[Motutu]

Can anybody help me understand why resonace structures are so important and they decide almost all the properties or a organic compound. There are 2 rules in writing resonace structures, 1) Don't break the single bond, 2) not more than 4 electron pair on the 2nd period atoms.

This is simple but magically these three things decide everything from stability of compunds to the reaction dynamics.

We say a compound with large number resonance structures are more stable than with a low number. Why ?

We say that a group is meta or ortho/para directing on the basis of the fact that in some of the resonance structures there is a -ve charge at -o and -p positions. I don't get why because -ve charge can accumulate those position in resonance but what is the proof that it happens in reality ?

 

[Enthalpy]

Someone (Adidex?) came recently with a complex where resonance breaks single bonds.
More stable: if you compare among compounds that have multiple bonds. But multiple bonds increase the heat of formation. Even the aromatic benzene has a positive one.

[Motutu]

Where can I find that complex in which resonance breaks single bond ? Do you have any link to it ?

I don't get the stability part in your answer ? can you expand on it please. 

And can you explain why in compounds the electron rich areas corresponds to the electron rich areas in the resonance structures of the compound ? like meta position for Nitrobenzene or ortho/para for halobenzene.

[clarkstill]

It's important to realize that resonance mesomers do not exist! Resonance is just a convenient "back of the envelope" way to calculate the properties of the frontier molecular orbitals of a conjugated system without solving any equations. For example, 'resonance' shows that an enolate is nucleophilic at both the oxygen and the alpha-carbon, but the real reason for this is that the HOMO has large lobes on these two atoms (i.e. a great deal of the negative charge lies on them) - resonance tells us the answer, but doesnt explain the reason why it is so.

You also ask why a compound with many resonance structures is more stable than those with a low number. Again resonance tells us the answer, but not the reason why. In order to understand this, you would need to take a molecular orbital, and try 'mixing' it with an additional pi bond or p-orbital - the interaction between occupied and unoccupied orbitals leads to a net lowering of the energy of the occupied MOs, hence the lower energy.

Regarding your query about ortho/para directing groups, the same argument applies as for the enolate above - the HOMO has large lobes at these positions due to the interaction of the aromatic pi system with the high energy electrons (e.g. lone pair) of the directing group. There is a great deal of evidence for this, including the ortho/para directing effect itself, as well as NMR chemical shift data, and computation. There are probably loads more, just not off the top of my head.

Hope this helps

[Babcock_Hall]

I seem to recall seeing SF₆ written with some resonance structures having a fluoride ion and others in which there was a S-F bond.

[Enthalpy]

Resonance breaking single bonds: I don't find the post again, but it was few months ago. The complex had a metal at the centre of two or four organic compounds around, with single bonds and amines (or hydroxyls?), symmetric enough that the compounds could exchange bonds with the metal, charges, maybe protons too. Sorry, that's all I remember.

[clarkstill]

If you draw the resonance forms they are talking about you'll find you never touch the sigma bond network, you just change between what looks like a series of alkynes linked by single bonds, and a series of alkenes directly connected (ie a cumulene).

Isn't the normal example of sigma bond resonance in non-classical carbocations, e.g. the 2-norbornyl cation?

[Motutu]

Oh thank you. Good to know that Resonance is just a cheating method of no real value in chemistry.

[clarkstill]

Oh thank you. Good to know that Resonance is just a cheating method of no real value in chemistry.

That isn't quite what I said! Resonance is very useful, it's just a mnemonic rather than any sort of fundamental principle.

[Babcock_Hall]

Oh thank you. Good to know that Resonance is just a cheating method of no real value in chemistry.

A MO analysis is not appropriate for every student, and a lower level theory that gives a qualitatively correct picture is desirable in those circumstances.

[Motutu]

What does real chemist uses ? Resonance or MO ?