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
Concerted Reactions
bond breaking and bond making occur simultaneously
Diels-Alder Stereochemistry
mechanism is concerted and stereospecific so stereochemistry at original double bond of dienophile is retained in the product
Exo Adducts
has two substituents on the same side, cis, of the shorter bridge
Endo Adducts
have two trans substituents that point towards the diene
favored so the products with the activating e- withdrawing group of dienophile is located in the endo position
endo transition state is slightly lower in energy
Visible Spectroscopy
wavelength range of 400-800 nm
describes the color of organic compounds
Ultraviolet Spectroscopy
wavelength range of 200-400 nm
can be used with visible spectrum to investigate electronic structures of unsaturated molecules and to measure the extent of conjugation
Electronic Spectra
records changes in electron energies using electromagnetic radiation at UV and visible wavelengths to excite e- from filled bonding or nonbonding orbitals to unfilled antibonding MOs
