Alpha-beta unsaturated carbonyl

[deleted388502]

Hi,

EK Organic 559 has a picture of an alpha-beta unsaturated carbonyl asking which carbons are possible electrophiles and I wanted to clear up why the alpha carbon is NOT considered a possible electrophile. The solution has the carbonyl carbon and beta carbon considered to be electrophiles, but I'm conceptually confused as to why the alpha carbon is not considered to be an electrophile as well considering that the hydrogens on the alpha carbon are acidic and if deprotonated, that would seemingly form a good electrophile?

I have also read that the seemingly "good" nucleophiles, i.e. a grignard, will attack at the carbonyl carbon, and a "soft" nucleophile (not as strong of a nucleophile) will attack at the beta carbon, and wanted to verify that this was correct.

Thanks!

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[Czarcasm]

Hi,

EK Organic 559 has a picture of an alpha-beta unsaturated carbonyl asking which carbons are possible electrophiles and I wanted to clear up why the alpha carbon is NOT considered a possible electrophile. The solution has the carbonyl carbon and beta carbon considered to be electrophiles, but I'm conceptually confused as to why the alpha carbon is not considered to be an electrophile as well considering that the hydrogens on the alpha carbon are acidic and if deprotonated, that would seemingly form a good electrophile?

I have also read that the seemingly "good" nucleophiles, i.e. a grignard, will attack at the carbonyl carbon, and a "soft" nucleophile (not as strong of a nucleophile) will attack at the beta carbon, and wanted to verify that this was correct.

Thanks!

The alpha carbon is what makes ketones and aldehydes a good nucleophile. We're so use to seeing their carbonyl carbon act as a good electrophile that the nucleophilicity of the alpha carbon is sometimes neglected entirely. The proton on the alpha carbon is barely acidic (pKa around 20) but if we use a strong enough base, we can get it to deprotonate and we can do this under acidic or basic conditions. Upon deprotonation, a pair of electrons reside on the carbon. If you recall any trend in organic, it's that an atom with a partial negative charge makes a reasonably good nucleophile.

Alpha beta unsaturated carbonyl compounds is another special unique compound. When a double bond resides between the beta and alpha position, it's resonance stabilized. The pi bond can shift over to share with the carbonyl carbon (while the carbonyl carbon's electrons shift up to oxygen). In reality, the structure is a combination of these resonance structures, but the idea is, sharing this pi bond with the carbonyl carbon results in a temporary positive charge on that beta carbon which makes it a good electrophile. Unlike nucleophiles, good electrophiles generally have positive charge.

 

[Czarcasm]

Hi,

EK Organic 559 has a picture of an alpha-beta unsaturated carbonyl asking which carbons are possible electrophiles and I wanted to clear up why the alpha carbon is NOT considered a possible electrophile. The solution has the carbonyl carbon and beta carbon considered to be electrophiles, but I'm conceptually confused as to why the alpha carbon is not considered to be an electrophile as well considering that the hydrogens on the alpha carbon are acidic and if deprotonated, that would seemingly form a good electrophile?

I have also read that the seemingly "good" nucleophiles, i.e. a grignard, will attack at the carbonyl carbon, and a "soft" nucleophile (not as strong of a nucleophile) will attack at the beta carbon, and wanted to verify that this was correct.

Thanks!

Also, that last point about hard and soft nucleophiles is correct. Generally, bulky nucleophiles tend to attack the beta position.

 

[deleted388502]

I was definitely confusing the two. Thanks so much!