Question

One of the following compounds is much more stable than the other two. Classify each as aromatic, antiaromatic, or nonaromatic

(a)Methyl heptalene

(b) Methyl azulene

(c) Methyl pentalene

Short answer

(a) Methyl heptalene

(b) Methyl azulene

(c) Methyl pentalene

Step-by-step solution

Aromaticity

Enrich Huckel demonstrated a rule which predicts that the given annulene or related compounds are aromatic or antiaromatic. If the number of electrons in a pi-electron system is (4n+2), the system is aromatic and if it is 4n then the system is antiaromatic where n is an integer.

Classification of the aromatic and nonaromatic molecule

Heptalene is antiaromatic, azulene is aromatic because it follows pi electrons and it is planar. Pentalene is antiaromatic. Azulene is aromatic, hence it is most stable than the other two which are antiaromatic.