Rank the following in order of acidity of the most acidic hydrogens?
#1)# methyl acetoacetate
#2)# dimethyl propanedioate
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
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
Acetylacetone (3) has a
From there, you would have to consider what makes it even more (or less) acidic: an acetyl group (
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 (
Because the hydrogen is made electropositive to a lesser extent, that should decrease the acidity by a little bit, relative to acetylacetone (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).
So indeed, (2) is less acidic than (1).
We've noted the following
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).