Chapter 15: Dienes, Resonance, and Aromaticity Flashcards
Conjugation
When pi electrons can be shared over 3 or more atoms
- requires unbroken chain of available p-orbitals
- p-orbitals need to be parallel to eachother
Relative stability of conjugated systems
conjugated > non-conjugated > cummulated
non-conjugated
conjugated
cummulated
Less obvious conjugated systems
Why are conjugated systems so stable?
- MO’s
- pi bond overlap
- resonance
UV-Vis spectroscopy
- λ increases with conjugation
- λ is determined by the energy between HOMO and LUMO
- As conjugation increases, gap between HOMO and LUMO decreases
- large λ = lower energy
- small λ = higher energy
How does λ relate to visible color?
Reactivity across conjugated double bonds
Kinetic product = 1,2 addition (because formation is faster)
Thermodynamic product = 1,4 addition (because slower but more stable)
The kinetic product forms at ___ temps.
low
The thermodynamic product forms at ___ temps
high
Extended Conjugation
- bromine can attack wherever a carbocation can be resonated to
- products that break conjugation (like the middle one) are unfavorable
Diene
dienophile
Diels-Alder Reaction
-pericyclic reaction
Stereochemistry of dienophiles
- to the right of the dienophile = wedge
- to the left of dienophile = dash
Stereochemistry of dienes
- inside substituents go up (wedge)
- outside substituents go down (dash)
What products are formed and which product is preferred?
-endo preferred
A compund is aromatic if ___
-if it contains a ring of continuously overlapping planar p-orbitals
Huckel’s rule
- set the number of pi electrons equal to 4n+2
- solve for n
- if n is an integer number, compound is aromatic
- if the number of pi electrons set equal to 4n, and n ends up being an integer, the compund is antiaromatic
