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1: cyclohex‐2‐en‐1‐one; 2: cyclohex‐3‐en‐1‐one

I can see that in the first case a resonance would occur, and in the second case, there would be no resonance effect. I have read the fact that resonance reduces bond length due to partial double bond character.

According to that, shouldn't the $\ce{C=O}$ bond length in 1 be lesser than that in 2? The answer in my book is that the $\ce{C=O}$ bond in 1 is longer (reason is simply given as: "Resonance"). Could someone help me understand what's going on here?

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    $\begingroup$ Think of it this way: resonance shortens some bonds and elongates others, all that due to partial double bond character. $\endgroup$ – Ivan Neretin Oct 22 at 5:15
  • $\begingroup$ @IvanNeretin How to know which are shortened and which become longer? $\endgroup$ – Techie5879 Oct 22 at 5:16
  • $\begingroup$ Why, that's simple. What is shorter, a single or a double bond? $\endgroup$ – Ivan Neretin Oct 22 at 5:16
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    $\begingroup$ @IvanNeretin Ahh i think i got it. In case 2, it is a double bond, no partial character, thus shorter and in case 1, it is a partial double bond character only, so it will be longer relative to the 2nd case. Is my reasoning accurate? $\endgroup$ – Techie5879 Oct 22 at 5:29
  • $\begingroup$ Yes.$\mathstrut$ $\endgroup$ – Ivan Neretin Oct 22 at 5:36
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The case 1 has partial double bond character (due to resonance) while case 2 has total double bond character of the C-O bond. So, relative to case 2, the C-O bond length in case 1 is longer.

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