CH 20 - Conjugated Systems

unconjugated (nonconjugated):

one or more sp3 carbons between alkenes

• no interaction between alkenes

  • react as individual alkenes

conjugated

alternating double-single bond pattern with all sp2 atoms

• alkenes interact via resonance

• react as a unit, rather than individual alkenes

cumulated

• diene in which two alkenes share same carbon (sp-hydrixed)

  • no resonance because pi-bonds are perpendicular

the stability of alkenes can be determined by

measuring the heat of hydrogenation

hydrogenation of alkenes to alkanes is

always an exothermic process

recall that spectroscopy is the interaction of

electromagnetic radiation with molecules to provide structural information

NMR

radio waves interact with nuclear spin transitions in a magnetic field

IR

infrared radiation interacts with molecular vibrations energy levels l

light in the UV (200-400nm) and visible regions (400-800nm) causes

electronic transitions

• usually from HOMO to LUMO

conjugation causes

longer wavelengths

conjugated dienes undergo

electrophilic addition similar to unconjugated dienes

thermodynamic control

conditions that favor the most stable product

• requires conditions allowing for equilibrium

kinetic control

conditions that favor the fastest forming product

• requires conditions where equilibration does not occur

low temperature

1,2 favored (kinetic) → no equilbrium

high temperature

1,4 favored (thermodynamic) → equilibrium

kinetic is not always

1,2-product and thermodynamic is not always 1,4-product

kinetic - will be formed from the

most stable carbocation resonance form

thermodynamic - will be the most

stable product = most substituted alkene

dials-alder reaction involves the reaction of

a 1,3-diene with an alkene or alkyne to give a cyclohexane product

FMOT: for a reaction to occur,

the orbitals involved in the reaction must have matching symmetries

• HOMO and LUMO orbitals must overlap in way that the orbital phases match

to have favorable FMO overlap, pericyclic reactions generally require that

4n+2 electrons be involved

DA reaction - the diene must be able

to adopt the s-cis conformation

DA reaction - diene

EDG makes the diene more reactive

DA reaction - alkene (dienophile)

EWG make the dienophile more reactive

DA reaction - is concerted, to the sterochemistry

of the starting materials are retained