TBR vs. AAMC Acidity Trends

[missdoctor]

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TBR and AAMC reasoning for acidity trends (within the periodic table) seem to be contradicting each other.

TBR says:

In order to determine acid strength-

When comparing within in a column only:

Atomic size matters more than electronegativity. As you go down a column, acidity inreases. And they say larger molecules make stronger acids because the conjugate bases of these acids are large and their negative charge is spread out more, making it harder for protons to be accepted. (So with this logic the larger acids are stronger because their conjugate base are more stable).

When comparing within a row only:

Electronegativity is most significant, so from left to right acidity increases across a row.


With that said, I was doing problems in the AAMC Official Guide to the MCAT Exam, and one question was:

Which of the following carboxylic acids will have the lowest pKa?

A. 2-Methylbutyric acid
B. 3-Chlorobutyric acid
C. 2-Ethylbutyric acid
D. 3-Bromobutyric acid

With TBR's reasoning, I would have said D. because comparing Br and Cl which are in the same column, size matters more then electronegativity. But AAMC said Cl because it is more electronegative. Do TBR's trends not apply to this specific problem for some reason, or is electronegativity ALWAYS the most significant factor for determining acid strength?

Thanks for any help!

 

[starbaduk]

I think: in binary acids like HF, HCl, and HBr, the atomic size of the conjugate base is important. Comparing HF and HI for example, after they donate proton, they would become F- and I-.

F- is smaller, so its negative charge would be less evenly distributed -> more unstable. I- is bigger, so it's more stable. So HI doesn't really mind losing a proton, making it a stronger acid.

You can also argue that since F is more electronegative, so F- doesn't mind being -1. But empirically, this isn't the case. So we say atomic size and bond strength matters more than electronegativity (except within the same row)

But when considering acidity between 3-Chlorobutyric acid and 3-Bromobutyric acid, you are considering acidity of hydrogen on the carboxylic group. Neither Cl or Br will gain a negative charge. So I don't think atomic sizes of these two halogens are relevant. But since Cl is more electronegative, it will withdraw electrons from the alph-carbon more strongly => Alpha carbon will become even more + => due to inductive effect carbonyl carbon will become more + => negative charge on the carboxylic group will be better stabilized, making this a stronger acid

 

[Rabolisk]

The TBr reasoning does not apply in this case. The trends apply when The acidic proton is bonded to the element in question. Then the conjugate base will be this element with a negative charge. However, in the aforementioned problem, the proton is bonded to an oxygen in all cases. Now a more electronegative substituent on the molecule withdraws electrons away from the oxygen, thus delocalizing the charge and making it, the conjugate base more stable, and the acid stronger. If what I said doesn't make sense to you, I would review acidity and resonance/inductive effects.

 

[missdoctor]

Ohhh okay I understand now. That's right those trends are applying to acidic protons in particular. Thanks so much for clearing that up!