# C=O bond length comparison among cyclohex‐2‐en‐1‐one and 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?

• Think of it this way: resonance shortens some bonds and elongates others, all that due to partial double bond character. – Ivan Neretin Oct 22 '19 at 5:15
• @IvanNeretin How to know which are shortened and which become longer? – Techie5879 Oct 22 '19 at 5:16
• Why, that's simple. What is shorter, a single or a double bond? – Ivan Neretin Oct 22 '19 at 5:16
• @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? – Techie5879 Oct 22 '19 at 5:29
• Yes.$\mathstrut$ – Ivan Neretin Oct 22 '19 at 5:36