Which is more acidic- diketone or carboxyllic acid?

[virtualmaster999]

Hey everyone!

Quick question. When comparing acidities for carbonyl compounds (especially those with alpha protons), which would be more acidic, a carboxyllic acid or a diketone? I know the diketone has more resonance forms but the electrons are shared between 2 oxygens in the acid. Kinda confusing.

Can someone help me out? Thanks!

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[Thanhn]

Draw the deprotonated compound and then follow the rule of CAR(D)IO
1. Charge - the more positive formal charge on the atom losing the proton, the more acidic.
2. Atom - more electronegative atom can better stabilize the "-" charge once deprotonated
3. Resonance - more resonance = more stable
4. Inductive Effects - look for any NEARBY electronegative elements to draw/disperse the negative charge
5. Orbital - sp is most s characteristic (50% s, 50% p) meaning its most sphere-like so the negative charges is closer to positive nucleus = more stable than sp2 (33% s, 66% p)

Both a diketone and carboxylic acid would have the same formal charge so negligible. However, diketone loses a proton at the alpha carbon whereas the carboxylic acid loses proton from an OH (oxygen). Oxygen is more electronegative so it can handle the negative charge once deprotonated better -- therefore, carboxylic acid has a lower pKa.

Just found this after I typed it out, it explains the concept better than I can.
http://www.masterorganicchemistry.com/2010/09/22/five-key-factors-that-influence-acidity/

 

[DAT Destroyer]

Hey everyone!

Quick question. When comparing acidities for carbonyl compounds (especially those with alpha protons), which would be more acidic, a carboxyllic acid or a diketone? I know the diketone has more resonance forms but the electrons are shared between 2 oxygens in the acid. Kinda confusing.

Can someone help me out? Thanks!

As a nice general rule carboxy acids are very well resonance stabilized and will be more acidic than a diketone or diester, both of which have less resonance stabilization.

Here's a few helpful pKa values carboxy acids =5, phenols =10, alcohols = 16 ,adelhydes and ketones 17 and 18 respectively. Esters and alkynes about 25, alkenes 44, and alkanes over 50. The lower the pKa value the more acidic the compound.

The David Klein textbook does a nice job explaining this in very easy to understand language with many examples.

Hope this helps

Dr. Roma

 

[510586]

As a nice general rule carboxy acids are very well resonance stabilized and will be more acidic than a diketone or diester, both of which have less resonance stabilization.

Here's a few helpful pKa values carboxy acids =5, phenols =10, alcohols = 16 ,adelhydes and ketones 17 and 18 respectively. Esters and alkynes about 25, alkenes 44, and alkanes over 50. The lower the pKa value the more acidic the compound.

The David Klein textbook does a nice job explaining this in very easy to understand language with many examples.

Hope this helps

Dr. Roma

I've heard that in ochem, carboxylic acids are the most acidic of any organic compound. Is this true, or are there exceptions?

 

[510586]

Draw the deprotonated compound and then follow the rule of CAR(D)IO
1. Charge - the more positive formal charge on the atom losing the proton, the more acidic.
2. Atom - more electronegative atom can better stabilize the "-" charge once deprotonated
3. Resonance - more resonance = more stable
4. Inductive Effects - look for any NEARBY electronegative elements to draw/disperse the negative charge
5. Orbital - sp is most s characteristic (50% s, 50% p) meaning its most sphere-like so the negative charges is closer to positive nucleus = more stable than sp2 (33% s, 66% p)

Both a diketone and carboxylic acid would have the same formal charge so negligible. However, diketone loses a proton at the alpha carbon whereas the carboxylic acid loses proton from an OH (oxygen). Oxygen is more electronegative so it can handle the negative charge once deprotonated better -- therefore, carboxylic acid has a lower pKa.

Just found this after I typed it out, it explains the concept better than I can.
http://www.masterorganicchemistry.com/2010/09/22/five-key-factors-that-influence-acidity/

Do you find it easier to compare acidity or basicity when approaching these questions?

 

[DAT Destroyer]

I've heard that in ochem, carboxylic acids are the most acidic of any organic compound. Is this true, or are there exceptions?

Many organic compounds are more acidic than carboxy acids. Protonated ketones, protonated alcohols, protonated nitriles and protonated ethers are much more acidic than carboxy acids and have negative pKa numbers.

Dr. Romano

 

[510586]

Many organic compounds are more acidic than carboxy acids. Protonated ketones, protonated alcohols, protonated nitriles and protonated ethers are much more acidic than carboxy acids and have negative pKa numbers.

Dr. Romano

Thanks! Are there protonated carboxylic acids?

 

[DAT Destroyer]

Thanks! Are there protonated carboxylic acids?

Yes, it is indeed possible to have a protonated carboxy acid, the pKa would be about -6.