DAT destroyer, regarding alpha hydrogen deprotonation

[zellyen]

i've always learned in rxns like aldol and claisen in class that the alpha hydrogens are deprotonated by acids like NaOH or NaOCH3 and the like, but DAT destroyer says that a (dilute) acid can also deprotonate the alpha hydrogens of a ketone? this is number 7 and 8 in the book. please advice

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[solstice]

1st NaOH and NaOCH3 are not acids! These are bases. Any base that is stronger than your object molecule will deprotonate it. The alpha-H of Keton and aldehyde is acidic enough for the base to remove it and when this happens they go under Aldol condensation RxNs where the newly created c: will attack the carbonyl of another Ke/Ald.

I do not have the book, so I hope I underestood your question right.

 

[zellyen]

i know those are bases but it stats in the books that the acids will DEprotonate too

 

[solstice]

it is impossible for the Acid stronger than your molecule to deprotonate. I do not have the book. State the exact sentense.

 

[wantVCUdental]

1st NaOH and NaOCH3 are not acids! These are bases. Any base that is stronger than your object molecule will deprotonate it. The alpha-H of Keton and aldehyde is acidic enough for the base to remove it and when this happens they go under Aldol condensation RxNs where the newly created c: will attack the carbonyl of another Ke/Ald.

I do not have the book, so I hope I underestood your question right.

i agree! it's common sense, perhaps DAT destroyer is wrong?
and if anything, NaOH is a strong base

 

[zellyen]

propanal is an aldehyde with an alpha hydrogen, when in dilute acid or base, do the Aldo Condensation

 

[sixkiller]

propanal is an aldehyde with an alpha hydrogen, when in dilute acid or base, do the Aldo Condensation

It's not commonly done, BUT....

aldol condensation CAN occur in dilute acid.

Protonate propanal at the oxygen....that's your electrophile.

Recall the mechanism for acid catalyzed enolization.

The double bond of the enol essentially acts as an electron donor.

Draw a propanal protonated at the oxygen and the enol tautomer of propanal. Push the electrons in the appropriate manner to generate the condensation product....

If you still can't "see" it...look it up.

 

[sixkiller]

If you push the electron in the appropriate manner, the hydroxyl carbon of the enol yields a carbocation (stabilized by the electron donating resonance effect of the oxygen) AFTER the double bond donates its electrons to the carbonyl carbon of the aldehyde.

Then...don't forget that the acid is regenerated...

 

[solstice]

propanal is an aldehyde with an alpha hydrogen, when in dilute acid or base, do the Aldo Condensation

Now I know what you are talking about. Read this statement carefully. It does not say that acid will remove the alpha-H, this never happens if your acid is stronger than your electrophile. It says that you get aldol condensation if you have the catalysts (acid or base) to either prepare your enolate ion or make one molecule a better electrophile.