"You cannot reason a person out of a position he did not reason himself into in the first place." Jonathan Swift

Chapter 16 Conjugated double bonds 2,6,10,14,19,23,27,31-Octamethyl-dotriaconta-2,6,8,10,12,14,16,18,20,22,24,26,30-tridecaene

3 Conjugated Alkenes Conjugated Not Conjugated Red Shift in electronic spectra More stable than corresponding non-conjugated molecules sp 2 -sp 2  bonds are shorter and stronger than sp 3 -sp 2 bonds 5.5 kJ/mole more stable than linolenic acid (below)

4 Identifying Conjugated Organics

Conjugated C=C Bonds and Electronic Spectra

Conjugated Alkenes Crocetin (from Saffron; 422 nm) Lycopene (505 nm) Indigo Blue (605 nm) 430 nm 500 nm 560 nm 580 nm 400 nm 420 nm 465nm 530 nm 600 nm 620 nm 640 nm 680 nm 720 nm Chlorophyll a (675 nm)

Structure Determination in Conjugated Systems: UV Spectroscopy The electrons in the highest occupied molecular orbital (HOMO) undergo a transition to the lowest unoccupied molecular orbital (LUMO)

Conjugated Alkenes 430 nm 500 nm 560 nm 580 nm 400 nm 420 nm 465nm 530 nm 600 nm 620 nm 640 nm 680 nm 720 nm

Butadiene: Molecular Orbitals 4 p-orbitals

Conjugated C=C Bonds and Electronic Spectra

430 nm 500 nm 560 nm 580 nm 400 nm 420 nm 465nm 530 nm 600 nm 620 nm 640 nm 680 nm 720 nm

Conjugation, Color and the Chemistry of Vision  -Carotene is converted to Vitamin A, which is converted to 11-cis-retinal:

Conjugation, Color and the Chemistry of Vision 11-cis-retinal is converted to rhodopsin in the rod cells of the retina. Visual pigments are responsible for absorbing light in eye and triggering nerves to send signal to brain

SUNSCREENS

Special Properties of Conjugated Dienes Length of the central single bond is shorter than non- conjugated similar molecule Comparison of 1,3-Butadiene and Butane with C-13 NMR H2C=CH-CH=CH2CH3-CH2-CH2-CH3 148 pm153 pm 1,3-ButadieneButane Shorter Bond

Why more stable (thermodynamics) than non- conjugated? p-p overlap is better than hyperconjugation

More reactive (kinetics) than non-conjugated LUMO is lower – more electrophilic HOMO is higher– more nucleophilic

Faster reactions, less ΔG for reactions (start lower)

Diene Preparation Cyclohexene3- Bromocyclohexene 1,3- Cyclohexadiene Based Induced Elimination of HX

Electrophilic Additions to Conjugated Dienes: Allylic Carbocations Electrophilic addition to 1,3-Butadiene yields a mixture of two products: 1,2 addition 1,4 addition

Non-conjugated alkene addition reactions 2- Methylpropene Tertiary Carbocation 2-Chloro-2- methylpropane

At room temperature, electrophilic addition to a conjugated diene leads to a product mixture where the 1,2 adduct predominates over the 1,4 adduct. At high temperatures, the product ratio changes and the 1,4 adduct predominates 1,2 adduct1,4 adduct At 0 o C:71% 29% At 40 o C:15% 85%

Reacting Butadiene with Electrophiles

Allyl Cations

Allylic and Benzylic Cations

Diene polymerizations

Vulcanization:

Diels-Alder Reaction

Dienes Dienophiles

Diels-Alder Reaction:Diene Reactivity Generally more reactive than acyclic dienes

Diels-Alder Reaction No way of telling which approach is used with this system

Diels-Alder Reaction

Stability of exo product is greater Higher temperature is required to reach product for exo path Running reaction at higher temp gives thermodynamic product (exo) Running reaction at room temp gives kinetic product (endo)

Diels-Alder Reactions

Diels Alder in Synthesis Look for cyclohexene = product of DA

38 The key Diels–Alder reactions used to prepare the C ring of estrone and the B ring of cortisone are as follows: Steroid Synthesis

Diels-Alder Reactions

NMR of Dienes

Need to know Diene nomenclature Recognize conjugated versus non-conjugated systems Know reaction and mechanism: -addition of Br 2 or Cl 2 to dienes -addition of HCl or HBr to dienes -Diels Alder cycloaddition (exo and endo) Understand kinetic versus thermodynamics products for diene reactions and Diels Alder cycloaddition reactions Be able to synthesize dienes from alkanes through bromination and elimination reactions. Extra special: predict regiochemistry of Diels Alder cycloadditions 45