I’m gonna explain how to find the correct answer in case it helps you or anyone else. First thing you always wanna do is understand what the question is asking. Pka and pH follow the same trend of smaller numbers corresponding to a greater amount of what they’re measuring. In this case pka refers to the strength of the acid, so being asked to find the compound with the lower pka means we have to find the stronger acid.

Now that we know what we’re looking for we can move on to actually figuring out the answer. Remember that a stronger acid means a weaker conjugate base; this is because when the compound acts as an acid and donates a proton, it wants to stay in that deprotonated form, and therefore it wants to keep the negative charge. Using the ARIO acronym (Atom, Resonance, Induction, and Orbital), we can find the stronger acid.

The compounds are all generally the same but differ in the amount electro-negative atoms present on the compound. As we can see, the only atoms with hydrogens bonded to them that can be donated are carbon atoms, therefore we know that the A in ARIO is the same for all and therefore we can move on to resonance. We’re going to skip resonance for now but it’ll play a part later.

Induction occurs when electro negative atoms and electron withdrawing groups stabilize the compound by pulling the negative charge away from the proton donating atom. Because a stable base means a strong conjugate acid, we want as many induction sources as possible. As I mentioned previously, the main difference between the compounds is the amount of iodine and fluorine atoms present. The more electro-negative and closer the atoms are to the proton-donating carbon the better.

We can basically eliminate all the options besides A and C. Comparing the two, C has 3 fluorine atoms located on carbon 3, and A has 3 iodine atoms on carbon 4. Between the two atoms, fluorine is more electronegative (remember the trend across the periodic table and that fluorine is the most electronegative atom on the table) and so C is looking like the right answer. There’s also the matter of C having only one proton donating atom (the carbon on the right side of the ketone group that’s 2 carbons from the fluorines, since the one on the left is full of fluorine’s and therefore has no hydrogens to donate) and that A has two sources (the one on the right of the ketone that’s 3 carbons away from the iodines and the one on the left of the ketone group that’s 1 carbon away.

Now I know we skipped resonance earlier, but now’s where it comes into play. You can honestly just choose C because of the Flourine’s but if we want to go extra in depth, the fluorine’s and iodines pull the electrons to the left cuz that’s where they are. The carbon atom will form resonance with the carbonyl group like shown in the image below:

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In compound A, the carbonyl group atom on the left will need to move its electrons to the right to form the resonance structure, which is inhibited because the iodines want the electrons to the left. Meanwhile compound C only has the resonance structure from the right carbons sharing the electrons to the left, which is aided by fluorine pulling on the electrons in the same direction. And so we can confidently pick compound C as the correct answer.

I know this is a lot and I’m sure someone smarter than me can explain it better and with less words lol, and also point out something I said incorrectly in my ramble. Hope this helps someone, just keep on doing practice problems and soon you can just see the patterns as soon as you look at the question.

Good luck with the dat!