Conjugation and Stability of Dienes and Allyl Cations

Conjugation

Overlap of p orbitals on adjacent atoms.

p Orbital

A type of atomic orbital involved in bonding.

1,3-Diene

A conjugated system with alternating double bonds.

Delocalization

Spread of electrons across multiple atoms.

Isolated Diene

Diene with π bonds too far apart.

1,4-Pentadiene

An example of an isolated diene.

Allyl Carbocation

A cation stabilized by conjugation.

Resonance Structures

Different Lewis structures for the same molecule.

Delocalized Hybrid

Actual structure combining multiple resonance forms.

Stability of Allyl Cation

Comparable stability to a 2° carbocation.

Electrostatic Potential Map

Visual representation of electron density distribution.

Cyclic Conjugated Rings

Rings like benzene with multiple resonance forms.

Formal Charge

Charge assigned based on electron distribution.

Resonance Hybrid

Combination of all resonance structures.

Rule 1 of Resonance

More bonds and fewer charges are preferred.

Rule 2 of Resonance

Structures where all atoms have octets are better.

Rule 3 of Resonance

Negative charge on electronegative atoms is preferred.

Common Resonance Examples

Allyl cation and acetate anion are examples.

Conjugated Double Bonds

Double bonds that allow electron delocalization.

Electrostatic Potential Plots

Graphs showing electron-rich and electron-poor areas.

Resonance Contributions

Stability based on structure and charge distribution.

Hybridization

The mixing of atomic orbitals to form new ones.

sp2 Hybridization

Involves one s and two p orbitals.

p Orbitals

Required for overlap in conjugation.

Trigonal Planar Geometry

Shape of sp2 hybridized carbon atoms.

Conjugated Dienes

Compounds with two double bonds linked by σ bond.

Stereoisomers

Discrete molecules with different spatial arrangements.

Conformations

Interconvertible arrangements of the same molecule.

Isoprene

A plant compound released during heat stress.

Lycopene

Antioxidant responsible for red color in tomatoes.

Cholecalciferol (D3)

Vitamin D3, important for calcium metabolism.

Simvastatin

Medication used to lower cholesterol levels.

Calcitriol

Treats hypocalcemia, increasing blood calcium levels.

Features of Conjugated Dienes

Distinct properties compared to isolated dienes.

C-C Single Bond

Unusually short in conjugated dienes.

Stability of Conjugated Dienes

More stable than isolated dienes due to conjugation.

Heat of Hydrogenation

Lower value indicates greater stability of diene.

1,2-Addition Product

Product formed in electrophilic addition to conjugated dienes.

1,4-Addition Product

Alternative product formed in electrophilic addition reactions.

Reaction Conditions

Influence the ratio of addition products formed.

Kinetic Product

Formed faster but less stable than thermodynamic product.

Thermodynamic Product

More stable product formed at equilibrium.

Energy of Activation (Ea)

Determines the reaction rate.

Ultraviolet Light Absorption

Conjugated dienes absorb longer wavelengths than isolated dienes.

1,4-Product

More stable due to two alkyl groups.

1,2-Product

Less stable with only one alkyl group.

Proximity Effect

Influences stability of 1,2-products.

Energy Diagram

Illustrates energy changes in reactions.

Two-Step Mechanism

Lower activation energy at low temperatures.

Kinetic Energy

Higher temperatures allow reaching both transition states.

Diels-Alder Reaction

Addition of 1,3-diene and alkene to form a ring.

Six-Membered Ring

Product of Diels-Alder reaction.

π Bonds

Three π bonds break during the reaction.

σ Bonds

Two σ bonds and one π bond form.

Energy Release

Typical release of ~40 kcal/mol.

s-cis Conformation

Diene must adopt this for reactivity.

s-trans Conformation

Diene is unreactive in this form.

Electron-Withdrawing Groups

Increase dienophile reactivity in Diels-Alder reaction.

Nucleophile

Diene acts as this in the reaction.

Electrophile

Dienophile acts as this in the reaction.

Carbonyl Group

Effective electron-withdrawing group in dienophiles.

Cis Fused

Bicyclic product with cis hydrogen atoms.

Bridged Ring System

Shares nonadjacent carbon atoms in two rings.

Dienophile

Alkene that reacts with 1,3-diene.

Bicyclic Product

Formed from cyclic dienophile in Diels-Alder.

Reactivity of Dienophile

Increases with stronger electron-withdrawing substituents.

Endo Orientation

Preferred arrangement in Diels-Alder products.

Dienophile

Substituted alkene reacting with diene.

Endo Product

Product with substituent oriented towards diene.

Exo Product

Product with substituent oriented away from diene.

Transition State

High-energy state during chemical reaction.

Dicyclopentadiene

Dimer formed from cyclopentadiene via Diels-Alder.

Retro Diels-Alder Reaction

Heating dicyclopentadiene to regenerate cyclopentadiene.

Steroid

Tetracyclic lipid with four fused rings.

C Ring of Estrone

Formed using Diels-Alder reactions in steroid synthesis.

B Ring of Cortisone

Constructed via Diels-Alder reactions in steroid synthesis.

Ultraviolet Light

Light that promotes electron to higher energy state.

UV Light Wavelength

Useful range for absorption is 200-400 nm.

Ground State

Lowest energy state of an electron.

Excited State

Higher energy state after light absorption.

Conjugated Dienes

Compounds with alternating double bonds.

λmax

Wavelength of maximum light absorption.

UV Spectrum

Plot of absorbance versus wavelength for UV light.

Visible Light

Light absorbed by compounds with many conjugated bonds.

Lycopene

Pigment absorbing light at λmax = 470 nm.

SPF Rating

Measure of sunscreen effectiveness against UV radiation.

PABA

Sunscreen ingredient that absorbs UV light.

Padimate O

Another sunscreen compound that absorbs UV light.