Question

Rank the following in order of acidity of the most acidic hydrogens?

`1)` methyl acetoacetate
`2)` dimethyl propanedioate
`3)` acetylacetone
`4)` 2-propanone

Is it `(3)`?

Answer 1

Well, the correct answer is the one you chose first (or had the answer to). :-)

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Good question - you're dealing with:

1) methyl acetoacetate
2) dimethyl propanedioate
3) acetylacetone
4) 2-propanone

COMPARING (4) WITH (3)

Now, the most acidic hydrogen would be the central hydrogen in the dicarbonyl compounds because it is closest to both carbonyl oxygens, which are electron-withdrawing groups.

So, you know it's not 2-propanone (4) that's the most acidic; it only has its 'end' hydrogens; each one's `"pKa"` is around `\mathbf(color(blue)(20))`, which is fairly high, and it is only a monocarbonyl compound.

Acetylacetone (3) has a `"pKa"` of around `\mathbf(color(blue)(9))`, so that's a good possibility right now.

From there, you would have to consider what makes it even more (or less) acidic: an acetyl group (`"H"_3"C"("C"="O")-`) or an acetoxy group (`"H"_3"C"-"O"-("C"="O")-`).

So, consider how the increasing ester character instead of ketone character changes things. This means comparing (1) with (3) and (2) with (1).

COMPARING (1) WITH (3)

A methoxy group (`"CH"_3"O"-`) is electron-donating, so I'd expect an acetoxy group's carbonyl oxygen to withdraw more electron density from the methoxy oxygen than from the central hydrogen.

Because the hydrogen is made electropositive to a lesser extent, that should decrease the acidity by a little bit, relative to acetylacetone (3).

The actual `"pKa"` of methyl acetoacetate (1) is around `\mathbf(color(blue)(11))` (that of ethyl acetoacetate, instead of methyl acetoacetate, is `10.68`). That means it is less acidic indeed than (3).

COMPARING (2) WITH (1)

Since dimethyl propanedioate (2) has one more acetoxy group than methyl acetoacetate, I would predict based on comparing (1) with (3) that (2) is less acidic than (1).

Actually, the `"pKa"` of (2) is about `\mathbf(color(blue)(11.80))` according to SciFinder:

So indeed, (2) is less acidic than (1).

CONCLUSIONS

We've noted the following `"pKa"`s:

• (1): `~~11`
• (2): `~~11.80`
• (3): `~~9`
• (4): `~~20`

That means the acidities of the compounds above are ordered from most acidic to least acidic as follows:

`color(blue)((3) > (1) > (2) > (4))`

which correlates with the trend that:

• More carbonyl groups around a `"C"-"H"` makes that hydrogen significantly more acidic. Therefore, it makes sense that (4) is the least acidic.
• Replacing a ketone group with an ester group slightly decreases the acidity. (2) has one ester group more than (1), and (1) one more than (3).