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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Lonn on May 30, 2011, 02:47:22 PM

Title: Why are these resonance structures significant?
Post by: Lonn on May 30, 2011, 02:47:22 PM

After learning the concepts of drawing resonance structures I started doing some problems in a book and it had the resonance structures I circled (in the image attached to this post) as valid/significant.

I don't understand why, because in the concepts it said that structures with positive formal charges on electronegative atoms aren't significant unless by doing them they help complete more octets of other atoms in the overall structure.

I though I understood the general logic about drawing resonance structures but I guess I don't. It would be greatly appreciated if someone could tell me why they are valid, I have read 3 online pages explaining resonance structures and the chapter in the book that explained them and I still can't figure this out.

in A why are the circled structure valid if they are adding formal charges and the positive one is on a electronegative atom?

in B why is the circled structure valid if the negative formal charge is on a less electronegative atom(carbon) than it was in the previous structure?

in C [same thing as A]

in D why is the circled structure valid if the positive charge is on a more electronegative atom(oxygen) than the negative one?

Title: Re: Why are these resonance structures significant?
Post by: azmanam on May 30, 2011, 04:27:09 PM

valid just means "legal" - it doesn't break any rules of resonance structures.  Valid does not mean "greatest contributing."

An interesting rule of thumb: the 2nd best resonance structure usually defines a molecule's reactivity.

indeed, enamines are nucleophilic at the alpha carbon - and extended enamines are nucleophilic at the gamma carbon... etc

Title: Re: Why are these resonance structures significant?
Post by: Lonn on May 30, 2011, 05:53:57 PM

I'm sorry I worded it wrong with "valid" I meant significant, the exercise said to draw only the significant resonance structures.

I don't understand why those structures(the ones I circled) are significant if in A and C they add two formal charges and the positive one is on a electronegative atom. In B they "shift" the negative formal charge to a less electronegative atom. And in D (the circled structure) the positive formal charge ends up on a more electronegative atom than the negative formal charge.