OChem question about Acidity

[jlt0530]

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Can you guys arrange formic acid, benzoic acid and acetic acid in order of increasing acidity? I know formic acid is the most acidic one b/c it doesn't have any EDG attached to the carbonyl carbon unlike the other two. But, between acetic acid and benzoic acid, which one is more acidic?

I know EWG increases acidity and EDG decreases acidity, and since methyl group and benzyl group are both EDG (right?), they are less acidic than formic acid. So, I'm guessing that I suppose to compare their electron donating ability, but at the same time, I thought i should also think of their resonance effect, and there, i'm confused. Can anyone explain this clearly?

Any help would be appreciated!! 🙂

 

[nze82]

Any group that donates electron to C=O decreases the acidic strength of the compound. Methyl group has a greater electron donating ability than the benzene ring, which in turn has a greater electron donating ability than the H of formic acid. So the acidic trend goes like:

Most Acidic: Formic Acid> Benzoic Acid> Acetic Acid :Least Acidic

 

[pbure9]

^ don't aromatics increase acidity by inductive effect or am I wrong?
So wouldn't benzoic acid be more acidic than acetic acid since the CH3- on acetic acid actually destablizes the anion?

 

[nze82]

^ don't aromatics increase acidity by inductive effect or am I wrong?

No you're right! The order of acidity you wrote is correct. I got confused when writing the first post. So, I had to revise it.

 

[pbure9]

No you're right! The order of acidity you wrote is correct. I got confused when writing the first post. So, I had to revise it.

lol, yea i checked the pka's just now and its pretty close. But I know that as a rule of thumb, aromaticity increases acidity.

 

[achytooth]

Also, you can ingest acetic acid .. (ie vinegarette dressing)

 

[jlt0530]

Aromaticity increases acidity..alright.. Thanks guys!! 👍

But what if we compare benzoic acid vs. benzoic acid-like acid that has another methyl group attached to the benzyl group. (Can you picture what i'm talking about? something like aniline is attached to the carbonyl carbon in ortho position) Then, since there is additional electron donating effect, bezoic acid alone would be more acidic, right? Trying to nail this thing down! Thanks for all of you guys' input!

 

[achytooth]

Aromaticity increases acidity..alright.. Thanks guys!! 👍

But what if we compare benzoic acid vs. benzoic acid-like acid that has another methyl group attached to the benzyl group. (Can you picture what i'm talking about? something like aniline is attached to the carbonyl carbon in ortho position) Then, since there is additional electron donating effect, bezoic acid alone would be more acidic, right? Trying to nail this thing down! Thanks for all of you guys' input!

that's where your electron withdrawing and electron donating stuff you were talking about comes in so yes, I believe you've got that part.

ortho substituent is stronger then para in Ar's I believe

 

[nze82]

Aromaticity increases acidity..alright.. Thanks guys!! 👍

But what if we compare benzoic acid vs. benzoic acid-like acid that has another methyl group attached to the benzyl group. (Can you picture what i'm talking about? something like aniline is attached to the carbonyl carbon in ortho position) Then, since there is additional electron donating effect, bezoic acid alone would be more acidic, right? Trying to nail this thing down! Thanks for all of you guys' input!

Now you're pushing it!

 

[jlt0530]

Now you're pushing it!

lol u know there is never enough for studying DAT!
Thanks for all of your help!