Acidic Alpha Carbons

[PrinceAli2786]

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Hey I have a question that I saw a while back but I have no idea what the answer is, I would appreciate the help😎

Which is the most acidic hydrogen in this compound?
H O H
l ll l
H3C-C-C=C-C-C-C-=CH
ll l l
O H H

or H3C=OCH=CH-C=O-C=-CH

*it messed it up when I drew it out


the -= at the end is a TRIPLE BOND, I forgot the exact molecule but I know it was testing when is the alpha carbon the most acidic, when next to a C=C or a C_=C triple bond or just near a regular C=O.

can someone please tell me how to solve this and give an explanation regarding all those circumstances, etc. I'd really appreciate it😎 thanks

 

[RPedigo]

Is this the molecule?

Just so I can make sure we're on the same page.

 

[Quantum Chem]

If the molecule drawn from RPedigo is correct, then I believe that the CH2 in between the second carbonyl and the carbon triple bond. That way when the enolate ion is formed the compound would resemble a conjugated system. Also, if I remember correctly I think that double and triple bonds reduce inductive effects on adjacent carbons, making those H's slightly more acidic.

You could also narrow down the choices. The hydrogens on the C=C bonds would not be the first ones to get plucked off because that would leave a negative charge on the double bond and if it tautormerizes then it would produce two doubles on the same carbon, which is not as stable as the conjugated one above. And while the methyl ketone has more H's to be plucked off giving it more of the kinetic approach, I don't think it would be more stable than my above opinion.

 

[PrinceAli2786]

yea that's the molecule from what I can remember, thanks for that! 🙂

thanks for the explanation, but just to make sure, so I can pretty much think:

Double and Triple bonds near a carbonyl's alpha carbon/hydrogen help to make it more acidic? with the Triple bond having more of an effect? Is this like a general rule I can keep/use for all problems like this?

once again, thank you very much for your help so far😎

 

[Quantum Chem]

To be honest, I don't know if there is a trend or not. I was basing my answer more on the fact that the enolate ion forms a conjugated compound. I could be wrong and/or my reasoning could be wrong. I tried to find a similar compound or explanation in my trusty old Organic textbook, but was unable to.

 

[PrinceAli2786]

To be honest, I don't know if there is a trend or not. I was basing my answer more on the fact that the enolate ion forms a conjugated compound. I could be wrong and/or my reasoning could be wrong. I tried to find a similar compound or explanation in my trusty old Organic textbook, but was unable to.

k np, thanks for the help though, I really appreciate it 🙂


if anybody else knows, can you please confirm/let me know I'd really appreciate the help. thanks

 

[161927]

The explanation is simple. When you abstract a hydrogen from the carbon alpha to the carbonyl and the alkyne, a carbanion is formed. This negative charge is doubly resonance stabilized. It can resonate with the pi bond of the carbonyl, and it can resonate with a pi bond of the alkyne. Because this resulting conjugate base is stabilized by resonance, the related acid is more acidic.

 

[PrinceAli2786]

The explanation is simple. When you abstract a hydrogen from the carbon alpha to the carbonyl and the alkyne, a carbanion is formed. This negative charge is doubly resonance stabilized. It can resonate with the pi bond of the carbonyl, and it can resonate with a pi bond of the alkyne. Because this resulting conjugate base is stabilized by resonance, the related acid is more acidic.

thanks😎