[student4life21]

http://img713.imageshack.us/img713/2508/molecule.png

The molecule with the F is apparently more acidic. However, I learned in class that because I is a bigger molecule, there is a weak bond strength which leads to I being more acidic than F. For example, H-I is a stronger acid than H-F. But how is it in this one the one with the F is a stronger acid.

[Doc Oc]

Read up on inductive effect.

[student4life21]

i understand induction, but apparently bond strength seems to outweigh induction because H-I is a stronger acid than H-F

[plankk]

But in this case you will not destroy these bonds. You only need to know which is more willing for electrons. And the flourine is indeed. It has the same charge as iodine but decidely less protons and it is much smaller, so it has great tendency to inductive effect.

[orgopete]

Oh, oh, a student reading and thinking. That is a dangerous combination.

Prediction, student is going to discover that the textbook contradicts itself.

Okay, lets play. If HI is a stronger acid than HF, which is a stronger acid CH₄ (109 pm) or HF (92 pm).

If fluorine is more electron withdrawing, then which is more dense, F₂ or I₂? Hint, electrons occupy the greatest volume of an atom. In which atom are the electrons the furthest from the nucleus? (This is a big hint.)

I cannot say with certainty, but of all the compounds that one can examine. The fluoroacetic acids are the one case that fluorine is the stronger electron withdrawing group. If you can look at the halomethanes, trihaloethanol, substituted phenols, benzoic acids, anilines, etc., and as electron withdrawing effects go, it is I~Br>Cl>F. Because of this, we must focus on fluoroacetic acid. It would be confusing to report that HCX₃ acidity decreases in this order: I>Br>Cl>F.

I argue that it will be simpler if as electron withdrawing groups are concerned, I>Br>Cl>>F and F>O>N>C. Then density, acidity, and leaving group ability follow. (Carbon can even be a better electron donor.) It is an application of the same principle. The contradictions will now largely disappear (except for why is fluoroacetic acid a stronger acid than chloroacetic acid?).

I have written a reasonably compatible explanation in A Handbook of Organic Chemistry Mechanisms. (You don't need to read it to understand the idea. You can easily think it up on your own. It just takes nerve. Because the textbooks tell you that fluorine is the most electron withdrawing group or that it has the most ionic character, predicting the reactions of methyl iodide, methyl fluoride, t-butyl iodide and t-butyl fluoride will incorrectly predict the result based upon principle. If you reject the principle being suggested, then you will be on the right track. Organic chemist know that iodine is more dense, and iodide is a better electron withdrawing group. This is only a problem if someone mentions "electroneg#%$@#*&". I tried to avoid using that word to avoid confusion with it.)