Most Stable Acid

[Donald Kimball]

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I understand that electron withdrawing groups tend to make a compound more acidic, but I'm confused when it comes to elements within the same group. Greater electronegativity helps stabilize the negative charge on the resulting anion, but larger molecules are better able to distribute the negative charge.

For example, Which of the following alkoxides is the most stable?

a. Cl3CCH2O-
b. F3CCH2O-
c. (CH3)3CCH2O-
d. CH3CH2O-

The correct answer is b owning to fluorine's electronegativity. However, chlorine is more massive than fluorine. So, what's the deal here?

 

[LoLCareerGoals]

Greater size helps with polarazability. This is helpful when stabilizing transition states (like in SN2 reaction) where an element has to be polarizable while still partially attached to the parent compound. Stability as a separated anion is about electronegativity.

 

[Temperature101]

I understand that electron withdrawing groups tend to make a compound more acidic, but I'm confused when it comes to elements within the same group. Greater electronegativity helps stabilize the negative charge on the resulting anion, but larger molecules are better able to distribute the negative charge.

For example, Which of the following alkoxides is the most stable?

a. Cl3CCH2O-
b. F3CCH2O-
c. (CH3)3CCH2O-
d. CH3CH2O-

The correct answer is b owning to fluorine's electronegativity. However, chlorine is more massive than fluorine. So, what's the deal here?

Do they mean most acidic? If yes,...Definitely B is the answer because of electronegativity....Can someone explain why they use the words "most stable"?

 

[Donald Kimball]

If electronegativity were the only consideration, wouldn't HF be the strongest acid? However, despite flourine's significant electronegativity, F- is less stable than I-. Consequently, wouldn't Cl3CCH2O- be more stable than F3CCH2O-?

 

[LoLCareerGoals]

Where did you see that F- is less stable than I-?
Also we aren't told how these ions were formed, so I don't think you can use arguments that involve conjugate acids.
Sure HF is weaker and thus dissociates worse than HI, but we aren't given any info as to wether these ions are in equilibrium with conj acids or anything like that.

Btw: Binary acid trend you are describing is reversed for oxyacids! So F containing oxyacid is the strongest. I am not sure these weird ass ions are oxyacids' anions, but just giving you a counterexample here because using binary acids as a counterargument does not capture all the factors.

 

[Temperature101]

If electronegativity were the only consideration, wouldn't HF be the strongest acid? However, despite flourine's significant electronegativity, F- is less stable than I-. Consequently, wouldn't Cl3CCH2O- be more stable than F3CCH2O-?

For these type of acid (binary acid HF, HCl, HBr, HI), Atomic radius dictate acidity....Greater atomic radius( weaker bond) makes the acid dissociate in water more easily...

 

[Donald Kimball]

Ah, I see. Thanks guys. I didn't consider that a different trend might hold for non-binary acids.

 

[LoLCareerGoals]

I have a question myself actually (I am not far enough into orgo): Are alkoxides conjugate bases of oxyacids? If not then what class of acids are they conjugate bases off (if any) ?

 

[Donald Kimball]

Alkoxides are conjugate bases of alcohols. E.g., methoxide (CH3O-) is the alkoxide of methanol (CH3OH).