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Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

21. Enolate Chemistry:Reactions at the Alpha-Carbon



Unusual Acidity of the Alpha Carbon

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Hey, guys, In this video, I'm going to introduce two very important new concepts that we need to know for this section. And that's the concept of Alfa carbons and taught memorization. So previously to this chapter, we've discussed how Carbonell carbons are very reactive because they have a partial positive on them. And so far, we've always been talking about reactions at the carbon carbon. What turns out that carbon eels have another reactive component other than the carbon carbon? This is evidenced by P. K s. Okay, Now, you know, it's been a long time since your acid and base chapter in or go one. But does anyone happen to remember What is the peak? A oven S p three hybridized C h bond. Okay, basically an Al Kaine. Do you remember what the Pekka is of? Just a typical Al cane. Okay, I heard someone say Great job. It's been forever since he's mentioned that it's 50. Okay, if you said 45 anything above its 50 it's something really high. Something crazy high. So the acidity of a normal Al Kane is 50. But Alfa carbons are uniquely a sick Alfa carbons don't have a PKF 50. Guess what? They're Pekka is 20. What is responsible for this crazy difference? Just you guys know this on a log scale. So that means that it's 10 to the 30 times more acidic than out came. That's an indescribable number. Okay, that's a huge, huge number. So what could possibly be, you know, responsible for this difference? Okay, The answer is taught memorization, and that's what we're gonna show. I'm gonna show you guys in the next video.

Tautomerization Mechanisms

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So, guys, Todd memorization is a phenomenon that naturally happens toe all ketones and Alba hides in an aqueous environment. Okay, And there's base catalyzed versions, and there's acid catalyzed versions. We're gonna learn both, but essentially, what happens with Todd memorization is that whenever you taught him, arise a carbon. Neil, you're gonna switch the locations of a pie bond and a hydrogen. Okay, So what winds up happening is that the hydrogen jumped up to where the pi bon waas and the pi bon jumps down to where the hydrogen waas. Okay, now these the relationship between these molecules is called ta tumors. Okay, so these air called ta tumors to each other. And just to be clear, these air constitutional ice members of each other, right, their constitutional ice swimmers, because Adams are moving. So please don't call these resonant structures right, Because resident structures, you can't move atoms here. You are moving atoms. Okay. Like I said, this happens in acquis environments, and it happened specifically to key tones and all guides. Okay, so how does this work? Okay. Oh, actually. And then there's the 200 m have names. I'm sorry. I forgot to tell you that. So the Tanamor that normally looks like a Carbonell is called the Keto totem. Er. Okay, now it's funny is that it's called the Keto Tanamor, even if it's not a key tone. So it's an alga hide. You would still call it the Keto Todd Beamer. Okay, once you taught, um, arise what you're gonna make is an alcohol on a double bond of vinyl alcohol. Okay, vinyl alcohols or special guys? Because vinyl alcohol's Cantat um, arise. Okay, so because of a vinyl alcohol, this is called the Enel totem. Er, okay, because you've got alcohol directly attached to a Taliban and e no. All right, So you got Keto Tanamor. You got Enel Tanamor these air in equilibrium with each other in Equus environments. So let's look at the acid catalyzed mechanism of how this happens. Okay? So, in an acid environment, what we're gonna do is we're gonna protein it first. Okay, that's going to give me a carbon Neil. That now has a positive charge. And what can happen is that the conjugate base of the acid can wind up deep protein eating an Alfa proton. Okay, I'm gonna put the Alfa down here, actually, an Alfa Proton. So what happens is that it deep rotates the alfa makes a double bond and kicks the electrons up to the O h. Okay, this makes the Enel totem er Okay, so I haven't Eagle one side. I have a keto on another, and I used acid to make this exchange happened. Okay, So, guys, from now on, this is revolutionary. Any time that you see a key toner and Aldo Hide, you need to be thinking about Todd memorization because this happened. Whether you like it or not, it's gonna happen. Okay, let's look at the base catalyzed version in a base catalyzed mechanism. We go straight for the alpha proton right away. So we're gonna take my own negative. We're going to remove the Alfa Proton. We're gonna make the double bond and kick up the electrons that Oh, this is going to give us a negatively charged Enel or a very special intermediate called and in a late and I on. Okay, that makes sense, because it's the negatively charged version of Enel. So it's in a late Okay, There is an entire branch of chemistry around immolates, and we're going to spend a lot of time dealing with immolates this semester. Okay? They're very special. Because of that, we're gonna go through the based catalyst mechanism very often because we want to achieve the anally. The Emily. Okay, then two protein eat. We would just use the congee acid, and this would give us the Enel. Okay, so once again, we have keto We have, you know, the biggest difference with this one being that with the base catalyzed mechanism, you pass through this intermediate. That's actually very reactive and very important. Okay, so base catalyze gives you an email late, and that's what Todd Memorization is. This helps to explain the acidity, guys, because the reason that that Alfa carbon is so acidic with a PK of 20 is because you conform a stable conjugate base or a stable molecule if you remove it because you can always just form the Enel. All right, Awesome. So let's move on to the next video

Draw the enol tautomer for cyclopentanone