Comparing Acidity with pKas

Most professors will give you their own list of pKa values to memorize. Most of the time, those values aren’t extremely useful (due to professors not giving much  thought to it).

Here is a list of the MINIMUM number of pKa values to memorize. By the time you get to your exam, you should at least know these values. 

Clutch Student: “Johnny, my professor said I don’t have to memorize any pKa values for the exam”.

Me: It doesn’t matter. You’ll need to understand these values for the rest of Organic Chem I and II (even Orgo III in some schools). By not memorizing this easy list, you are doing yourself a huge disservice. Just believe me and go with the flow! 

Pop quiz: If I were to ask you what the overall pKa for this molecule is, what pKa would you say? How do we figure that out?

The overall pKa of a molecule is equal to the pKa of the most acidic hydrogen. A chain is only as strong as its weakest link; if there is a proton that can easily dissociate, that will be the one that a base will choose to remove.

So the answer is approximately 10.

Problem: Rank the following organic compounds in the order of increasing pKa.


Remember that the pKa of all these molecules is equal to the pKa of their most acidic hydrogen. 

Error: In the video I actually made a mistake when I was ordering these values! Since the question is asking to order in terms of increasing pKa, just reverse the order that I gave you. Everything else is correct though!

Problem: Rank the following compounds in the order of increasing acidity.