Problem
Compare stability of 1 and 2:
Answer
1 is more stable than 2.
My approach
- Both have the same number of π-bonds.
- Both have one resonance structure.
- Both are non-aromatic.
- In 1 there are three α-hydrogens, whereas in 2 there are four α-hydrogens.
As extent of hyper-conjugation is more in 2 than in 1, making 2 more stable, which contradicts the answer.
Addressing the comment:
A factor you haven't considered is ring strain. I'm not sure which of the two structures would have more ring strain, though.
I think that 2 has more strain because there are three $\mathrm{sp^3}$ carbon atoms which have been forced to be in a 120° angle, while their ideal angle is 109°28′. So, there is more strain in 2 compared to 1 because in 1 only two carbon atoms are forced to do so.