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Why do we deprotonate that specific hydrogen as opposed to any other one? What information does the table on the right give us? It seems unrelated to citric acid - where does the fluorine come from?

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    $\begingroup$ Welcome to Chemistry.SE. This feels like a homework-style question, so I'll go ahead and give it the homework tag. I'm also adding acid-base tag, since this is what the question is about. The tags bond, hydrogen, and molecules do not really match the question. $\endgroup$ – Ben Norris Sep 28 '13 at 2:10
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The table on the right looks like a standard group of acids shown to illustrate the effect of inductive withdrawing on carboxylic acid pKa. The first two structures are relevant to the question. The table may have been copied from somewhere else because it had both structures on it. Two of the pKa values in the table are every similar to two of the pKa values given in the question. The fluoroacetic acid is a fine distraction; you are correct in thinking that it is not involved.

As for figuring out which of citric acid's three carboxylic acids is deprotonated first...

Citric acid has three carboxylic acid groups of two different types. Can you see that? One of the acids is closer to the OH group than the other two. Which acid or acids look more like the first entry in the table (except jammed onto the citric acid structure)? Which acid or acids look more like the second entry on the table? Once you have made that distinction, you should be able to use the pKa data on the table to determine which of the three carboxylic acid groups in citric acid is the most acidic (goes with the 3.13 pKa).

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  • $\begingroup$ I see what you mean. So the table on the right just provides us with information to get a close approximation to the most acidic group in the molecule? $\endgroup$ – Bob Dylan Sep 28 '13 at 2:42
  • $\begingroup$ @BobDylan That is correct $\endgroup$ – Ben Norris Sep 28 '13 at 12:54
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It will deprotonate the acid group in the middle as is more acid the base can easily take it´s proton away, rathe than having problems with the protons in the other acid groups.

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    $\begingroup$ Welcome to Chemistry.SE! Would you be willing to elaborate? What types of problems would pyridine have with the other protons? $\endgroup$ – Ben Norris Feb 3 '14 at 12:25
  • $\begingroup$ It helps here if you explain the why of your answer rather than just stating it. $\endgroup$ – matt_black Feb 3 '14 at 20:02

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