Question

What are acidic H atoms in these molecules?

b) CH3CH2CH(CH3)-C=O-CH3
c) HOCH2-CH2-C=O-C`-=`C-CH3
d) CH3 CH2 C=O C(CH3)=CH2
e) 1-COOCH3-2-CH2Cn benzene
f) COCl-cyclopentyl

Answer 1

Well, I can only go off of the molecules I can understand from what you wrote, which are `(a), (b), (c), (d), (f)`. I assume you want the MOST acidic protons.

Acidic `"H"` atoms in carbonyl compounds are on an `alpha` carbon, and naturally, in alcohols they must be the hydroxyl proton. But examine the `"pKa"`s, then consider the protons.

`a)` 1,3-cyclopentadione

As mentioned, the acidic protons are on the `alpha` carbon, which are adjacent to a carbonyl carbon. In fact, being adjacent to two carbonyl carbons makes it even more acidic!

This is because the carbonyl oxygen(s) are electron-dense electron-withdrawing groups, thus making the `alpha`-carbon electropositive and the `alpha`-proton acidic. In cases where hydroxyl groups are also present, carbonyl oxygens are preferentially protonated.

`b)` 3-methyl-2-pentanone

You can tell you have two choices here. Which proton is more acidic, left (more substituted) or right (not substituted)?

My bet is on the right-hand one, since two alkyl groups are donating electron density into carbon-3 (methyl and ethyl), which promotes the ability of carbon-3 to share electron density with the proton and makes the left-hand proton less acidic.

`c)` 1-hydroxy-4-hexyn-3-one

Well, it's either the `alpha` proton or the hydroxyl proton...

• `alpha` protons on acetone have a `"pKa"` around `20`
• on regular alcohols (e.g. ethanol, methanol, propanol, . . . ), it's typically around `15 - 17`.

We expect further that the carbonyl oxygen withdraws electron density away from the hydroxyl oxygen, weakening the bond with its proton even more.

So, it is likely the hydroxyl proton here that is most acidic. A similar compound here has a comparable `"pKa"` that clearly correlates much better with the hydroxyl proton than with the `alpha` proton.

`d)` 2-methyl-1-penten-2-one

At this point, this one should be pretty obvious. There are no `alpha` protons on the right-hand side at all. The closer the hydrogens are to the carbonyl carbon, the more acidic.

`f)` cyclopentanecarbonyl chloride

This one should now be similarly obvious; it's the only `alpha` proton there.

Try drawing the enolate resonance structure. Where would the added electrophile attach?