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)
