Kaplan Practice Test Question-Acidity

[Sally0627]

Advertisement - Members don't see this ad

Practice Test I on Kaplan #82. It asks about which is more acidic with COOH on a benzene and the options are Cl, NH2,Br,F,I.
I had picked I but it was the incorrect answer and they said F is more acidic than I.
Which one is right? I thought acidity increases going down for the halogens.

 

[RSD2014]

Practice Test I on Kaplan #82. It asks about which is more acidic with COOH on a benzene and the options are Cl, NH2,Br,F,I.
I had picked I but it was the incorrect answer and they said F is more acidic than I.
Which one is right? I thought acidity increases going down for the halogens.

The halogens are Electron withdrawing groups. The more Electronegative the Halogen, the more it draws electrons to itself and make the Acid more acidic. We are not looking at Hydro-ic acids, such as HI>HBr>HCl>HF In this case, Acidity does increase going down the column.

 

[Kahr]

The halogens are Electron withdrawing groups. The more Electronegative the Halogen, the more it draws electrons to itself and make the Acid more acidic. We are not looking at Hydro-ic acids, such as HI>HBr>HCl>HF In this case, Acidity does increase going down the column.

Ahh, thank you for elucidating, I was thinking the same thing, but wasn't certain based on the wording whether it was just that one of the halogens was attached to the benzoic acid, or whether it was somehow interacting with it as a separate reactant.

 

[Sally0627]

Yes, thank you sooo much! I've been getting these questions wrong. Now i know why. thanks alot!!

 

[YakovY]

It has to do with charge delocalization - a more electron-negative atom on the benzene ring will be a better at spreading the negative charge that's left when the H+ is abstracted.