Chapter 14: Delocalized Pi Systems Flashcards
Delocalized Electrons
shared by three or more atomic centers with parallel p orbitals that participate in π-type overlap
Conjugated Dienes
alternating double and single bonds that have extended delocalization of π electrons
have a coplanar configuration due to increased barrier to rotation about the single bond due to overlapping delocalized π electrons
more reactive kinetically, more stable thermodynamically
Allyl
three carbon intermediates with a carbocation/carbanion/radical adjacent to the π framework of a double bond
has special stability due to contributing resonance forms with three p orbitals (a bonding one, an nonbonding one and an antibonding one)
Allylic Carbon
activated carbon in an allyl
Radical Allylic Substitution
free radical chain mechanism
common reagents:
- halogenation: X2, ROOR or hv
- bromination: NBS, HBr, hv or NBS in CCl4
X2, ROOR or hv
radical allylic halogenation
NBS in CCl4
NBS is insoluble in CCl4, produces a steady source of small amounts of bromine for radical allylic bromination
Initiation Step
two radicals formed from a stable compound using hv or ROOR
Propagation Steps
radical and stable compound form a new stable compound and a new radical
Radical Stability
1° > 2° > 3°
Mg, THF, 0°C
with an allylic halide, produces a Grignard reagent
Nonconjugated Isomers
have two double bonds separated by saturated carbons, has a heat of hydrogenation about 3.5 Kcal/mol
Allenes/Cumulated Dienes
have π bonds that share a single sp hybridized carbon and are perpendicular to each other
s-cis Conjugated Diene
two π bonds lie on the same side of the molecule, less stable
s-trans Conjugated Diene
two π bonds on opposite sides of the molecule, more stable
Thermodynamic Control
reaction whose ratio of products reflects their thermodynamic stability
Kinetic Control
reaction whose ratio of products reflects their relative rates of formation
less stable product will form with a lower activation barrier
Diels-Alder Cycloaddition
occurs when atoms at the ends of a diene add to an alkene double bond, closing a ring
bonds form simultaneously and stereospecifically
4 conjugated atoms containing 4 π e- react with a double bond and 2 π e-
Cycloaddition Reactions
reactions between π systems that produce cycloadducts
Diene
4 carbon molecule with 2 π bonds, 4 conjugated atoms and 4 π e-
Dienophile
alkene with 2 conjugated atoms, 2 π e-
“diene loving”
Diels-Alder Substitution on Reactants
dienophile favors electron attracting groups
diene favors electron donating groups
Dienophile Reactivity with Substituents
CH3
Diene Reactivity with Substituents
none
