# Homework Help: Rank acidity of organic compounds

1. Mar 18, 2014

### get_physical

1. The problem statement, all variables and given/known data
Rank by acidity

2. Relevant equations

Br is lower on the periodic table, hence more acidic. But in the first compound, it is also farthest away from the carboxylic acid, which one gets priority ?

3. The attempt at a solution

I think The one with the bromo group closest to the carboxylic group is most acidic and then next would be the di-fluro, then one fluro and finally, the bromo since it is farthest away from the group? Or does bromo come first before the fluorine?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Mar 18, 2014

### get_physical

3. Mar 18, 2014

### Staff: Mentor

In HX compounds I would agree, but here what matters is the electronegativity - whatever is more electronegative will pull the electrons stronger, lowering electron density on the -COOH group and making the acid stronger.

4. Mar 18, 2014

### get_physical

how do I know that here is electronegativity that matters and not the rule where going down is more acidic?

5. Mar 18, 2014

### Staff: Mentor

Can you list examples where this rule works?

6. Mar 18, 2014

### get_physical

BR-CH2-COOH vs. F-CH2-COOH

Would the one with fluorine be more acidic because it is more electronegative? But I thought when they are in the same column, acidity in the haloacids increases as we move down the column. Does that mean only if we are comparing haloacids on their own without attached to a COOH group.

7. Mar 18, 2014

### Staff: Mentor

I have a feeling you are mixing up several separate trends (each limited to another kind of compounds). As far as I can tell "haloacid" is a name used for halogenated organic (carboxylic) acid, so how can you have haloacid that is "without COOH group"?

Strength of HX acids is HI > HBr > HCl > HF - that's where the acidity grows down the period. But it doesn't mean haloacids will behave the same way, as their acidity depends on completely different effects.

8. Mar 18, 2014

### get_physical

Yes I am quite confused at this moment.

So for Br-CH2-COOH and F-CH2-COOH, which one is more acidic? We said that F-CH2-COOH is more acidic because F is more electronegative, but at the same time, "Strength of HX acids is HI > HBr > HCl > HF "
????

9. Mar 18, 2014

### Staff: Mentor

I would expect fluoroacteic acid to be stronger. But this is just applying rule of thumb, so the reality can be s little bit more complicated.

FCH2COOH is not the same as HF, these are completely different classes of compounds.

10. Mar 18, 2014

### get_physical

Is F-CH2-COOH a haloacid??

11. Mar 18, 2014

### Staff: Mentor

Yes.

But HF is not. It is a hydrogen halide.

12. Mar 18, 2014

### get_physical

If it is a haloacid, then shouldn't the haloacid with Br be more acidic? according to the "Strength of HX acids is HI > HBr > HCl > HF "
???

13. Mar 18, 2014

### Staff: Mentor

These are not haloacids. You have ignored what I wrote earlier:

14. Mar 18, 2014

### get_physical

I know HF is not a haloacid and that it is a hydrogen halide.

but it says ""Strength of HX acids is HI > HBr > HCl > HF " "

Strength of HX acids = haloacids? no??

If so, Can you give me an example where ""Strength of HX acids is HI > HBr > HCl > HF " "

15. Mar 18, 2014

### Yanick

You are confusing the stabilization due to the inductive effect in the haloacid case versus the concept of anion charge density in the case of hydrogen halides.

16. Mar 18, 2014

### get_physical

Yes I am. Could someone please explain it? Thank you

17. Mar 18, 2014

### Staff: Mentor

HX was meant to mean "any molecule in which X is a halogen". So, X can stand for F, Cl, Br or I (or even At), but not anything more complicated.

That's rather common use of the notation, so I thought it will be obvious. Sorry if it wasn't.

18. Mar 18, 2014

### Yanick

I can't just give you an answer but I'll try and help guide you to a, hand-wavy, way to remember this stuff.

Take a look at the anions produced when a proton dissociates from some molecule/atom/ion. You can judge the strength of the conjugate acid by analyzing the stability of the anion. For the hydrogen halide case, you can look at the size of the anions produced, hand-wave a bit and convince yourself that charge density ~ energy. Remember that each halide will have a charge of +1.

For the case of the haloacids, you have to remember that the inductive effect falls off greatly with distance. Neglecting through-space interactions and considering only effects working through bonds, you can get a simple trend going right off the bat. Also consider that the explanation of the inductive effect relies on polarization of sigma bonds due to electronegativity differences. So try to consider these things and see if you can come up with the basic trends.

You can also look up a table of pKa's and compare chloroacetic acid, bromoacetic acid, fluoroacetic acid and the di- and tri- variants etc.

19. Mar 18, 2014

### get_physical

Yes, I know HX means any molecule in which X is a halogen. BUT they said "HX acids" ! so that means haloacids??

20. Mar 18, 2014

### Staff: Mentor

I have no idea what you are talking about. I have explained what I mean by haloacids and what I mean by HX, and I told you you can't use the same rules for both, yet you are still mixing these things. I don't know how to get more precise.

I can only repeat it again: HX is NOT a haloacid.

21. Mar 18, 2014

### get_physical

I know exactly what you are saying that HX is NOT a haloacid. What I am talking about is what you said earlier. You said

Strength of HX acids is HI > HBr > HCl > HF - that's where the acidity grows down the period. But it doesn't mean haloacids will behave the same way, as their acidity depends on completely different effects.

You said strength of HX acids . Doesn't that refer to HALOACIDS? HX ACIDS = HALOACIDS.

Thank you!

22. Mar 18, 2014

### Yanick

get_physical, please refer to my first response to this thread. The links I provided explain the difference between the two terms.

In summary: Haloacid refers to a halogen substituted carboxylic acid such as chloroacetic acid, for example, or more generally RCOOH where the halide will be somewhere in the "R" group. Hydrogen halides, or hydrohaloacids, refer to HX.

23. Mar 19, 2014

### Staff: Mentor

You are just trolling now.

EOT

24. Mar 19, 2014

### get_physical

No,I'm not trolling.

Like I said many times, I know HX is not a haloacid. But you didn't say HX. You said HX acids. That's where I got confused.

You said HX acids, not just HX. Since HX = halogen then I thought HX acids was referring to haloacids.

Anyway, I think I get it now.....