Ch.15 - Conjugated Systems, Orbital Symmetry, and UV Spectroscopy Flashcards
How many bonds are conjugated double bonds separated by?
one single bond
How many bonds are isolated double bonds separated by?
two or more single bonds
Which double bonds are more stable, isolated or conjugated?
conjugated
Heats of hydrogenation used to compare what?
the relative stabilities of alkenes
More ________ double bonds are usually more stable.
substituted
For conjugated double bonds, the heat of hydrogenation is _____ than the sum for the individual double bonds.
less
The more stable the compound, the ____ heat released during hydrogenation.
less
Which is more stable, a terminal alkyne or a cumulated diene?
cumulated diene
Which is more stable, a terminal alkyne of and internal alkyne?
terminal alkyne
Which is more stable, an isolated diene or an internal alkyne?
internal alkyne
Which is more stable, a conjugated diene or an isolated diene (cis)?
isolated diene (cis)
Which is more stable, an isolated diene (trans) or a conjugated diene?
isolated diene (trans)
The C2-C3 single bond of Buta-1,3-diene is shorter than ______A.
1.54 A
The small amount of overlap across the central C2-C3 bond of a Buta-1,3-diene gives it _______ _______ _______ character.
partial double bond
All atoms of buta-1,3-diene are _____ hybridized with overlapping p orbitals.
sp2
Each p orbital has ____ lobes.
2
How is the wavefunction of a p orbital indicated?
by plus (+) and minus (-) signs
What is formed when lobes overlap constructively?
a pi bonding (π) MO is formed
(+ and +, or - and -)
What is formed when lobes overlap destructively?
a pi antibonding (π*) MO (+ and -)
Which π MO for buta-1,3-diene has the lowest energy, all bonding interactions, electrons are delocalized over 4 nuclei.
π1
What are the characteristics of π2 MO for buta-1,3-diene?
1) 2 bonding interactions
2) one antibonding interaction
3) higher energy than π1 MO and not as strong
What are the characteristics of π3 MO for buta-1,3-diene?
1) 2 antibonding and 1 bonding interaction
2) 2 nodes
3) vacant in the ground state
What are the characteristics of π4 MO for buta-1,3-diene?
1) 3 nodes
2) strongly antibonding
3) highest energy MO
4) vacant at ground state
The bonding MOs of both buta-1,3-diene and ethylene are _____ and the antibonding MOs are _______.
1) filled
2) empty
Which has a higher energy, buta-1,3-diene or ethylene?
ethylene
The s-trans conformer is _____ stable than the s-cis.
more
When the kinetic control is at -80C which product is more favorable?
1,2-product
When the kinetic control is at 40C which product is more favorable?
1,4-product
Why does the transition state for the 1,2-addition have a lower Ea at -80C?
it is a more stable secondary carbocation
Because the kinetics of the reaction determines the product, the reaction is said to be under ______ ______
kinetic control
The 1,2-addition is still the faster addition at 40C, but what is different than the -80C?
the bromide attack is reversible
In 40C, the 1,2-product ionizes back to what?
allylic cation
Why is the 1,4-addition the most stable product at 40C?
it has more substiututed double bond
Because the thermodynamics of the reaction determines the product, the reaction is said to be under _______ ________.
thermodynamic control
What is the allylic carbon?
the one attached to an sp2 carbon
How are allylic cations stabilized?
resonance
The stability of 1° allylic is about the stability of what?
2° carbocation
The stability of 2° allylic is about the stability of what?
3° carbocation
What are the radical stabilities?
1° < 2° < 3° < 1° allylic
To encourage substitution, use a ____ concentration of reagent with _____,______, or ______ to initiate free radical formation.
1) low
2) light
3) heat
4) peroxides
During propagation ______ ______ is formed that is stabilized by resonance.
allylic radical
What 2 things react quickly by the SN2 mechanism?
1) allylic halides
2) tosylates
The transition state of the SN2 mechanism is stabilized through conjugation with what?
the p orbitals of the pi bond
The Diels-Alder reaction is a reaction between what?
a diene and an electron-deficient alkene (dienophile) which produces a cyclohexane alkene
What are the conditions for a diels-alder reaction?
change in heat
What are the sterochemical requirements of a diels-alder reaction?
1) the diene must be in s-cis conformation
2) diene’s C1 and C4 p orbitals must overlap with dienophile’s p orbitals to form new sigma bonds
3) both sigma bonds are on same face of diene (syn stereochemistry)
What cannot undergo the Diels-Alder reaction?
s-trans dienes
What is the endo rule?
the stereochemical preference for electron-poor substituents on the dienophile to assume endo positions in a bicyclic diels-alder product
The diels-alder reaction is an example of a _______ reaction.
pericyclic
Woodward and Hoffmann predicted reaction products using their theory of ______ _____ _______ ______.
conservation of orbital symmetry
HOMO
a diene contributes electrons from its highest energy occupied orbital
LUMO
the dienophile receives electrons in its lowest unoccupied orbital
What is the forbidden cycloaddition?
[2+2]
What is the [2+2] cycloaddition made up of?
two ethylenes to form cyclobutene has antibondin overlap of HOMO and LUMO
What happens when ethylene goes from the ground state to the excited state?
the absorption of correct energy photon will promote an electron to an energy level that was previously unoccupied
[2+2] cycloaddition is photochemically ______, but thermally ______.
1) allowed
2) forbidden
200-400nm photons do what?
excite electrons from a π bonding to a π* antibonding orbital
A compound that has a longer chain of conjugated double bonds absorbs light at a ______ wavelength.
longer
what does the spectrometer measure?
the intensity of a reference beam through solvent only (Ir) and the intensity of a beam through a solution of the sample (Is)
absorbance is the log of the ratio ____/_____
Ir / Is
what is beer’s law?
A = ecl
