In this video, I want to focus on a specific type of condensation reaction called an aldol condensation. From prior videos, we know that an enolate is a negatively charged species and it can attack electrophiles. But what we didn't fully realize up until this point is that these enolates can actually react with themselves to condense. Specifically, if it's a ketone or aldehyde, it's going to condense into this category of molecule called a beta-hydroxy carbonyl. The final products are called aldols. That's the reason that we have that name because they're part aldehyde and they're part alcohol. Hence the name aldol. What I'm going to do here is I'm going to show you the full mechanism for an aldol condensation. One of the most important things to realize about any condensation reaction is that a big part of getting these questions right isn't just knowing the mechanism. It's knowing how to line up your carbonyls because if you line them up incorrectly, you're going to be doing a lot more work and you're probably going to get it wrong. I'm going to teach you not only just the mechanism but I'm going to use my years of teaching experience to try to engrain this method into you of how is the best way to line up your carbonyls so that you can always visualize the product the easiest. The first step guys is super easy. All it is is that you're using some kind of base, usually OH negative, to pull off a proton and make your enolate. Remember that enolates have two different resonance structures. There's also a resonance structure with the negative on the O but we're not going to use that one. For this mechanism, you always want to be using the enolate on the carbon because that's going to help predict what our product looks like. The reason that we would get an aldol condensation is when you have a base, but you have no other electrophile. I'd be wondering how would I know that something is an aldol and how would I know that something is let's say a reaction from another chapter because there's a lot of reagents that you know now. But guys, if you are forming an enolate without an electrophile to react with, then you know this is going to be a self-condensation, which is exactly what we're going to draw. We've got our enolate. Notice that my enolate, I'm drawing it on the left-hand side. I'm always going to draw the enolate on the left. I don't have an electrophile on the right. I don't have a halide or an acyl chloride, whatever. That means I'm forced to react with just another carbonyl. How does that work? I like to call them; there's the enolate and there's the electrophile. Why? The enolate is the one with the negative charge. The electrophile is the one that you're going to attack the partial positive of. This one should not be in an enolate form. You should just draw this the normal way that it was before the base reacted. A few other pointers here. You want to make sure that your enolate is drawn on the left, but that your anion is toward the electrophile because you want to have the electrophile and the anion as close to each other as possible. Here, I'm drawing the enolate on the left but I want to make sure my negative charge is as close to the electrophile as possible. Also, any R groups on it face downwards. This is going to come up more later. But if I had bulky R groups on that CH2, I should actually face them facing down to clear the way for the electrophile that I'm about to attack. My electrophile, you've actually got some special rules about that one. The electrophile you draw on the right. Always draw on the right and you should draw your smallest group toward anion. The reason is because we're going to have to attack with the anion and it's going to be easiest to visualize if I have my small group close to the anion and my large group away from it. I think that's enough to get going. This mechanism is going to look like follows. This is a nucleophilic addition mechanism. My negative charge is going to attack my partial positive. I'm going to break a bond and I'm going to make a tetrahedral intermediate. It's a big tetrahedral intermediate. I understand. But you still have an O negative with now what groups? Now you've got the extra CH2 that's attached here. We still have that H. The H that was here is still here. It's just I'm not drawing it. Then there's the CH3. This is called a nucleophilic addition mechanism because we're going to protonate the O at the end. We're not going to kick out a leaving group. We use water or acid to protonate and to give us our beta-hydroxycarbonyl because I have a beta-hydroxy on my carbonyl. The reason I call it carbonyl is because it could be either a ketone or an aldehyde, just depending on what you started with. That's really it. It's a really easy mechanism. It's nuc...
25. Condensation Chemistry
Aldol Condensation
25. Condensation Chemistry
Aldol Condensation - Online Tutor, Practice Problems & Exam Prep
An aldol reaction gets its name from the 2 functional groups that make it up. Let's take a look at how we can distinguish this condensation from others.
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ProblemWhat product can be isolated from the following aldol condensation reaction?
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B
C
D
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ProblemProvide the mechanism for the following transformation.
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PRACTICE PROBLEMS AND ACTIVITIES (35)
- A student tried to prepare the following compounds using aldol condensations. Which of these compounds was she...
- Describe how the following compounds can be prepared using an aldol addition in the first step of the synthesi...
- Describe how the following compounds can be prepared using an aldol addition in the first step of the synthesi...
- Describe how the following compounds can be prepared using an aldol addition in the first step of the synthesi...
- Identify A–C. (Hint: A shows three singlets in its 1H NMR spectrum with integral ratios 3 : 2 : 3 and gives a ...
- What aldol addition product is formed from each of the following compounds? b.
- What aldol addition product is formed from each of the following compounds? a.
- An aldol addition can be catalyzed by acids as well as by bases. Propose a mechanism for the acid-catalyzed al...
- Draw the products of the following reactions: b.
- The following compounds can be synthesized by aldol condensations, followed by further reactions. (In each c...
- The following compounds can be synthesized by aldol condensations, followed by further reactions. (In each c...
- The following compounds can be synthesized by aldol condensations, followed by further reactions. (In each c...
- Predict the products of aldol condensation, followed by dehydration, of the following ketones and aldehydes. (...
- Predict the products of aldol condensation, followed by dehydration, of the following ketones and aldehydes. (...
- Predict the products of aldol condensation, followed by dehydration, of the following ketones and aldehydes. (...
- Give the expected products for the aldol condensations of (c) pentan-3-one.
- Give the expected products for the aldol condensations of (b) phenylacetaldehyde.
- Give the expected products for the aldol condensations of (a) propanal.
- The Knoevenagel condensation is a special case of the aldol condensation in which an active methylene compound...
- The Knoevenagel condensation is a special case of the aldol condensation in which an active methylene compound...
- Propose a mechanism for the aldol condensation of cyclohexanone. Do you expect the equilibrium to favor the r...
- Propose a complete mechanism for the acid-catalyzed aldol condensation of acetone.
- Predict the products of the following aldol condensations.Show the products both before and after dehydration....
- Propose a mechanism for the dehydration of diacetone alcohol to mesityl oxide (a) in acid. (b) in base.
- Show how each compound can be dissected into reagentsjoined by an aldol condensation, then decide whetherthe n...
- Using cyclopentanone as the reactant, show the product ofb. an aldol addition. c. an aldol condensation.
- Show how each compound can be dissected into reagentsjoined by an aldol condensation, then decide whetherthe n...
- Show how each compound can be dissected into reagentsjoined by an aldol condensation, then decide whetherthe n...
- The Knoevenagel condensation is a special case of the aldol condensation in which an active methylene compound...
- The Knoevenagel condensation is a special case of the aldol condensation in which an active methylene compound...
- The Knoevenagel condensation is a special case of the aldol condensation in which an active methylene compound...
- Predict the products of these reaction sequences. < of reactions>
- Show how you would use an aldol, Claisen, or another type of condensation to make each compound. (c)
- Propose mechanisms for the reactions shown inProblems 22-62 parts (a)
- Many of the condensations we have studied are reversible. The reverse reactions are often given the prefix ret...