are normal Diels-Alder reactions

Ch ●Type C and Type D are Inverse Demand Diels-Alder reactions

Ch  Relative rate:

Ch  Relative rate:

Ch  Steric effects:

Ch B.Stereochemistry of the Diels – Alder Reaction 1.The Diels–Alder reaction is stereospecific: The reaction is a syn addition, and the configuration of the dienophile is retained in the product.

Ch

Ch The diene, of necessity, reacts in the s-cis rather than in the s-trans conformation. X

Ch  e.g.

Ch  Cyclic dienes in which the double bonds are held in the s-cis conformation are usually highly reactive in the Diels–Alder reaction.  Relative rate:

Ch The Diels–Alder reaction occurs primarily in an endo rather than an exo fashion when the reaction is kinetically controlled. longest bridge R is exo R is endo

Ch  Alder-Endo Rule: ●If a dienophile contains activating groups with  bonds they will prefer an ENDO orientation in the transition state.

Ch  e.g.

Ch  Stereospecific reaction:

Ch  Stereospecific reaction:

Ch  Examples:

Ch  Diene A reacts 10 3 times faster than diene B even though diene B has two electron-donating methyl groups.

Ch  Examples:

Ch  Examples ●Rate of Diene C > Diene D (27 times), but Diene D >> Diene E ●In Diene C, t-Bu group  electron donating group  increase rate ●In Diene E, 2 t-Bu group  steric effect, cannot adopt s-cis conformation

Ch  END OF CHAPTER 13 