Ranking Acidity Question

[batman09]

So I've attached a picture of the question from Qvault and after reading their explanation, I still don't quite understand why the correct answer is right.

Here's what I was thinking for this question:
-Using Chad's C.A.R.D.I.O mnemonic, I thought answer choice C was correct because it follows the "A" in CARDIO. In other words, it is the largest atom and therefore the weakest base. Because it is the weakest base, it will be the strongest acid. (The rule of size takes precedence over electronegativity right?)

Someone please help!

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[Ogoz]

i believe is more stable to have F rather than I because F is more electronegative. If you think of it this way, F is the most electronegative so its a strong EWG( Electron withdrawing droup). So a strong EWG adjacent to the O- will help stablizing the charge better because is partially pull electron away from O-.

I is a good leaving group if you were to do an SN2 reaction and F will be a bad leaving group in halide.

 

[batman09]

Hmm okay so in other words, when ranking acids and bases of the same molecule but with different halogens attached, electronegativity takes precedence over halogen size?

I have in my notes from Chad that when ranking the acids HF, HCl, HBr, and HI in order increasing acidity, the answer would be HI > HBr > HCl > HF because the I- anion is the biggest (making a stable and therefore weak base) while F- anion is the smallest (making the least stable and therefore strongest base).

Why does the same logic not apply here or do I have my notes from Chad wrong? Orrrr do you go by the context of the molecule? Like in my example from Chad, the halogens were attached to a hydrogen so you didn't need to consider inductive effects while my original question from Qvault had the halogens attached to an OH group, so inductive effects played a part?

Ahhhhhhhhhhhhh lol

 

[ktran17]

The A in Chad's CARDIO mnemonic is applied only when the negative charge of the respective base is on the halogen itself, and therefore the main thing we're focusing on. In this case, size is more important than electronegativity.

However, when applying the inductive effect, electronegativity takes precedent over size. the negative charge isnt on the halogen, but instead on an oxygen.

This problems utilizes the R in CARDIO rather than the A. I hope this makes sense =)

 

[batman09]

Ohh okay I get it now, thanks guys!

 

[Golfer21]

Yeah just use the cardio method and it should make sense then.