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Total number of pairs of resonating structure in which second is more favorable than first:

Set of resonance pairs

I know (d) and (e) have second structure more stable.

(f) has both same.

According me to, (a) and (c) should have second structure unstable as oxygen has positive charge. I don't have a clue about (b) and (g). Please help me out.

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  • (a) Positive charge on $\ce{C}$ versus positive charge on $\ce{O}$: Even though $\ce{O}$ is more electronegative than $\ce{C}$, its octet is fulfilled while $\ce{C}$'s is not (only 6 $e^-$s on positively charged $\ce{C}$). Therefore, second structure is more favored over the first one.
  • (b) Negative charge on $\ce{O}$ versus negative charge on $\ce{C}$: Here, both negatively charged $\ce{O}$ and $\ce{C}$ have their octet fulfilled. Yet, it is always stable when the negative charge is on more electronegative atom ($\ce{O}$) than on comparatively less negative atom ($\ce{C}$). Therefore, first structure is more favored over the second one.
  • (c) Positive charge on $\ce{C}$ versus positive charge on $\ce{O}$: Here, same argument of (a) applied here. Therefore, second structure is more favored over the first one.
  • (d) Charge separation versus neutral: it is always stable when the compound has no charge (neutral) than on has charge separation (with net charge is zero). Therefore, second structure is more favored over the first one.
  • (e) and (f) Kekulé structures of naphthalene and benzene, respectively: Of each pair, first and second structures have the same energy. Therefore, both structures of each pair have same stability equally favored.
  • (g) Positive charge on $2^\circ$-$\ce{C}$ in both structures: Since positive charge is on a $2^\circ$-$\ce{C}$ of each compound, the relative energy due to carbocation is same on both structures. Yet, the first structure has its double bond conjugated to the carbonyl carbon while the second structure has isolated double bond to the carbonyl carbon (cross-conjugated). Therefore, first structure is more favored over the second one.
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    $\begingroup$ (+1) but in (g) double bond is not isolated, it's in conjugation with vacant orbital (carbocation). Hyperconjugation, inductive effect, cross conjugation would be better, ig. $\endgroup$
    – Zenix
    May 3 '20 at 12:38
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    $\begingroup$ With (e), the two Kekule structures are not exactly equivalent. Huckel MO calculations on naphthalene indicate that the shared edge has more pi bonding than the left and right edges in the structures as drawn, so the second structure is favored. $\endgroup$
    – Oscar Lanzi
    May 3 '20 at 13:42
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    $\begingroup$ There is no such thing as a more stable resonance structure. $\endgroup$
    – Martin - マーチン ♦
    May 3 '20 at 14:22
  • $\begingroup$ @ Martin - マーチン: I know that but but that's what asking for. $\endgroup$
    – Mathew Mahindaratne
    May 3 '20 at 15:06
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    $\begingroup$ @math then that option should not be included until the student's education is appropriate to arrive at the correct answer. That's my ulterior motive. $\endgroup$
    – Oscar Lanzi
    May 3 '20 at 16:00
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In (b) part negative charge is on less electronegative atom carbon so it will hold negative charge loosely in comparison to oxygen. So resonance can happen more betterly in second one.

In (g) part according to me a should me more stable as in a part there is linear conjugation but in b part there is cross conjugation.

Sorry for the English.

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