Organic II - Aromaticity Flashcards
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
Is cyclobutadiene aromatic, anti-aromatic or non-aromatic?
Anti-aromatic.
Is cyclooctatetrane aromatic, anti-aromatic or non-aromatic?
Non-aromatic.
(Can form tub formation)
What is an aromatic compound?
A cyclic, planar, completely conjugated compound with 4n + 2 pi-electrons.
What is an anti-aromatic compound?
A cyclic, planar, completely conjugated compound
with 4n pi-electrons.
What is a non aromatic compound?
A compound that lacks either one of the following requirements for aromaticity; cyclic, planar, and completely conjugated
How many basic nitrogens does pyrimidine (C4H4N2) have?
2
How many of purine’s (C5H4N4) nitrogen’s is not basic?
1
What is the relationship between pKa value and base/acid?
The lower the pKa value, the weaker the base/stronger acid.
Two p orbitals of similar phase overlap side-by-side forms…
a pi bonding MO.
Two p orbitals of opposite phase overlap side-by-side forms…
a pi anti-bonding MO.
F attached to a benzene is called…
Fluorobenzene
NO2 attached to a benzene is called…
Nitrobenzene
CH3 attached to a benzene is called…
Toulene
OH attached to a benzene is called…
Phenol
NH2 attached to a benzene is called…
Anniline
SO3H attached to a benzene is called…
Benzene-sulfonic acid
OCH3 attached to a benzene is called…
Anisole
COOH attached to a benzene is called…
Benzoic acid
COCH3 attached to a benzene is called…
Acetophenone
Dimethylbenzenes are often called…
Xylenes
1,3-dimethylbenzene is also called…
meta-xylene
Of the 3 groups, which is the highest ranked?
Alcohol
Halogen
Methyl/Ethyl
Alcohol
