[limabeans45]

Chapter 1 in my organic textbook primarily has to deal with resonance structures, and drawing out the structures. But I am really lost with my homework problems(which will probably mimic the quiz/test problems I would assume). Problems like this:

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

I don't want to know how to solve specific problems, I am just going to post a lot of questions in the following order.
I'm very confused about how to draw a specific compound out from scratch. Take CH3NO2CH3, what is the logic in drawing out the compound the way it is drawn out? I know that H's must always go on the outside, but other than that I am kind of lost.

I'm very confused about why certain resonance structures are okay, and why some others aren't. It seemed a lot easier to me in Chem 1, but now looking at it with organic compounds I feel completely lost. Some problems in my book show Nitrogen becoming positively charged, even though it's highly electronegative. They might also show a double bond breaking, but not breaking enough to become a single bond. The only rule I can think of off the top of my head are that the amount of electrons on the outer shell must remain the same. I'd write out an example from the book but I don't know how to format it properly in here. I could draw some out later on and post them on and image hosting site and link them here.

The book always seems to stop at 3 or 4 resonance structures, but it seems like some of these compounds could have more than that.
The positive/negative charges are really confusing me, like in one compound there is a resonance form where Nitrogen is positive, and Carbon is uncharged, even though Nitrogen is more electronegative. Things like that are just confusing me.

Maybe i'm just overreacting, but looking at the resonance structures for compounds like H3CN2 is really blowing my mind.

[Vidya]

okay you start like this.
first practice how to draw a lewis structure and follow the rules while making the lewis structure .Practice calculating formal charges also. practice to find out  which lewis structures are good or better and which one are not good.

[james_a]

There's really two components to evaluating resonance structures - whether it is "legal" and whether it is "significant".
You will see that there are "significant' resonance structures that have a FORMAL positive charge of +1 on oxygen and nitrogen, but these will also have a full octet around the atom. However, resonance structures where there is a positive charge on a nitrogen or carbon and less than a full octet are not considered significant.
 For a good 5-part overview of evaluating resonance structures in organic chemistry  you might want to check out this series of videos.

http://www.youtube.com/watch?v=3UmglHJDijc&feature=relmfu

hope this helps!