conjugation in carbonyl compounds...

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samsam66

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Hi Folks,

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the screen shot is from kaplan chemistry ch.9.......i have no clue what they're talking about:

1) "stabilization of a +ve charge once the nucleophile has bonded" !!!!....really! what +ve charge? when a nuc binds you should get -ve on the O? Da **** is he talking about????

2) it seems the key msg of the paragraph is that Conjugated carbonyl compunds are more stable...but don't you want that +ve partial charge on carbonyol C to be strong and localized to get a nuc to hit it? what good is it having it resonate around?

@aldol16 ...where art thou

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1) "stabilization of a +ve charge once the nucleophile has bonded" !!!!....really! what +ve charge? when a nuc binds you should get -ve on the O? Da **** is he talking about????

I believe they mean stabilization of negative charge once nucleophile has bonded. Simple check is conservation of charge. The initial molecule has no charge. Say the nucleophile has a negative charge, as is common in nucleophiles. The resultant complex must have a negative charge. That negative charge is indeed stabilized by resonance.

2) it seems the key msg of the paragraph is that Conjugated carbonyl compunds are more stable...but don't you want that +ve partial charge on carbonyol C to be strong and localized to get a nuc to hit it? what good is it having it resonate around?

So this molecule is stable because of resonance but remember that resonance also distributes charge. So one resonance form, for example, is obtained by drawing an arrow from the C=O bond to the O, making a (+)C-O(-) center. Another resonance form derived from this form is obtained by drawing an arrow from the alkene double bond. This puts the positive charge on the terminal carbon above. Therefore, you have partial positive character on both the carbonyl and terminal carbons, making the whole thing reactive towards nucleophiles. Hence the so-called conjugate addition reactions.
 
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