Question

Which of the following compounds is not aromatic?
A. $1,3 -$ cyclobutene
B. Pyridine
C. Furan
D. Thiophene

Answer 1

Aromatic compounds are made up of conjugated planar ring systems with delocalized $\pi$ electron clouds in place of individual alternating double and single bonds. Aromatics and arenes are other names for them. Toluene and benzene are the greatest examples. Aromatic compounds must comply with Huckel's guidelines.

Complete Step By Step Answer:
When the total number of $\pi -$ electrons in a ring-shaped cyclic molecule can be equivalent to the formula $4n{\text{ }} + {\text{ }}2$ where $n$ can be any positive integer, the molecule is said to follow the Huckel rule (including zero).
An aromatic compound must be planar, with a cyclic cloud of electrons below and above the molecule's plane. It must follow the Huckel rule since it contains $s{p^2}$ hybridised carbon atoms.
Let us see which of the given compounds does not follow Huckel rule.
In $1,3 -$ cyclobutene,
Number of $\pi$ electrons $= 4$
It is not equal to $4n + 2$
It is not an aromatic compound.
In Pyridine,
Number of $\pi$ electrons $= 6 = (4n + 2)$
Hence, we can say that pyridine follows the Huckel rule.
(Here lone pair of electrons of $n$ does not involve in delocalisation)
In Furan,
Number of $\pi$ electrons $= 6 = (4n + 2)$
Hence, we can say that furan follows the Huckel rule.
In Thiophene,
Number of $\pi$ electrons $= 6 = (4n + 2)$
Hence, we can say that thiophene follows the Huckel rule.
Hence, the correct option is A. $1,3 -$ cyclobutene.

Note:
In aliphatic molecules, the carbon compounds are linked in a straight chain. The carbon molecules in aromatic compounds are linked together by conjugated $\pi$ electrons in a ring structure. Cyclic compounds can be aromatic or non-aromatic; for example, benzene is an aromatic cyclic compound, but cyclohexane is not.