laczlacylaci

2+ Year Member
Jun 20, 2016
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Cheese State
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Pre-Medical
upload_2016-8-26_10-54-55.png
Increasing pKA=decreasing acidity. this tests the inductive effect.
The more electronegative the atom=the more it makes the overall compound want to donate a proton=strong acid. So the compound with the F will be most acidic. O is second most electronegative so 2 and Br is less electronegative than O. So F>O>B and least acidic C.

Applying this to HF, HBr, H2O, HI, H2S we would instead do:
HI>HBr>HF>H2S>H2O
Comparing down a group, the bigger the atom, the stronger the acid. (HI, HBr, HF)

Comparing across a period (HF and H2O), the more electronegative, the more acidic, correct?
 

TheLongGame

2+ Year Member
Sep 4, 2016
68
49
Status
Medical Student
Yes, conceptually you are correct. For the purposes of the MCAT, stick to what you have.

In reality...
The correct order is

2-fluoropropanoic, 2-bromopropanoic, methoxyacetic, propanoic... This answer choice isn't available, but it is indeed correct.

The pKa of the above compounds in order:

2.68, 2.97, 3.54, 4.87