Resonance (organic chem)

[MelonBar]

Hey all, would appreciate any help with this resonance problem from 1st year organic chem. Basically the question asks you to draw the 3 resonance structures of methyl phosphate anion/CH3OPO3 2-.

why cant there be a 4th structure which has: a double bond from the O (bonded to CH3) to P and the other 3 phosphate O's can have negative formal charges. This seems to be consistent with the rules so why isnt that in the answers?

Cheers

( iam working on attaching a photo)

[nino quincampoix]

You can't create charge with resonance. You can only move/distribute it, or if you do "create" charge, it has to be balanced so that it cancels out and the net charge remains unchanged.

Obviously phosphorous can have a maximum of 5 bonds and oxygen can have a maximum of 4 bonds.

Placing a negative charge on O (the one bonded to CH3 and P) will violate its octet.

[MelonBar]

Thanks nino. Is there a way of systematically working out all resonance structures? I find that I can only get them through trial and error with the harder molecules especially.

side note, I understand what youre saying about 'creating' charge but wouldn't O still have a full octet with a neg. charge? 1 lone pair, double bond to P, single bond to C = 8 electrons around it.

[nino quincampoix]

Well, for starters, you can't break single bonds to create resonance structures (technically you can, but it shouldn't appear in a 1st year chemistry paper). You can break double bonds (but form them elsewhere or distribute their constituent electrons).

Some things to look for:


I'm sure there are other "rules," but you only really need to know valence shell configurations so that you can push charges or lone pairs around.


Oxygen, without a formal charge, has two bonds and two lone pairs.

[nerdgasm]

You can't create charge with resonance. You can only move/distribute it, or if you do "create" charge, it has to be balanced so that it cancels out and the net charge remains unchanged.

Obviously phosphorous can have a maximum of 5 bonds and oxygen can have a maximum of 4 bonds.

Placing a negative charge on O (the one bonded to CH3 and P) will violate its octet.

(Image removed from quote.)

I think the structure MelonBar was referring to, is the one at the bottom of the picture. Here, the methyl O should have a positive charge as it has three bonds coming from it, and the third bond was formed from its own lone pair being donated to form a P-O pi bond. So, charge is technically conserved in this structure (as now you have three negative charges and one positive charge for a net charge of -2, as MelonBar alluded to).

Perhaps the answer could lie in the fact that there are four 'formal' charges in this potential resonance contributor, as opposed to two in all the others. Keeping in mind that resonance contributors aren't really 'real' (as the molecule exists in a form that is the average of the contributors), I would suggest that the higher number of formal charges means that your suggested contributor is less stable than the other contributors, so would be significantly less 'seen' in the final average, which is why we don't count it. I suppose for experimental evidence, one might try to determine the length of the methyl O-P bond (how much shorter is it than a normal methyl O-P single bond?). I'm guessing this may have been done in the past, thereby helping us to work out which resonance structures are found?

I'm not entirely sure this fourth resonance structure is completely 'illegal' - but maybe it is just a lot more improbable.

[nino quincampoix]

I think the structure MelonBar was referring to, is the one at the bottom of the picture. Here, the methyl O should have a positive charge as it has three bonds coming from it, and the third bond was formed from its own lone pair being donated to form a P-O pi bond. So, charge is technically conserved in this structure (as now you have three negative charges and one positive charge for a net charge of -2, as MelonBar alluded to).

Perhaps the answer could lie in the fact that there are four 'formal' charges in this potential resonance contributor, as opposed to two in all the others. Keeping in mind that resonance contributors aren't really 'real' (as the molecule exists in a form that is the average of the contributors), I would suggest that the higher number of formal charges means that your suggested contributor is less stable than the other contributors, so would be significantly less 'seen' in the final average, which is why we don't count it. I suppose for experimental evidence, one might try to determine the length of the methyl O-P bond (how much shorter is it than a normal methyl O-P single bond?). I'm guessing this may have been done in the past, thereby helping us to work out which resonance structures are found?

I'm not entirely sure this fourth resonance structure is completely 'illegal' - but maybe it is just a lot more improbable.

You're right in that there is a fourth possible resonance structure. I forgot to put the + charge on the oxygen in the picture.

But as you said, it won't be significant because there are too many formal charges. See below for an explanation I found in a textbook.