Question

Identify the total number of compounds which are unstable at room temperature.
(A)
(B)
(C)
(D)

Answer 1

We will study each option and determine which compound is least stable among them. We will use the Huckel Rule which says if a compound contains $4n(n = 0,1,2,3....)$ delocalised $\pi$ electrons in a closed loop, then it is considered as Anti-Aromatic and if a cyclic, planar compound contains $4n + 2(n = 0,1,2,3...)$ $\pi$ electrons it is said to be Aromatic compounds. Compounds that do not satisfy the above 2 conditions are known as non-Aromatic Compounds. We have an order of stability between Aromatic, Anti-Aromatic, non-Aromatic compounds.
$Aromatic > Non - Aromatic > Anti - Aromatic$

Complete answer:
Let’s study the first option, this compound is known as Cyclobutadiene. This compound is cyclic and contains $4\pi$ electrons, now according to the Huckel Rule the cyclic compound that contains $4n(n = 0,1,2,3....)$ $\pi$ electrons is Antiaromatic.
The compound in the second option contains $8\pi$ electrons but it does not have delocalised electrons so resonance is not possible in this compound. so, this compound is non-Aromatic.
Let’s see the third option the compound is known as Cyclopentene it contains $2\pi$ electrons and it is an alicyclic compound it does not have cyclic structure and contains only single double bond so, it is non-Aromatic
Now, the last option remains: this compound is not cyclic and due to its bonding, it is not planar as well. Resonance cannot be achieved in this compound so as it is neither cyclic nor planar so it is not aromatic and also not anti-aromatic, it is a non-aromatic compound.
Besides the first compound all other compounds are non-aromatic and by checking the stability order of aromatic non aromatic and antiaromatic compounds we found out that anti-aromatic compounds are the least stable compounds.
So, the correct option is option (A).

Note:
It is important to satisfy all conditions of Huckel Rule for an compound to be aromatic must be cyclic, planar and have $4n + 2(n = 0,1,2,3...)$ $\pi$ electrons, for anti-aromatic compound must be cyclic and contain $4n(n = 0,1,2,3...)$ $\pi$ electrons.