Which of the following is most stable?

Solution

Chemistry ma’am · 3 steps

Check each ring against Hückel's rule: the one with (4n+2) π electrons wins, even with a charge.

1. The ranking rule. For cyclic species, stability goes aromatic > non-aromatic > antiaromatic. Aromatic means the ring is cyclic, planar, fully conjugated (every ring atom sp²) and has (4n+2) π electrons. A fully conjugated ring with 4n π electrons is antiaromatic and strongly destabilised.

   Option	Species	Fully conjugated?	π electrons	Type
   (a)	cyclopropenyl cation	yes, empty p orbital on C⁺	2, which is 4n+2 with n = 0	aromatic
   (b)	cyclopropenyl anion	yes, lone pair in conjugation	4, which is 4n	antiaromatic
   (c)	cyclopentadienyl cation	yes, empty p orbital on C⁺	4, which is 4n	antiaromatic
   (d)	1,3-cyclohexadiene	no, two sp³ CH₂ carbons	4, but not cyclic π	non-aromatic

2. Why the charge helps. In (a), the positive charge is spread equally over all three carbons by resonance, and the 2 π electrons fill a bonding orbital that covers the whole ring. This aromatic stabilisation outweighs the strain of the small ring.
3. So. Only the cyclopropenyl cation is aromatic, so option (a) is the most stable.

The idea to keep. Never judge ring stability by charge alone. Count the π electrons in a fully conjugated ring: 4n+2 makes it aromatic and very stable, 4n makes it antiaromatic and very unstable.

Answer from the NTA final answer key. Solution by PrepWiser.