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?

  • 5
    $\begingroup$ 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 '19 at 5:15
  • $\begingroup$ @IvanNeretin How to know which are shortened and which become longer? $\endgroup$ – Techie5879 Oct 22 '19 at 5:16
  • $\begingroup$ Why, that's simple. What is shorter, a single or a double bond? $\endgroup$ – Ivan Neretin Oct 22 '19 at 5:16
  • 4
    $\begingroup$ @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 '19 at 5:29
  • $\begingroup$ Yes.$\mathstrut$ $\endgroup$ – Ivan Neretin Oct 22 '19 at 5:36

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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.