Organic II - Aromaticity Flashcards
(46 cards)
What is the simplest aromatic hydrocarbon (or arene)?
Benzene
How many degrees of unsaturation does benzene have?
4
Is benzene highly unstaturated? Why or why not?
It is unsaturated because it has 4 degrees of unsaturation.
List some saturated hydrocarbons.
Alkenes, alkynes and dienes.
What kind of reactions do saturated carbons go through?
Addition reactions.
How does benzene react with bromine?
Reacts with bromine only in the presence of FeBr3 (lewis acid) and results in substitution, not addition.
What does a low heat of hydrogenation of benzene mean?
Benzene is stable (more than conjugated polyenes).
Define annulenes.
Hydrocarbon with alternating single and double bonds.
Give examples of annulenes.
Cyclobutadiene, [4]annulene
Benzene, [6]annulene
Cyclooctatetraene, [8]annulene
Why is cyclooctatetraene less stable than benzene?
Because it can’t form resonance structures.
Cyclooctatetraene with Br2 and CCl4 goes through which reaction and produces what?
Addition reactions and produces constitutional isomers. (Ring is changed/touched)
Styrene with Br2 and CCl4 goes through which reaction?
Bromination on the non-cyclic bond. (Ring is untouched)
To have pi MOs what needs to happen with the carbons in benzene?
Each carbon is sp2 hybridized.
6 p-orbitals give rise to __ pi MOs, __ bonding and __ antibonding.
6;3;3
What is the best indication of aromaticity?
The presence of a ring current produced by circulating pi electrons.
For a compound to be aromatic, it must have _________ pi electrons.
4n + 2
n is the indicator of aromaticity.
Hukcle # = 2, 6, 10, 14
What are the values for aromaticity of the Huckle Rule?
2, 6, 10, 14…
If a compound have 3 double bonds, how many pi bonds and pi-electrons does it have?
3 pi bonds and 6 pi-electrons.
Do lone pairs count as pi-electrons?
Yes.
Why is pyrrole (C4H4NH) aromatic?
Because the lone pair on the nitrogen is delocalized.
Is the N-protonated pyrrole aromatic or nonaromatic? Why?
Nonaromatic because the nitrogen is sp3.
What does PAH represent?
Polyaromatic hydrocarbons
As the number of fused aromatic rings increases, what happens to the resonance energy per ring?
It decreases, so compound becomes more reactive and can undergo addition reactions.
A compound that is classified as “anti-aromatic” has unusual _________.
instability
