Acidity/Basicity

[frab83]

I came across 2 questions testing similar concepts but I think that they're explanations contradict...or I don't fully understand. Please help.

1. Which is the strongest acid of the following?

A. H3CCOOH
B. F3CCOOH
C. Cl3COOH
D. FCl2COOH

ANS: B. Electronegative atoms increase acidity through the inductive effect. Since flourine is more electronegative than chlorine (and hydrogen), choice B is the strongest acid.

2. Which is the strongest acid of the following?

A. HF
B. HBr
C. HI
D. H2O\

ANS. C. Water is less acidic than HF, since flourine is more electronegative than oxygen (eliminating choice D). Of the three hydrogen halides listed, HI is the most acidic since IODINE IS LARGER THAN BROMINE AND FLOURINE (increasing the size of the halogen atom makes the compound more acidic).

Heres my concern...Based on Explanation for Q. 2, shouldnt the answer to 1 be C. Cl3COOH??? since Cl is bigger in size than Fl?

...TOTALLY CONFUSED PLEASE HELP.

[admin]

Good question.

These concepts must be kept clearly in 2 different departments: general (inorganic) chemistry and organic chemistry.

Just so you get an overview: acids in organic chemistry (specifically, carboxylic acids like the ones in the first question) are no where near as acidic as the acid halides in the second question. In fact HCl is many millions of times more acidic than a carboxylic acid.

Note that the halides in the case of the carboxylic acids are not attached to hydrogen (this is the key difference). Thus they have an "inductive" effect that relies solely on their electronegativity. In other words, since they are so removed from the point of action (where the H will dissociate) then size does not matter.

You can read more in the Gold Standard MCAT or here is a helpful link: http://www.vias.org/genchem/acidbase_equ_12591_06.html

[frab83]

So in the first one there is an inductive effect due to the electronegative "substituent" on the carboxylic acid. But on the second one, we just base the acidity level on the size of the electron cloud. I understand it more now. There is no inductive effect occurring in Q2 because the atom is directly bonded to the Hydrogen, whereas inductive effect occurs in Q1 since the electronegative atom is bonded to the carboxylic acid.

I think I understand it more now.

Thank you.

[admin]

Sounds like you've got it.

Glad to help!