Acidity of protons on alpha carbons

[thechairman]

Say you have a ketone that has a methyl on one side, and an ethyl group on the other side. Which alpha hydrogens are more acidic (which more likely to be deprotonated by base)? The methyl hydrogens, or the alpha hydrogens on the ethyl group?

i ask this because there is a question that asks you to predict the major product of a crossed aldol condensation.

i've always thought primary carbanions were the most stable, compared to secondary. but the answer has the formation of the secondary carbanion (the alpha hydrogen on the ethyl group gets abstracted) as the nucleophile to attack the carbonyl.

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[SeventhSon]

Say you have a ketone that has a methyl on one side, and an ethyl group on the other side. Which alpha hydrogens are more acidic (which more likely to be deprotonated by base)? The methyl hydrogens, or the alpha hydrogens on the ethyl group?

i ask this because there is a question that asks you to predict the major product of a crossed aldol condensation.

i've always thought primary carbanions were the most stable, compared to secondary. but the answer has the formation of the secondary carbanion (the alpha hydrogen on the ethyl group gets abstracted) as the nucleophile to attack the carbonyl.

you may have to consider sterics of the actual nucleophile and electrophile in determining the most kinetically favored product.

All carbanions are very unstable compared to the other configuration of the compound, but this effect is possibly is less important than the fact that the more substituted compound has more electron density at the carbanion and thus is the stronger nucleophile, causing this configuration to lead to the dominant product.

Hope someone else can offer some help, ochem is not my forte

 

[Psychotropic]

Carbanion stability is the opposite of what you believe. Tert>Sec>Primary. The sp3 bonds of the adjacent methyl groups of the carbanions help spread some of the charge of the anion, making deprotonation at a more hindered position more favorable. Think of SN1 reactivity.

 

[thechairman]

Carbanion stability is the opposite of what you believe. Tert>Sec>Primary. The sp3 bonds of the adjacent methyl groups of the carbanions help spread some of the charge of the anion, making deprotonation at a more hindered position more favorable. Think of SN1 reactivity.

so in this case you are saying don't think methyl groups in terms of electron donating or withdrawing, but whether or not their size distributes the negative charge?

 

[Psychotropic]

so in this case you are saying don't think methyl groups in terms of electron donating or withdrawing, but whether or not their size distributes the negative charge?

So when determining acidity you should keep in mind:

1.) Resonance

2.) Electron donating/withdrawing nature of atom bonded to the proton (hydrogen) of interest

3.) Adjacent Groups


Point 3 is key to the acidity of alpha carbons. In this case, the resonance is the same in both cases, and the hydrogens of interest are both bonded to carbons. The difference lies in each carbons ability to spread the negative charge. By having a methyl group adjacent to the alpha carbon group, upon deprotonation, the negative charge can be "spread" to adjacent sp3 orbitals. This is getting complex and perhaps is too much to think about for the MCAT. Just know tert>sec>primary. The above reasoning is the specifics as to why. I highly doubt anything on the MCAT will get this complicated.

 

[quiksilver87]

yeah, but wouldnt the methyl group beside the alpha protons donate electrons (inductive effect) to the carbanion? Thus, making the methyl group protons on the other side more favorable to be removed?

 

[fileserver]

Carbanion stability is the opposite of what you believe. Tert>Sec>Primary. The sp3 bonds of the adjacent methyl groups of the carbanions help spread some of the charge of the anion, making deprotonation at a more hindered position more favorable. Think of SN1 reactivity.

In general,
Stability for radical and carbocation: tert>sec>primary
Stability for Carbanion: primary>sec>tert

However, when you extract the alpha proton, you will form a negative charge on the alpha carbon on either side. Then, because O maintain the charge better then C, the molecule form an enolate (the dominate form). Then, we consider their alkene stability. That is why alpha proton on the ethyl is better to extract. Sorry for the bad command in English.

 

[Psychotropic]

In general,
Stability for radical and carbocation: tert>sec>primary
Stability for Carbanion: primary>sec>tert

However, when you extract the alpha proton, you will form a negative charge on the alpha carbon on either side. Then, because O maintain the charge better then C, the molecule form an enolate (the dominate form). Then, we consider their alkene stability. That is why alpha proton on the ethyl is better to extract. Sorry for the bad command in English.

This is correct, I apologize for my explanation. I confused carbocation/carbanion stability.