[menjah]

Hello, I've done quite a bit of research on resonance but can't seem to find out what it's caused by or how to apply it to Lewis diagrams.
Basically, I know that in real observation, certain atoms have bonding strengths in-between whole bonds. This poses a problem to the traditional electron diagram, and that you overcome this with multiple possible but incorrect lewis diagrams. The nature of the real atom is supposed to be the average of the possibilities, yadda yadda.
I think from what I've read that the resonance is caused by more than one place for a double/triple/etc bond to occur for the octet rule to be complete, but I've also heard things about resonance being the constant switching between covalent bonding and ionic bonding.
Also, if I get a compound, how will I know it's resonant? Please sort me out; my textbooks and teacher don't seem to have my answer 

[nj_bartel]

Organic Chemistry, John McMurry

1) Individual resonance forms are imaginary, not real.
2) Resonance forms differ only in the placement of their pi or nonbonding electrons.
3) Different resonance forms of a substance don't have to be equivalent.
4) Resonance forms obey normal rules of valency.
5) The resonance hybrid is more stable than any individual resonance forms.

****Any three-atom grouping with a p-orbital on each atom has two resonance forms****

[cliverlong]

Hi,

   I think the following might help - but I'm not 100% sure it is correct ! So anyone who knows please amend what I have written if necessary.

   Regarding resonance bondiong I would start with benzene. A simple analysis of the 4-valency on benzene leads us to think the molecule is a 6-member hydrocarbon ring with alternate double and single bonds between the carbons and single hydrogens bonded to each carbon on the "outside" of the carbon ring. Thinking the carbon-carbon bonds are of two types, single and double would lead us to think there would be different bond energies and different inter-carbon spacing. None of this is found in benzene - the C-C bonds are identical.

   Now the idea of "resonance" of the C-C bonds is introduced to explain these findings.

http://en.wikipedia.org/wiki/Resonance_(chemistry)

I'm going to ignore resonance and go to orbital hybridization.

http://en.wikipedia.org/wiki/Orbital_hybridisation

If we first think about C-H double bonds in methane some "merging" of the 2s and 2p orbitals creates sp³ hybrids so that all the C-H bonds are equivalent. These are sigma bonds.

Now if we consider ethene, the merging of some of the 2s and 2p orbitals create sp² hybrids for the sigma bonds. These form the C-H and the first C-C bonds and are arranged in a plane at 120 degree orientation. The remaining non-hybridized 2p orbitals form the second bond between the carbons. This is the pi bond. Now I guess this will have different properties to the sigma bonds.

Now consider benzene

http://www.chemguide.co.uk/basicorg/bonding/benzene2.html

sp hybridization again and unhybridized p-orbitals. The electrons in the p-orbitals form the delocalised pi bond system that float in rings above and below the plane of the C ring. The presence of the pi electrons make benzene attractive to electrophilic groups

http://www.chemguide.co.uk/mechanisms/elsub/whatis.html

Jim Clarke and chem guide - love it.

I think my approach is molecular orbital theory. No "resonance" - not needed.

Any good?


Clive

[cliverlong]

Can anyone give me information on bicarbonate (hydrogen carbonate)?

http://en.wikipedia.org/wiki/Bicarbonate

Can this be considered a candidate for "resonance" since we have three oxygens bound to a central carbon? We have a lot of "asymmetry" in the representation in the diagram given, with C-O single and double bonds and one of the oxygens being negatively charged.

I feel the C-O bonds should be equivalent and the H somehow bonded to the three oxygens. Is that right or wrong? Is there a diagram for the hydrogen carbonate ion or is the one given in the wikipedia article correct?


Thanks

Clive

[macman104]

http://web.chem.ucla.edu/~harding/tutorials/resonance/draw_res_str.html

[azmanam]

@ macman: that's true for the carbonate ion, but for the bicarbonate ion, there are really only two major resonance contributors.  Thus, for the carbonate ion, the three C-O bonds are equivalent.  For the bicarbonate ion, only 2 of the C-O bonds are equivalent.

http://academic.reed.edu/chemistry/alan/ED/JCE/figlist.html
http://courses.cm.utexas.edu/jsessler/ch310m/fall2004/Class_notes/Lecture3-as%20given.pdf

[menjah]

It's great that there's a website like this that knowledgeable people actually go to.

From macman's link, one of my previous thoughts, that the resonance is actually caused by the possibility having a double/triple bond in different places, has been confirmed.

I was kind of thinking that, but I had read so many slightly different versions of the theory that I got confused.

It's also good to know about that orbital hybridization thingy that changes the shape of the molecule; I find all these things quite interesting, provided I understand what's going on.

Thank you all so much for helping =D

[macman104]

I wasn't linking specifically for that structure, just resonance structures in general.