Examples of antiaromatic compounds are pentalene (A), biphenylene (B), cyclopentadienyl cation (C). The prototypical example of antiaromaticity, cyclobutadiene, is the subject of debate, with some scientists arguing that antiaromaticity is not a major factor contributing to its destabilization.

A molecule is aromatic if it is cyclic, planar, completely conjugated compound with 4n + 2 π electrons. It is antiaromatic if all of this is correct except it has 4n electrons, Any deviation from these criteria makes it non-aromatic.

Well, because the quinone fragment is not aromatic. Its bond length are different from each other. These C=O. bonds are always double, and the C−C bonds next to them are always single; you can’t draw a resonance structure where that would be otherwise.

Thiophene is considered to be aromatic, although theoretical calculations suggest that the degree of aromaticity is less than that of benzene. The “electron pairs” on sulfur are significantly delocalized in the pi electron system.

Non Aromatic particles are each non-cyclic, non-planar, or do not hold a comprehensive conjugated π system inside the ring. A compound in a cyclic form that does not demand a continuous form of an overlapping ring of p-orbitals need not be considered as aromatic or even antiaromatic.

(d) Cycloheptatriene is a non-aromatic, conjugated 6 π-electron system. Remember that conjugated only means interacting π system so B is still conjugated, but it is not a cyclic conjugated system required for aromaticity. All the carbons in AD are already involved in a π system so losing a H- would not change that.

The conjugation does not extend all the way around the ring, so the molecule is not aromatic. In addition, the single methylene group (CH2) sticks out of the plane of the rest of the molecule.

Stronger acids have more stable conjugate bases. Thus, the conjugate base with six pi electrons is aromatic and should be more stable than the ring with eight pi electrons, which cannot be aromatic. Based on this analysis, cyclopentadiene must be more acidic than cycloheptatriene. Comparing acidities of rings.

That said, every resonating structure (except the uncharged one) for pyrrole and furan will create a formal positive charge on the hetero-atom. Since N is less electronegative than O, it will be slightly more stable than O with that positive charge. Hence, pyrrole will be more aromatic than furan.

Hence thiophene (B) is most aromatic in nature.

In this example, pyridine is the more potent base. In compared with pyrrole and furan thiophene is more stable. Due to the fact nitrogen is much less electronegative than oxygen it will be barely greater stable than oxygen with that effective rate. pyridine > pyrrole > furan > thiophene.

Oxygen attracts its electrons more strongly than sulfur, so delocalization in thiophene is more prominent. Therefore, aromatic stabilization is stronger in thiophene which makes it less reactive than furan. Indeed, furan can react as a diene, for example in a Diels-Alder reaction, whereas thiophene cannot.

The most common heterocycles are those having five- or six-membered rings and containing heteroatoms of nitrogen (N), oxygen (O), or sulfur (S). The best known of the simple heterocyclic compounds are pyridine, pyrrole, furan, and thiophene.

Furon,pyrole and thiopene are more reactive than benzene.. Benzene has two equivalent resonance structures, with a high degree of electron delocalization. Simply put, it is incredibly stable because the pi electrons in reality spend their time evenly across all six carbon atoms.

Pyrrole, furan, and thiophene are all much more reactive than benzene in electrophilic aro- matic substitution. For example, a less reactive acylating reagent is used in the acylation of furan than in the acylation of benzene.

The computed vertical resonance energy (or quantum mechanical resonance energy) in benzene is 88.8, 92.2, or 87.9 kcal/mol with the basis sets of 6-31G(d), 6-311+G(d,p), or cc-pVTZ, respectively, while the adiabatic resonance energy (or theoretical resonance energy) is 61.4, 63.2, or 62.4 kcal/mol, exhibiting …

Both are aromatic in nature both have delocalised electrons but naphthalene has more resonance structures and more delocalisation so overall it must be more stable for a single ring.