
Conjugation
Conjugation
 Pi bonds consist of the interaction of electrons in adjacent and
parallel p orbitals like a transverse wave.
 When p orbitals are adjacent and parallel we say they are conjugated but
we usually don't emphasis it until we have more than two orbitals
interacting, an extended Pi system.
 Our models do not adequately represent these interactions.
 Can you give all adjacencies of an extended Pi system in an adjacency
matrix? How many ones?
 Three conjugated p orbitals in a linear system are part of an allylic
system.
 Linear transverse waves have a fundamental motion and a series of
harmonics.
 The full range of motion is called the envelope.
 The + and  signs or color coding indicate the phases of the waves and
not charge!
 The mathematics used to model the behavior of waves is called wave
mechanics or quantum mechanics because wave motion is digital (breaks down
into smaller states) not analog (doesn't just get bigger)!
Maximum Wavelength of Absorption
 For linear systems, the energy of Pi systems is spread out like the
rungs of the latter with a lower and upper bound of ± 2.
 For even systems half of the molecular orbitals are bonding and half are
antibonding.
 For odd systems, one orbital is nonbonding.
 Absorption of electro magnetic radiation moves electrons from one
molecular orbital to another.
 The smallest jump which leads to the longest wave length of absorption
is due to moving an electron from the highest occupied molecular orbital
(HOMO) to the lowest unoccupied molecular orbital (LUMO).
 Because of the upper and lower bounds, and number of rungs in the latter
requires that the HOMOLUMO gap get smaller the longer the Pi system, the
longest or maximum wave length of absorption also increases.
 For isolated double bonds the absorption is below 200 nm (far
ultraviolet).
 Conjugated Pi systems absorb above 200 nm (near ultraviolet).
 Ten alternating unsatuarations (20 carbons) and longer absorb above 400
nm, the visible.
 Color in organic molecules is due to conjugation.
 Because white light is made of several frequency components, RGB, the
appearance of color is the complement of what it absorbs.
 Examples include pH indicators and laundry.
 The HOMOLUMO gap can be halved by having the Pi system interact with
nonbonding electrons.
Aromaticity
 The stability of benzene over cyclohexatriene.
Resonance energy of 34 kcal/mole.
 The energy diagram of mono cyclic pi systems looks like the
corresponding circumscribed polygon standing on one vertex. The zero
energy level goes directly through the center of the polygon or circle.
 Some mono cyclic pi systems are more stable than alkenes, aromatic, flat
4n+2 Pi electrons, Huckel's rule for mono cyclic pi systems.
 Some mono cyclic pi systems are less stable than alkenes, antiaromatic,
flat 4n Pi electrons, rings size 35.
 Some mono cyclic pi systems are just alkenes, nonaromatic, not flat
with 4n+2 or 4n Pi electrons.
 Some cyclic pi systems may be charged.
 Some cyclic pi systems may not be monocyclic, benzenoid or nonbenzenoid.
 Some cyclic pi systems include other atoms besides carbon and are called
aromatic heterocycles,

