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In my book, to explain the structure of benzene, it referred to the idea of resonance:

"The idea of resonance is that actual molecule is a definite structure that is a hybrid of two theoretical structure"

I partially understood this sentence, could someone please elaborate

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The confusion may have been caused by ambiguous use of phrase structure. "Definite structure" means the actually electronic structure of benzene, while "theoretical structure" means the two contributing lewis structures, but it remains theoretical because either one of them along cannot correctly describe a real molecule. As confusing as the double arrow may be, the real molecule do not oscillate between two structures; rather, the molecule is a constant superposition of these two structures. –  Xiaolei Zhu Mar 3 at 15:44

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It means that what we usually consider to be the 'structure' of benzene is not the actual structure. See below

Benzene

Source

While we say that there are three distinct pi bonds in the structure there is actually resonance, or electron delocalization. This means that the electrons are not always in the double bonds, but distributed throughout the ring. This is one of the most important characteristics of an aromatic system.

In the above image the lower structure is the "hybrid of two theoretical structures" - notice the inscribed circle. It represents that those 6 electrons are shared between all of the Carbon atoms. The two structures above that are the "two theoretical structures" that are in resonance with one another

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Use LCAO to draw the pi orbital at each trigonal center. Where are the double bonds? LCAO will 1,3-shift charge or radicals with near no effort at all, getting you through the course with a useful, functional understanding of aromatic substitution and carbonyl condensation reactions. LCAO will not tell you about aromaticity versus anti-aromaticity. Woodward-Hoffmann rules for the orbitally fancy stuff.

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The core feature of the benzene molecule that is 'caused' by resonance is the bond length.

You know from your previous studied that double bonds are shorted than single bonds. In the 'resonating' Lewis structures for benzene, you would thus observe two kinds of bonds, three single bonds and three double bonds. However spectroscopic studies show only one kind of carbon-carbon bond in benzene. Hence 'resonance' implies some 'averaging' of structure between the two Lewis structures. You can't really draw a Lewis structure for the true representation of benzene, which is why the circle motif was developed.

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