3
$\begingroup$

This question already has an answer here:

Draw the resonance structures of Fluorobenzene.


In the solutions the structures below were given, I wonder why these structures are not just same. Both of these structures have a negative formal charge on the ortho position.

enter image description here


  • Are the above structure of Fluorobenzene same ?
$\endgroup$

marked as duplicate by Jan, Klaus-Dieter Warzecha organic-chemistry Feb 10 '17 at 18:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • 1
    $\begingroup$ They are not "really" different, but there are two ortho carbons in fluorobenzene so you draw a resonance structure showing charge delocalization to each ortho carbon. There is also a third resonance structure with the charge on the para carbon - 3 resonance structures in all. $\endgroup$ – ron Feb 10 '17 at 17:56
  • $\begingroup$ @ron Does that mean that when we draw resonance structures we aim to show all the different places where electron can delocalize ? does not matter if it is resonance structures are similar or different. $\endgroup$ – A---B Feb 10 '17 at 18:03
  • 1
    $\begingroup$ Yes, that's correct. $\endgroup$ – ron Feb 10 '17 at 18:04
  • $\begingroup$ @ron Thank you $\ddot \smile$. That instantly solved many of my questions about resonance theory. $\endgroup$ – A---B Feb 10 '17 at 18:05
  • 1
    $\begingroup$ Simply - When you're drawing resonance structures you effectively number the atoms to break symmetry. $\endgroup$ – MaxW Feb 10 '17 at 18:11
5
$\begingroup$

There are called equivalent resonance structures since the number of pi bonds, lone pairs and sigma bonds are the same. On the other hand, resonance is the delocalization of pi electrons. These structures show how pi electrons are delocalised from ortho position to para position and then back to ortho(on the other side of the molecule). So both the structures you mentioned are different in this context and do have a purpose in explaining the resonance phenomenon.0

Note: This rule only applies to hydrocarbons. In other cases, the two resonance structures need to have the same structural formula to be equivalent.

$\endgroup$
  • $\begingroup$ "There are called equivalent resonance structures since the number of pi bonds, lone pairs and sigma bonds are the same. " Does not resonance structures ought to have number of pi, sigma and lone pair equal ? I maybe wrong. $\endgroup$ – A---B Feb 10 '17 at 18:07
  • 1
    $\begingroup$ Two resonance structures must obey this condition: they must have the same number of electrons involved in sigma bonds, pi bonds and lone pairs. If they also happen to have the same number of pi bonds, lone pairs and sigma bonds, they are called equivalent resonance structures. Every resonance structure need not be a equivalent resonance structures but every equivalent resonance structures must be a resonance structure. $\endgroup$ – Abhishek Mhatre Feb 10 '17 at 18:13
  • $\begingroup$ What about these? Are they equivalent? They have the same number of pi bonds, lone pairs and sigma bonds, but they are most certainly not equivalent. $\endgroup$ – orthocresol Feb 10 '17 at 18:51
  • $\begingroup$ @orthocresol Oh yes I think you are correct. So what is the criteria for equivalence ? $\endgroup$ – A---B Feb 11 '17 at 11:15
  • $\begingroup$ That both resonance forms should have the same structural formula. $\endgroup$ – Abhishek Mhatre Feb 11 '17 at 15:32

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