Edited Invalid date
1.3 Criteria for Aromaticity
The stability of benzene and its derivatives is associated with the ring of six p electrons.
We will see that aromatic stabilization is more than just benzene and its derivatives.
There are a lot of compounds that exhibit aromatic stabilization.
Even five-membered rings and seven-membered rings can be aromatic if they meet certain criteria.
The two criteria for aromaticity will be explored in this section.
3 were hybridized to interrupt the overlap.
The first compound has two pairs of p electrons, and the last compound has four pairs of p electrons.
MO theory is required to understand the requirement for an odd number of pairs of p electrons.
You should consult your textbook or lecture notes for any coverage of MO theory.
It is possible to say that there is an odd number of pairs of p electrons.
There are six p electrons in the middle structure of benzene.
Benzene is aromatic.
The other two compounds have the same number of pairs of p electrons.
Both cyclobutadiene and cyclooctatetraene have 8 p electrons.
The numbers 4 and 8 are not the same as the Huckel numbers.
An observation that can be justified is that these compounds are remarkably unstable.
The compound behaves as if it were nonaromatic.
It can be isolated, unlike compounds that are too unstable to be isolated.
There are structures that have a net charge.
We will be able to identify ion types that are aromatic and antiaromatic.
The structure of the following anion is resonance-stabilized.
The stability of this anion is not explained by resonance alone.
Both criteria for aromaticity were met with this anion.
The delocalized lone pair represents two p electrons, and the rest of the ring has four p electrons, for a total of six p electrons.
The stable conjugate base renders cyclopentadiene acidic.
You often think of hydroxide as a strong base, but remember that basicity and acidity are relative concepts.
When compared with other weak bases, such as the acetate ion, hydroxide is a strong base.
When compared with other strong bases, such as the amide ion or carbanions, hydroxide is a relatively weak base.
There are only a few examples of carbanions with such stability.
This stability is due to its aromatic nature.
An anion was the previous example.
An example of a cation can be found here.
The tropylium cation is resonance-stabilized.
The space provided has resonance structures.
The stability of this cation is not fully explained by resonance.
We will find that this cation does satisfy both criteria if we consider both criteria.
This cation is aromatic because both criteria are satisfied.
Let's see if we can determine whether the ion is aromatic, nonaromatic, or antiaromatic.
It needs to fail the first criterion for aromaticity in order to be nonaromatic.
We don't have a Huckel number when we count the number of p electrons.
We expect this structure to be anti-aromatic with 4 p electrons.
This ion is expected to be very unstable.
Review flashcards and saved quizzes
Getting your flashcards
Privacy & Terms