CHEM 2020_chapter 14_Revised_Conjugation, Resonance, and Dienes (1)

Organic Chemistry Overview

  • Title: Conjugation, Resonance, and Dienes

  • Author: Janice G. Smith, 6th Ed

  • Course: CHEM 2020

Chapter 14: Conjugation, Resonance, and Dienes

Conjugation

  • Conjugation occurs when p orbitals overlap on three or more adjacent atoms.

  • The p orbital at the allylic position conjugates with the double bond.

  • A 1,3-diene exhibits a conjugated system due to the overlapping p orbitals on adjacent atoms.

Delocalization

  • Delocalization refers to the spreading out of electrons across three or more p orbitals on adjacent atoms, a result of their overlapping.

Dienes

  • 1,4-Pentadiene:

    • An example of an isolated diene where π bonds are too distant to be conjugated, being separated by an sp3 carbon.

Electrostatic Potential Plots for Dienes

  • Electrostatic potential plots illustrate that in 1,3-butadiene, the electron-rich area is spread out rather than being localized.

Conjugation of Allylic Cations

  • The allyl carbocation represents a conjugated system, where conjugation enhances its stability.

Delocalized Hybrids

  • Resonance structures for allyl cations demonstrate how conjugation results in electron delocalization,

    • The true structure of the allyl cation is a hybrid of its resonance forms, stabilizing it more than a normal 1° carbocation.

Stability of Allyl Cations

  • Experimental data show allyl cation stability is similar to a more substituted 2° carbocation.

Common Examples of Resonance

Type [1]: The Three Atom “Allyl” System

  • Examples include the allyl cation and acetate anion, showing resonance structures that vary in bond and charge location.

Type [2]: Conjugated Double Bonds

  • Cyclic, completely conjugated rings (e.g., benzene) can form multiple resonance structures.

Type [3]: Cations with Adjacent Lone Pairs

  • Positive charge shifts based on formal charge principles across resonance structures.

The Resonance Hybrid

  • The resonance hybrid reflects the most stable resonance structure.

  • Rules for Stability:

    1. Structures with more bonds and fewer charges are better.

Resonance Hybrid Contributions

  • Rule [2]: Structures where all atoms have an octet are preferable, even with charges on electronegative atoms.

  • Rule [3]: Structures that place negative charges on more electronegative atoms are favored.

Determining Resonance Structure

  • Two Lewis structures show resonance stabilization for the anion (CH3COCH2)−, using sp3 and sp2 hybridization differences.

p Orbitals Required for Conjugation

  • The adjacent carbon's electron pair can only delocalize with available p orbitals allowing for conjugation.

Conjugated Dienes

  • Conjugated dienes have two double bonds joined by one σ bond, allowing three possible stereoisomers.

Conformation of Conjugated Dienes

  • Rotation around the C–C bond leads to two conformations; stereoisomers are distinct molecules, while conformations interchange.

Interesting Dienes

  • Isoprene: Released by plants to increase heat stress tolerance.

  • Lycopene: Antioxidant contributing to the red color in tomatoes and other fruits.

Biologically Active Compounds with Conjugated Double Bonds

  • Simvastatin: Used to lower cholesterol.

  • Calcitriol: Treats hypocalcemia.

Features of Conjugated Dienes

  1. C–C single bond between double bonds is shorter than in isolated dienes.

  2. More stable than similar isolated dienes.

  3. Reactions differ from those of isolated double bonds.

  4. Absorb longer wavelengths of ultraviolet light.

Percent s Character vs. Bond Length

  • Each carbon in 1,3-butadiene is sp2 hybridized, influencing bond length and stability.

Stability of Conjugated vs. Isolated Dienes

  • Conjugated dienes have lower heat of hydrogenation and are thus more stable by 7 kcal/mol.

Products of Electrophilic Addition

  • HBr Addition:

    • Isolated dienes yield one product; conjugated dienes yield 1,2- and 1,4-addition products.

Kinetic vs Thermodynamic Products

  • At equilibrium, heating a mixture favors the 1,4-product.

Greater Stability of 1,4-Products

  • 1,4-products are favored due to increased bonding from alkyl groups compared to 1,2-products.

Proximity Effect for 1,2-Products

  • The proximity of substituents influences the faster formation of 1,2-products.

Energy Diagram for Reaction Mechanisms

  • Energy diagrams illustrate stability pathways during reactions, showing differences based on temperature.

Diels–Alder Reaction

  • A key addition reaction between a 1,3-diene and an alkene (dienophile) forming a six-membered ring.

  • Typical reactions yield significant energy release (~40 kcal/mol).

How To Draw the Diels-Alder Product

  1. Arranging reactants in the correct conformation.

  2. Cleaving π bonds and forming new bonds with arrows.

Rules of Diels–Alder Reaction

  • Dienes require the s-cis conformation to react; s-trans is unreactive.

Electron-withdrawing Substituents in the Diels–Alder Reaction

  • Increase reactivity by making dienophiles more electrophilic.

Dienophiles in the Diels–Alder Reaction

  • Carbonyl groups serve effectively as electron-withdrawing groups, influencing reactivity.

Formation of Fused and Bridged Bicyclic Systems

  • Cyclic dienophiles lead to bicyclic products; positions of atoms maintained during reaction.

Diels–Alder Reaction Orientation

  • Endo orientation is preferred due to favorable interactions during transition states.

Finding the Diene and Dienophile

  • Identify ring structures and reactants for completing a Diels-Alder product.

Diels–Alder Reaction Dimers

  • 1,3-Cyclopentadiene rapidly reacts with itself to produce dicyclopentadiene.

The Retro Diels–Alder Reaction

  • Heating dicyclopentadiene regenerates cyclopentadiene, allowing further reactions with other dienophiles.

Steroid Synthesis via Diels–Alder Reactions

  • Diels–Alder reactions are pivotal for synthesizing steroids.

Common Steroids and Biological Activity

  • Steroids vary in biological functions based on the substitution patterns on their rings.

Ultraviolet Light Absorption

  • Absorption leads to electron excitation, primarily targeting wavelengths of 200–400 nm.

Conjugated Dienes and Ultraviolet Light

  • Longer wavelengths promote excitation in conjugated dienes, indicated by λmax.

Increasing Conjugation and Ultraviolet Light

  • Increased conjugation shifts light absorption from UV to visible wavelengths.

Lycopene and Color Absorption

  • Lycopene appears red due to selective light absorption at λmax = 470 nm.

Sunscreens

  • Composed of conjugated compounds which absorb UV light to protect skin; SPF rated for effectiveness.

  • Common active ingredients include para-aminobenzoic acid (PABA) and padimate O.