Chapter 16 Flashcards
Why is benzene less reactive than its non-cyclic counterpart?
unusually large resonance energy, closed shell arrangement of pi electrons in low energy orbitals.
What happens in an attempted addition reaction of benzene with a dihalide?
Nothing! unless you add a catalyst (FeBr3) which causes a substitution reaction of an H for X.
What interactions occur in Pi2 and Pi3 of Benzene?
2 bonding interactions.
Pi2 has 4 bonding, 2 anti bonding, Pi3 has 0 antibonding, 4 nonbonding, and 2 bonding.
What does the energy diagram for Cyclobutadiene look like (where are the electrons) and what is the affect on reactivity?
two electrons in the bonding Pi1 orbital, the other two are split between Pi2 and Pi3 higher-energy nonbonding orbitals. The effect is an increase in reactivity compared to benzene.
What the four criteria for a compound to be considered aromatic?
- must be cyclic with conjugated pi bonds.
- each atom in ring must have unhybridized p-orbital(s)
- structure must be planar/nearly planar for overlap of p-orbitals to occur.
- delocalization of pi electrons must lower energy.
If a cyclic compound is strongly acidic, what does that tell us about its conjugate base?
conjugate base is aromatic, and the acid loses a proton easily to regain stable aromaticity.
What characteristic of cyclooctatetraene makes it non-aromatic?
It is most stable in a non-planar “tub” conformation.
What type of orbital does the lone pair of electrons on pyridine reside in?
sp2 orbital of nitrogen - they do not interact with the pi system.
Is pyrrole an aromatic compound?
Yes - the lone pair of electrons on the sp2 nitrogen are in the pi system, and are delocalized about the nitrogen and carbons.
What type(s) of orbital(s) are the lone pairs in furan held in?
one pair is in a p-orbital, the other is in an sp2 orbital.
Why is pyridine basic?
the sp2-contained lone pair easily picks up a hydrogen, as it does not affect the aromaticity or interact with the pi system at all.
Why is the nitrogen in pyrrole not basic like the one in pyridine?
the lone pair is involved in the pi system - unfavorable to use these electrons to pick up a hydrogen.
What happens to the NMR of a hydrogen that is closer to an acyl group?
The acyl group deshields the hydrogen, shifting it to a higher ppm.
