organic chemistry help

[RING12]

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which one of the following molecules, has the highest pka, value?

FCH2-COOH

CLCH2-COOH

BR-CH2-COOH

ICH2-COOH

CH3-COOH

I thought it should beF-CHECOOH BC OF INDUCE EFFECT from F, BC ITS MOST ELECTRONEGATIVE ATOM. but the answer is CH3-COOH?

Can some please explain it to me why?

 

[kanudo]

i am not sure about it so if i am wrong some one plz correct me .
larger pka means less acidic and
Electron donating group makes it (less acidic) more basic: so CH3 makes it basic.
As Chad says

"WAS"= " withdrawing groups makes acidic"

 

[RING12]

no you are right, I was just looking at the question again and I understood it now, the question is looking for more basic molecule. Im doing all princeton ochem stuff and Im freaking out, they are really tricky!

 

[MrWilson]

I believe you have it backwards. The lower the pKa, the stronger the acid (and the more stable the conjugate base). In essence, the higher the Ka, the stronger the acid. You're on the right track regarding the answer containing the most electronegative group will be the strongest acid. The answer with the least electronegative group (CH3) will have the highest pKa.

http://en.wikipedia.org/wiki/Acid_dissociation_constant - 4th paragraph