Conjugated Unsaturated Systems 46 Chapter 13 allylic substitution & allylic radicals allylic bromination sabitility of allylic radicals allylic cations resonance theory - detailed (recall chapter 1 info) alkadienes, polyunsaturated hydrocarbons 1,3-butadiene, resonance delocalization stability of conjugated dienes electronic attack on conjugated dienes, 1,4-addition diels-alder rx, 1,4-cycloaddition Modified from sides of William Tam & Phillis Chang

Introduction conjugated system at least one p orbital adjacent to one (or more) π bond e.g.

Allylic Substitution vs Allyl Radical vinylic carbons (sp 2 ) allylic carbon (sp 3 ) mechanisms?

2A.Allylic Chlorination (High Temperature)

Radical chain reaction Chain propagation (r.d.s.) Addition rx

 Mechanism ●Chain propagation ●Chain termination

Allylic Bromination with N-Bromo-Succinimide (NBS) NBS (a solid insoluble in CCl 4 ) low concentration of Br

Examples

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

Resonance of Allyl Radicals

Allyl Cation (recall S N 1 intermediate) Relative order of carbocation stability

Rules for Writing Resonance Structures Resonance structures don’t exist But structures allow predictive description of molecules, radicals, & ions for which a single Lewis structure is inadequate Connect resonance structures by ↔ The hybrid (combined “weighted” avg.) of all resonance structures represents the real substance Resonance theory

writing resonance structures move only electrons resonance structures not resonance structures

All structures must be proper Lewis structures 10 electrons! X not a proper Lewis structure

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

All delocalized atoms of the π-electron system must lie roughly in a plane

A system described by equivalent resonance structures has a large resonance stabilization The energy of the hybrid is lower than the energy estimated for any contributing structure

The more stable a contributing structure the greater its contribution to the hybrid

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

Structures in which all of the atoms have a complete valence shell of electrons (“octets”) make larger contributions to the hybrid this carbon has 6 electrons this carbon has 8 electrons

Charge separation decreases stability

Alkadienes and Polyunsaturated Hydrocarbons Alkadienes (“Dienes”)

Alkatrienes (“Trienes”)

Alkadiynes (“Diynes”) Alkenynes (“Enynes”)

Cumulenes

Conjugated dienes Isolated double bonds

1,3-Butadiene: Electron Delocalization 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

Conformations of 1,3-Butadiene cis trans single bond single bond

7C.Molecular Orbitals of 1,3-Butadiene

The Stability of Conjugated Dienes Conjugated dienes are thermodynamically more stable than isomeric isolated alkadienes

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

The Electromagnetic Spectrum

UV–Vis Spectrophotometers

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 )

Absorption Maxima for Nonconjugated and Conjugated Dienes

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 spectroscopy, 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

Electrophilic Attack on Conjugated Dienes: 1,4 Addition

Mechanism (a) (b)

Kinetic versus Thermodynamic Control of a Chemical Reaction

Diels–Alder Reaction: 1,4 Cycloaddition Reaction of Dienes

e.g. dienedieophile addu ct

Factors Favoring the Diels–Alder RX Types A and B are normal Diels-Alder rxs

Types C and D are Inverse Demand Diels-Alder rxs

Relative rate

 Relative rate

Steric effects

Stereochemistry of the Diels–Alder RX Stereospecific: syn addition and the dienophile configuration is retained in the product

The diene reacts in the s-cis conformation (s-trans can’t cycloadd) X

e.g. (diene locked s-cis)

Cyclic dienes with the double bonds s- cis are usually highly reactive in the Diels–Alder rx, e.g.

The Diels–Alder rx occurs primarily in an endo fashion longest bridge R is exo R is endo DA rx can form bridged structures

Alder-Endo Rule For dienophiles with activating groups having π bonds, the ENDO orientation in the t.s. is preferred endo exo

e.g. = =

Stereospecific reaction

Sterics Diene A reacts 10 3 times faster than diene B

Examples Rate of Diene C > Diene D (27 times) t Bu group electron donating group and favors s-cis diene end

End Examples - steric effects Two t Bu groups cannot adopt s- cis conformation