resonance structure question

[pbehzad]

When you are doing resonance structures, is it better to have a neutral charge on the entire molecule and have one atom without an octet, or is it better for all the atoms to have an octet and there be a formal charge on an atom(s)? The molecule I am questioning is H2CN2.
The molecule will be neutral if the second N has 2 valence pairs, and is bonded singly to the other N, but it would not have an octet. If there is a double bond between the two N's, one of the Ns will be (+), and other will be (-) but both Ns will have octets. Got my first orgo test tomorrow Thanks for any help!

 

[Nik]

N can violate the octet rule I believe. Regardless, It is always better never to not have a charge (in most cases anyway).

 

[Raptor]

I should be the master of resonance structure after many dern practice. But about your question, the most stable resonance structure is the one with less distribution of charge (neutral will be very stable). Good luck on your test, hope you do great.

 

[Kirk]

Molecules will almost always have an octet. Any atom without an octet is a vicious, nasty, mean electron scavenger. It will NOT be stable. A molecule will be much more stable with a charge then without an octet.

 

[pbehzad]

Ok so which one is it? raptor and nik say one thing and kirk says the other? dont you love organic chemistry !

 

[kidsmd]

it should be the one that has the full octet. Here's a link from one of the professors at my school:

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

Hope this helps!!!

 

[Raptor]

What I am trying to say is that when molecules that has an octet, they are stable. A molecule with a charge isn't stable because its always trying to lose or gain e-. Now in resonance structure, if the instructur give a question like, "Which one of these resonance structures are the most stable?" then the resonance structure that has no charge or neutral is stable and the molecule that has a charge is less stable then the one that doesn't.

 

[dpark74]

Your specific example CH2N2 should have charges. There should be a negative charge (-1) on C and a positive charge (+1) on the N attached to the C.

Diazomethane people...come on; used to convert carboxylic acids (mostly) to methyl esters.

 

[dpark74]

Originally posted by Nik
N can violate the octet rule I believe. Regardless, It is always better never to not have a charge (in most cases anyway).

NO violates the octet rule. (only 7 around N, and 8 around O)

Also, CH2N2: no way it can be neutral (ie, no charges) and all atoms have octets.

 

[pbehzad]

my prof said that it is better to have octets than no charge on teh atoms. she said the H2CN2 molecule was neutral anyways cause the (+) and (-) charges cancelled each other out. Thanks for your help.

 

[dpark74]

Yeah..sorry for not making myself clear...overall CH2N2 is neutral but has formal charges on C and N

 

[Phaedrus]

It is ALWAYS better to have a full octet, than no charge. You MUST have a full octet, even if it means giving C and N charges.

 

[Harps]

Molecules that have no charges on any of the ATOMS, but don't have an octet can still be very very stable (some atoms violate the octet rule as has already been said in one of the above posts). But if you can get all atoms to have octets and have the molecule be OVERALL neutral then that is even better. Meaning some atoms will have charges but they cancel out. Anyways, the best resonance structures are usually easy to pick out of the choices because these are ones with complete octets or overall neutral charge--generally.
Hope this kinds of sums up the confusion and the related clarification regarding resonance structures. Good luck