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)# ?
Is it
1 Answer
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
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).
The actual
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
So indeed, (2) is less acidic than (1).
CONCLUSIONS
We've noted the following
- (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).