For alpha-beta unsaturated carbonyls, how do you predict where the nucleophile..

[ratatat]

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For alpha-beta unsaturated carbonyls, how do you predict where the nucleophile attacks (either the B-carbon or the carbonyl)? Which one is the major product?

Thanks 🙂

 

[inaccensa]

For alpha-beta unsaturated carbonyls, how do you predict where the nucleophile attacks (either the B-carbon or the carbonyl)? Which one is the major product?

Thanks 🙂

Isn't the double bond, electron rich and thus attacked by Electrophiles? Alkenes tend to undergo electrophilic addition. carbonyl is electron deficient and attacked by nucleophiles. It may undergo substitution normally for carboxylic acids and addition for aldehydes and ketones.

 

[inaccensa]

I think I didn't read ur q right. U r asking about alpha-beta unsaturated.

In in case of alpha, beta unsaturated ketone for ex, the carbonyl is strongly pulling the electrons from the unsaturated carbons. Hence the e- cloud density in this case decreases compared to a straight chain alkene. I think we also have to consider the resonance form with carbocation and a -ve charge on oxygen. If a nucleophile approaches in this case, it will add to the oxygen rather than the double bond. This is the basis of keto-enol tautomers.

 

[inaccensa]

but now I'm confused, why do we use a nucleophile for unsat. alkene? Should electrophiles attack the double bond?

 

[loveoforganic]

Hard nucleophiles attack at the carbonyl. Soft nucleophiles attack at the beta position. An example of a hard nucleophile would be a grignard. An example of a soft nucleophile would be a halogen.

http://en.wikipedia.org/wiki/HSAB_theory
http://www.meta-synthesis.com/webbook/43_hsab/HSAB.html

^^More on hard/soft acids/bases.

That's above mcat level by far.

 

[loveoforganic]

but now I'm confused, why do we use a nucleophile for unsat. alkene? Should electrophiles attack the double bond?

Draw a resonance structure popping the double bond from a,b to a,carbonyl and popping the carbonyl pi bond up to the oxygen. There's a partial positive charge on the b carbon.