aniline resonance

Based on the above picture, it seems that there are four. However, I have two questions:

  1. Aren't the top right and bottom left forms simply mirror images of each other, and thus considered the same?
  2. If not, would an alternate version of the top left form be a fifth structure, if the double and single bonds were flipped?

Your four structures are correct and you could also include your suggested fifth structure (your first and fifth structures are comparable to the resonance structures of benzene). While some of the structures are mirror images, we generally think of particular atoms as being fixed when writing out resonance structures. You can think of the carbons as having a number (which they technically would if you named the resonance structures IUPAC style), which leads to the "mirrored" structures not being identical since the double bonds are between different carbons.

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  • $\begingroup$ (+1) for a nice answer but I must comment that I was taught that resonance structures that are mirror images of each other (aka identical to each other) are not counted separately. That might have been a different context though. Do you know of any other context where duplicates are not counted separately? Or is what you said applicable in all contexts? Thank you! $\endgroup$ – Gaurang Tandon Feb 18 '18 at 12:40
  • $\begingroup$ @Guarang Tandon I had always counted mirrored structures under the assumption that we weren't rotating the atoms of the original structure. I guess you wouldn't necessarily have to draw out these resonance structures if you remembered that the corresponding mirrored structure also exists, but drawing it out for cases like benzene makes the resonance more explicit. $\endgroup$ – Tyberius Feb 18 '18 at 14:57
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    $\begingroup$ The way I view it is that resonance is a superposition of (localised) wavefunctions, so for every wavefunction that has a negative charge on C2, you need a corresponding wavefunction with a negative charge on C6 with the same weight. These wavefunctions are symmetry-related, just like how C2 and C6 are symmetry-related, but they aren't the same. $\endgroup$ – orthocresol Jan 2 at 14:34
  • $\begingroup$ @Tyberius: can I assume an isotopic carbon at C-6 (like we do in benzyne mech.) to explain this to myself? $\endgroup$ – Rahul Verma Jan 2 at 15:47

I believe there are only three resonance structures for aniline. The upper right and lower left structures are the same. Flipping the single and double bonds within the ring (your suggested fifth structure) would not make another resonance structure because the ring is a benzene ring. The pi bonds forming the double bonds are actually delocalized. They are "equally shared" throughout the ring.

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    $\begingroup$ "upper right and lower left are the same" That's not true: if mirror images were the same, then ozone would have no resonance structures. Or acetate ion, for that matter. $\endgroup$ – orthocresol Jan 2 at 14:29
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    $\begingroup$ "Flipping the single and double bonds within the ring (your suggested fifth structure) would not make another resonance structure because the ring is a benzene ring." This is equally wrong, because that would mean benzene cannot be described with resonance either. $\endgroup$ – Martin - マーチン Jan 2 at 15:04

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