Question About Pka of Propanol vs. Butanone (Dat Qvault ochem question)

[chouwee]

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Can somebody please explain to me why Butanone (pka = 14.7) is less acidic than Propanol (pka= 16)? I thought since in Propanol the negative charge resides on the oxygen, thus make it a stronger base, meaning weaker acid. The negative charge on Butanone would reside on the alpha carbon making it a weaker base than Propanol. Am i getting something mixed up?

 

[gooperwooper]

the lower the pka the stronger the acid right?

 

[chouwee]

Yes but the pka values were not given in the question. I had to research them. So you had to use the CARDIO method.

 

[Lnguyen7]

Basicity increases from left to right of the periodic table, so CH3- > OH- in basicity. Your pka values show butanone is more acidic than propanol somehow.

 

[Frank22]

Can somebody please explain to me why Butanone (pka = 14.7) is less acidic than Propanol (pka= 16)? I thought since in Propanol the negative charge resides on the oxygen, thus make it a stronger base, meaning weaker acid. The negative charge on Butanone would reside on the alpha carbon making it a weaker base than Propanol. Am i getting something mixed up?

I believe you meant to ask why is butanone more acidic than propanol, but great question nonetheless.

The oxygen on a carbonyl group is highly electronegative and therefore withdraws significant electron density from the carbon to which it is double-bonded; this in turn makes the neighboring alpha carbons more acidic than they would otherwise be. The carboanion that would result from the deprotonation of one of butanone's alpha carbons would be resonance stabilized by the neighboring oxygen. On the other hand, when propanol is deprotonated, the oxygen also possesses the negative charge, but there is no resonance stabilization. Because the conjugate base of butanone is more stable (i.e., less reactive or weaker) than the conjugate base of propanol, it is also more acidic.

Recall also that primary carboanions are more stable than secondary carboanions; as a result, a base would most likely deprotonate the methyl group of a butanone molecule rather than ethyl group.

 

[Lnguyen7]

I believe you meant to ask why is butanone more acidic than propanol, but great question nonetheless.

The oxygen on a carbonyl group is highly electronegative and therefore withdraws significant electron density from the carbon to which it is double-bonded; this in turn makes the neighboring alpha carbons more acidic than they would otherwise be. The carboanion that would result from the deprotonation of one of butanone's alpha carbon's would be resonance stabilized by the neighboring oxygen. On the other hand, when propanol is deprotonated, the oxygen also possesses the negative charge, but there is no resonance stabilization. Because the conjugate base of butanone is more stable (i.e., less reactive or weaker) than the conjugate base of propanol, it is also more acidic.

Recall also that primary carboanions are more stable than secondary carboanions; as a result, a base would most likely deprotonate the methyl group of a butanone molecule rather than ethyl group.

While resonance contributes to the acidity of a molecule, atom comes before resonance using CARDIO. If resonance is used to explain why butanone is more acidic than propanol, it cannot be used to explain why acetone is less acidic than propanol. There must be some combined factors that influence the acidity of butanone here. Perhaps because there are two alpha carbons that can be deprotonated?

 

[Frank22]

While resonance contributes to the acidity of a molecule, atom comes before resonance using CARDIO. If resonance is used to explain why butanone is more acidic than propanol, it cannot be used to explain why acetone is less acidic than propanol. There must be some combined factors that influence the acidity of butanone here. Perhaps because there are two alpha carbons that can be deprotonated?

Actually in both the deprotonated forms of propanol and butanone, the negative charge is on the oxygen. The difference is that butanone has some resonance stabilization with the neighboring alpha carbon, while propanol has no delocalization among its electrons.

EDIT: also note that while CARDIO has significant utility in determining the basicity of of a given molecule, it should by no means be regarded as an infallible algorithm for that purpose. For example, carboxylic acids are less basic than ammonium ions, even though the former is neutral while the latter is charged; a patent violation of the first rule of CARDIO.

 

[Lnguyen7]

Actually in both the deprotonated forms of propanol and butanone, the negative charge is on the oxygen. The difference is that butanone has some resonance stabilization with the neighboring alpha carbon, while propanol has no delocalization among its electrons.

How would you explain why acetone is less acidic than propanol?

 

[Frank22]

How would you explain why acetone is less acidic than propanol?

Ketones with different groups bonded to the alpha carbon can have varying pKas; the more electron withdrawing one group bonded to the ketone is, the more acidic will be the alpha hydrogen on the other carbon. For example, methyl phenyl ketone has an alpha hydrogen on the methyl group with a pKa of approximately 16.5 because the phenyl group enhances the electron withdrawing capacity of the carbonyl group, which is more acidic than 2-propanol, which has a pKa of approximately 17.1.

It also depends on the base that is used; an alkoxide ion will favor the thermodynamic product, i.e., deprotonate the ethyl group of the ketone, while a strong base like LDA will favor the least sterically hindered alpha carbon (i.e., the methyl group). In that case, acetone may be more readily deprotonated than 2-butanone if a strong base like LDA is used, while 2-butanone will be more readily deprotonated if an acid like sodium methoxide is used.

 

[Lnguyen7]

Ketones with different groups bonded to the alpha carbon can have varying pKas; the more electron withdrawing one group bonded to the ketone is, the more acidic will be the alpha hydrogen on the other carbon. For example, methyl phenyl ketone has an alpha hydrogen on the methyl group with a pKa of approximately 16.5 because the phenyl group enhances the electron withdrawing capacity of the carbonyl group, which is more acidic than 2-propanol, which has a pKa of approximately 17.1.

It also depends on the base that is used; an alkoxide ion will favor the thermodynamic product, i.e., deprotonate the ethyl group of the ketone, while a strong base like LDA will favor the least sterically hindered alpha carbon (i.e., the methyl group). In that case, acetone may be more readily deprotonated than 2-butanone if a strong base like LDA is used, while 2-butanone will be more readily deprotonated if an acid like sodium methoxide is used.

I agree with what you said, but resonance can't still explain why acetone is less acidic than propanol. There are just quite a few exceptions. I also saw your edit in previous post, no doubt about what you wrote there.