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It is known that benzene has two main resonance forms

Fig 1

Often we still draw benzene molecule as one of them

Fig2

My question is, actually when we draw the lower figure, do we refer to the whole benzene molecule or one of the resonance structure or depend on the context?

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    $\begingroup$ Since neither of the two resonance structures truly exists as shown, the bottom is taken to represent the resonance hybrid of the top unless suggested otherwise (I'm not sure I've ever seen it suggested otherwise, but it could happen?). $\endgroup$ – SendersReagent Apr 23 '16 at 3:23
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I approach this question from the opposite direction.

Benzene is commonly drawn as a ‘cyclohexatriene’ corresponding to the Kekulé structure, i.e. with three single bonds and three double bonds, despite the fact that the six bonds of benzene are actually indistinguishable from each other.

benzene

This graphical representation of benzene is in accordance with Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) as well as Graphical Representation Standards for Chemical Structure Diagrams (IUPAC Recommendations 2008).

Therefore, when you see such a structure diagram in the literature, it usually represents a real benzene ring and not a particular resonance form.

However, if a structure diagram is explicitly meant to show the individual resonance forms, they are commonly drawn with a two-sided arrow.

benzene

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  • $\begingroup$ me thinks you over sold the use of the double bond forms. See page 104 of the pdf file (page 380 of printed copy) lcso.epfl.ch/files/content/sites/lcso/files/load/… $\endgroup$ – MaxW Apr 23 '16 at 16:12
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    $\begingroup$ @MaxW Certainly, the shorthand form shown on page 380 which indicates all resonance forms of benzene by a circle is acceptable, especially to show the equivalence of the six bonds. However, the preferred style is the ‘cyclohexatriene’ structure diagram (see almost all other structure diagrams of benzene rings in that document, and “Conventionally, benzene is commonly drawn with three single bonds and three double bonds, (…)” on page 379). Anyway, the question is not about how to draw benzene; it is about how to interpret a given ‘cyclohexatriene’ structure diagram. $\endgroup$ – Faded Giant♦ Apr 23 '16 at 16:43
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    $\begingroup$ "Anyway, the question is not about how to draw benzene; it is about how to interpret a given ‘cyclohexatriene’ structure diagram" +1 That's exactly what I mean. $\endgroup$ – Rodriguez Apr 23 '16 at 16:53
  • $\begingroup$ I'm shocked, but section GR-6.5 on page 105 of pdf file goes into more detail and reinforces the idea that the double bond form is preferred unless resonance is specifically being discussed. Weird. I really like the circle form better. $\endgroup$ – MaxW Apr 23 '16 at 16:58
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The answer is you are referring to neither of them. That is because resonance structures don't actually exist in reality. We only use them to give us a rough idea what the actually molecule and bonds look like. A common way to explain resonance structures is this:

An explorer from a far distant land travels to a new continent and sees a strange animal that he has never seen before, a rhinoceros! When he comes back home, not really sure how to describe this strange animal, he tells everyone that it looked like an unicorn and a dragon mixed together.

enter image description here

So what the explorer did was he tried to explain some weird, not easily imagined object (a rhinoceros) by describing it as a mixture of two mythical, non-existent objects, but which are easily imagined by people.

That is exactly what the resonance structures of benzene (also for any other molecule) are. They don't actually exist, but rather benzene exists as a weird mixture of these resonance structures. The reason why we use resonance structures is that they give us an idea how to imagine what benzene looks like.

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    $\begingroup$ What a great analogy! I've never seen this before, but I'll be sure to use this to explain resonance in the future! $\endgroup$ – ringo Apr 23 '16 at 7:45
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    $\begingroup$ The analogy seems to originate from "Resonance in Organic Chemistry" by George Willard Wheland - 1955, page 4. In an earlier version (The Theory of Resonance and Its Applications to Organic Molecules, 1944) of the monograph he used a mule as an intermediate between a horse and a donkey which are both real creatures. $\endgroup$ – MaxW Apr 23 '16 at 22:27
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    $\begingroup$ @MaxW my organic lecturer explained tautomers using the above analogy. Where you explain a mule as a mixture of two real animals, a horse and a donkey (the individuals tautomers in this case). $\endgroup$ – Nanoputian Apr 24 '16 at 0:15
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    $\begingroup$ This is veering off-topic but I strongly disagree with the use of this analogy for tautomerism. Tautomerism is not the same as resonance. Tautomers independently exist and are interconverted via a rapid process. The relative abundance of each tautomer can be directly measured, e.g. via NMR spectroscopy. Resonance forms do not exist and are not interconverted. $\endgroup$ – orthocresol♦ May 8 '16 at 16:00
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I've seen the following drawing used instead of the three double bonds. The circle signifies that the ring is aromatic. It's an alternative that is used to show that all bonds are the same.

circle depiction of aromaticity

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    $\begingroup$ This. It reinforces the notion of the pi electrons all being in a weird circular flux of no fixed position. $\endgroup$ – Nij Apr 23 '16 at 12:32
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The way I would put it is that we are compensating for the inability of Lewis structures to satisfyingly depict electron delocalization, so instead we pick whichever "resonance" structure is most appropriate, with the understanding that these are only poor approximations of the real situation. (FWIW, I don't quite like the "circle" notation, as this is not very useful for mechanistic "electron pushing" analyses. How many electrons are there in a circle, anyway?)

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    $\begingroup$ Electron pushing is where the evil starts; the circle has 6 electrons. $\endgroup$ – Martin - マーチン♦ Apr 23 '16 at 15:57
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    $\begingroup$ If electron pushing is evil, my soul is lost for all eternity and destined to burn in the deepest and darkest six-electron circle of hell … @Mart $\endgroup$ – Jan Apr 23 '16 at 21:35

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