O. Chem as Second Language Acidity Problem

[hawkeye712]

The problem ask to predict which compound is more acidic. The answer is the second one.

Can anyone explain why?

 

[Xo1991]

Think about it in terms of conjugate base.

First, remove the proton H from both molecules and make the Oxygen negative charge (i'm sure you know this part but just to be thorough)

so you have a negative charge on oxygen, and you also know that the stronger acid has the weaker conjugate base (more stable base). comparing fluorine and chlorine, which one is more electronegative? Fluorine. So, fluorine will withdraw electrons from the negative charge on oxygen, which stabilizes the negative charge. Since the conjugate base is more stable, the acid with the fluorine is a stronger acid.

 

[hurley]

.

 

[hawkeye712]

Wouldn't you look at the size difference first? Chad said that size is a more important factor than electronegativity. Base on this, chlorine is larger than fluorine so it can stabilize the conjugate base better, making the first compound more acidic.

 

[athomas]

Size would be more important if it was just Cl- and F- floating around, but it's electronegativity that matters when you are talking about stabilizing through induction.

 

[hawkeye712]

Size would be more important if it was just Cl- and F- floating around, but it's electronegativity that matters when you are talking about stabilizing through induction.

Great! Thanks for your help everyone.