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Explain this chem question on TPR FL test?

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sfsn

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"Ionic liquids are made of large organic cations and inorganic anions and exist in the liquid phase at room temperature. Even weakly basic anions can deprotonate substrates in these highly concentrated ionic liquids, causing unwanted side reactions. Which anion would be the least likely to produce unwanted side reactions in an ionic liquid?

a) BF4-
B) AlCl4-
C) SO42-
D) SbF6-
upload_2016-3-31_18-52-11.png

(H0 = Hammett function = a measure of acidity that is used for very concentrated solutions of strong acids, including superacids)

The explanation is:
D. The question is asking for the weakest base. The weakest basic anion will be the conjugate of the strongest acid. Since the answer will come from Table 1 and the question is asking for an extreme, the middle value of the variable in choice A can be eliminated. All superacids are stronger than pure H2SO4, eliminating choice C. SbF6– is the conjugate of SbF5/HF, which is the strongest acid because it has the lowest Ho value. Therefore, choice B is eliminated and choice D is the correct answer.

Why does it say, "The question is asking for the weakest base"?

The question stem asks, "Which anion would be the least likely to produce unwanted side reactions in an ionic liquid?"
It also says that "weakly basic anions can deprotonate substrates...causing unwanted side reactions."

Wouldn't you want to avoid a weak base (and thus strongest acid) if you're trying to avoid unwanted side reactions?

Admittedly I'm weak on acid/bases. Can anyone explain this to me? Thank you!
 

aldol16

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You want something that produces the fewest unwanted side reactions. In other words, you don't want something that is really basic. That's because as it says, bases create side reactions by deprotonating things. So you want something that's the weakest base. The table should tell you which one out of those is the weakest base. I'm frankly surprised that they gave you a Hammett table. We usually don't teach the Hammett postulates until Phys Org in grad school.
 

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Look for the conjugate base of the strongest acid.
Plus you could also eliminate B and C because they contain oxygen and chlorine which both (due to atomic radius and electronegativity) are more likely to share their valence e- for chemical rxns. Leaving you with the two options that have Fl. But yeah, the conjugate base of the strongest acid is by definition the overall weakest base.
 

sfsn

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Oh I see, so it's saying that a weak base would deprotonate substrates, and a strong base would deprotonate even more/to a greater degree. So it's best to go with a weaker base (stronger acid) in order to avoid unwanted side reactions.

Now I realize that the underlying concept being tested here is the very basic (pun intended?) understanding of the function of a base (and how a stronger base would deprotonate more compared to a weaker base). And then connecting this understanding to the consequence that they presented (side rxns). This was not complicated at all; I wish I could reach this level of clarity in a few seconds during the pressure of the testing environment...

Just for completeness, here's the whole passage. They do explain enough of the Hammett function for the test.

upload_2016-3-31_20-7-39.png

Thanks for your help!
 

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A bronsted based is a substance that picks up proton. Look at the pic in equation 2. Rather than pick up the proton, SbF5 INSTEAD picked up the F atom. That also points to it being very weakly basic.
 
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