Question

Which of these species is anti-aromatic?

• A. I only
• B. II only
• C. III only
• D. both II and III

Answer 1

The correct option is A I only

The first structure is cyclobuta-1,3-diene. In this ring is planar, number of $\pi$$e^{-}$'s is 4. It does not follow Huckel's rule. Hence, the compound is anti aromatic.

The second structure is cyclobutenyl dication. In this, ring is cyclic, planar and 2$\pi e^{-}$'s system. It follows (4n+2) rule (n=0) and 2$\pi e^{-}$'s are in complete delocalisation with two positive charges. Hence, the compound is aromatic.

The third structure is cyclobutenyl dianion. In this ring is cyclic, planar, $6e^{-}$'s system $[2\pi e^{-}+4e^{-}\text{(from two negative charges)}]$. It follows Huckel's rule.
Hence, the compound is aromatic.