Organic Chemistry / Element Effect
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Solution: For each compound below, identify the most acidic proton in ...

Question

For each compound below, identify the most acidic proton in the compound: 

Video Transcript

Alright guys, so let's go ahead and figure out which one of these hydrogen's is the most acidic on the compound. So, here we've got these two amine hydrogen's, right? Then we've got these SH hydrogen's and then we've got this OH hydrogen, right? I just highlighted this SH on the S, there we go, alright guys. And we know that these other ones in here we can just kind of omit them since they're going to have pkaÕs of about 50, right? Same with this one, even though it's going to be a little bit different for each one, we know that there are much more acidic hydrogen's on this compound than anywhere else, right? Then theyÕre on these purple ones. So, we know we can go ahead and just approximate these to about 50, alright? But that still leaves us with yellow, green and blue, okay? So now guys, what is the pKa of the yellow hydrogen's? The pKa, it's on an amine, right? This hydrogen is on an amine, so we're going to have a pKa of about 38. But now, what about green? Well, that's a sulfur and we've got an alcohol, and guys wait a minute, we know what's going on here, right? If we go ahead and draw our camp seven, we know that our sulfur is right here, we know that as electronegative-, as it moves to the right and up the periodic table, our electronegativity increases, but sulfur and oxygen are very similar in other connectivity, so what really matters here is the trend of size, right? That goe-, that increases as we move from left to right and from top down. So guys, what's really happening here? That's more important, we've talked about this before. Sulfur is substantially bigger than oxygen, so it can essentially dilute the charge over a bigger surface than the oxygen can. So, our SH is going to be more acidic, alright? This is the atom effect. So, we can say that this alcohol has a pKa of 16, and this SH has a pKa of less than 16. Alright guys, so this is going to be the most acidic hydrogen on our molecule. Alright, let's move on.