# 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 $\left(3\right)$?

##### 1 Answer
Aug 11, 2016

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

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 $\text{pKa}$ is around $\setminus m a t h b f \left(\textcolor{b l u e}{20}\right)$, which is fairly high, and it is only a monocarbonyl compound.

Acetylacetone (3) has a $\text{pKa}$ of around $\setminus m a t h b f \left(\textcolor{b l u e}{9}\right)$, 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 ($\text{CH"_3"O} -$) 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 $\text{pKa}$ of methyl acetoacetate (1) is around $\setminus m a t h b f \left(\textcolor{b l u e}{11}\right)$ (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 $\text{pKa}$ of (2) is about $\setminus m a t h b f \left(\textcolor{b l u e}{11.80}\right)$ according to SciFinder:

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

CONCLUSIONS

We've noted the following $\text{pKa}$s:

• (1): $\approx 11$
• (2): $\approx 11.80$
• (3): $\approx 9$
• (4): $\approx 20$

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

$\textcolor{b l u e}{\left(3\right) > \left(1\right) > \left(2\right) > \left(4\right)}$

which correlates with the trend that:

• More carbonyl groups around a $\text{C"-"H}$ 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).