PROVE that benzene, #"NO"_3^(-)#, and #"CO"_3^(2-)# have resonance??

1 Answer
Truong-Son N.
Jan 29, 2018

Well, are their bonds not all equivalent within themselves?

We consult NIST, as usual...

For benzene,

#r_("C"stackrel(--)(-)"C")("benzene") = "1.3970 Å"#;

Its atom-atom distances are given:

http://cccbdb.nist.gov/expgeom2.asp?casno=71432&charge=0

and it has TWO resonance structures.

https://upload.wikimedia.org/

It is indisputable that benzene has identical bond lengths for all of its carbon-carbon bonds... and that is attributed to electron delocalization throughout the aromatic ring...

Nitrate is another one that is easy to prove. Its bond lengths are approximately the same as #"NO"_3#. Adding one electron merely slightly alters the electron spread, but not the symmetry.

It has

#r_("N"stackrel(--)(-)"O")("nitrate") ~~ "1.2377 Å"#

http://cccbdb.nist.gov/expgeom2x.asp?casno=12033497

and that means it has THREE resonance structures.

http://thompsona.free.fr/

Carbonate also has THREE resonance structures, with

#r_("C"stackrel(--)(-)"O")("carbonate") ~~ "1.2843 Å"#

http://cccbdb.nist.gov/geom3x.asp?method=24&basis=0

http://www.chem.ucla.edu/

And the only way all of these molecules have identical bond lengths within themselves is resonance.

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