Explain the following observation. (a) Cyclooctatetraene is not stable but when potassium is dissolved in cyclooctatetraene the dianion formed is very stable. (b) Cycloheptatrienone is stable, but cyclopentadienone is unstable and undergoes Diels-Alder reaction with itself.

Cyclooctatetraene (COT) is a molecule with eight carbon atoms and alternating single and double bonds in a ring. This molecule has a non-planar (tub-shaped) structure due to the steric strain caused by the planar structure. This distortion breaks the conjugation of this molecule, and as a result, it is not aromatic. Thus, COT is not stable. When potassium is dissolved in COT, two electrons are added to the π system, which leads to the formation of COT dianion. This dianion adopts a planar structure and the resulting molecule has 10 π-electrons. According to the Hückel's rule, a cyclic planar compound is aromatic if it has 4n+2 π-electrons. Considering n = 2, the dianion of COT has an aromatic character that contributes to its stability. So, the COT dianion is very stable.

Cycloheptatrienone is a seven-membered ring compound with three double bonds and a carbonyl group (C=O). This molecule has six π-electrons in a cyclic conjugated system. According to Hückel's rule (4n+2), cycloheptatrienone is aromatic (n = 1) and hence stable. On the other hand, cyclopentadienone is a five-membered ring molecule with two double bonds and a carbonyl group. It has four π-electrons in its cyclic conjugated system. This does not meet the conditions of Hückel's rule for aromaticity, which suggests that it is not aromatic, and thus, it is unstable. The instability of cyclopentadienone leads it to undergo a Diels-Alder reaction with itself. The Diels-Alder reaction is a [4+2] cycloaddition reaction where a diene reacts with a dienophile to form a new cyclic compound. In the case of cyclopentadienone, it acts as both the diene and the dienophile, reacting with itself to form a more stable cyclic compound (a dimer).