Chapter 15- Conugated Dienes, Orbital Symmetry, And UV Spectroscopy Flashcards
UV and visible spectroscopy are dependent upon the ___ of electrons
Excitation
What are the two important electronic transitions?
pi–> pi* and n–> pi*
The pi–> pi* transition involves promotion if the pi electron from a ____ pi MO to an _____ pi* MO
Bonding, antibonding
The n–> pi* transition involves promotion of a ____ electron to an ______ p MO
Nonbonding, antibonding
Only what two electrons are involved in UV Spec?
N and pi
Promotion if a _ electron to a _ orbital requires more energy than is available in the 750-200 nm range. Therefore, in almost all cases, the __ MO is populated
Sigma, sigma, pi
What are the two reasons for the unusual stability of conjugated dienes?
- Orbital hybridization- sp2- sp2 overlap
2. Interaction between the pi orbitals- resonance
Which is more stable, fhe s-trans or s-cis configuration?
S- trans is more stable
What makes allylic cations so stable?
The positive charge is delocalized over two carbons by resonance
The stability of a primary allylic is is th same as a __ carbocation
The stability of a secondary allylic is the same as a ___ carbocation
Secondary, tertiary
What products are formed from the electrophilic additions to conjugated dienes?
1,2 and 1,4
Thermodynamic product predominates at ___ temp and kinetic product predominates at ___ temp
High, low
Why does the 1,2 (kinetic) addition have a lower activation energy?
It is a more stable secondary carbocation
Which addition is faster, 1,2 or 1,4?
1,2
At -80 degrees C, the nu attack of the bromide on the allylic carbocation reversible or irreversible?
Irreversible (kinetic control)
The product that ____ predominates in electrophilic additions
Forms faster
At 40 degrees C, is the bromide attack reversible or irreversible?
Reversible (thermodynamic control)
Which is the more stable product, 1,2 or 1,4? Why?
1,4 because it has a more substituted double bond
What are highly reactive substrates for Sn2 reactions?
Allylic halides and tosylates
The transition state of allylic Sn2 reactions are stabilized through ___ with the p orbitals of the pi bond.
Conjugation
UV spectroscopy is applicable to only what?
Conjugated pi electron systems
