Intramolecular Acid-Base Catalysis - Online Tutor, Practice Problems & Exam Prep

Intramolecular acid-base catalysis involves an acidic or basic group within the reacting molecule that enhances reaction rates. For instance, a carboxylic acid can catalyze ester formation through protonation. In contrast, intramolecular general base catalysis facilitates hydrolysis by deprotonating water to form hydroxide ions, speeding up the reaction. The presence of a basic group on the same molecule as the ester results in the fastest hydrolysis, demonstrating the significance of intramolecular interactions in chemical reactions.

Intramolecular Acid-Base Catalysis Concept 1

Hey everyone. In this video, we're going to talk about intramolecular acid base catalysis. Now, here we're going to say that an intramolecular catalyst is just an acid or base or nucleophilic catalyst present within the reacting molecule. If we take a look here, we're going to look at intramolecular general acid catalysis. Here we're going to say an acidic group can catalyze reactions by intramolecular protonation.

And if we're talking about an acidic group, in this example we can use a carboxylic acid as that acidic group. Its presence would help with the formation of this ester here. So, we have it right here still. So again, the presence of that carboxylic acid, which is an acidic group, helps with this carboxylic acid and this alcohol interacting in order to make this Ester product at the end.

Intramolecular Acid-Base Catalysis Example 1

In this example, it says, write a plausible mechanism for the following esterification reaction. So if we take a look here, we have this carboxylic acid and this alcohol which together interact to make this ester group. We have this acidic carboxylic acid that is ortho to it which is helping to enhance this process in a current. So step 1 has to do with protonation and nucleophilic attack. We're going to say the carbonyl group is protonated through intramolecular protonation.

And we're going to say the methyl methanol molecule attacks the protonated carbonyl group. Alright. So what's gonna happen here is this double bond breaks and goes to the oxygen which causes this lone pair to grab this hydrogen and the bond breaks to this oxygen. So this oxygen here will be negative. This group here becomes an OH group.

Our methanol group is also attacking here causing this whole breakage. So that we're going to add it here. There goes the methanol. Oxygen is making 3 bonds so it's positively charged. And then the OH was there originally, this one is still there.

So we create this structure. Step 2 now is proton transfer. We're going to say H⁺ is transferred from the methoxy oxygen to the hydroxy oxygen. So if we take a look here, here's our structure.

We're going to say that this H here can be transferred to this OH here. That's going to create a water molecule here. Oxygen is making 3 bonds so it's positive. And then this oxygen finally becomes neutral. So remember water is a good leaving group.

So last step, step 3 is leaving group and proton transfer. So the carbonyl group is reformed and water is kicked out. What's going to happen here is this comes here to remake a double bond kicking out this water molecule. So here we have a protonated carbonyl oxygen group. So the final product is formed after an intermolecular proton transfer.

So we have this negative oxygen. We have this positive oxygen. We're going to have this hydrogen transferring over to this negative oxygen making it neutral. So what we're going to get at the end is this. Here goes our carboxylic acid reconstituted and then we have this portion here with our carbonyl oxygen.

So this will represent the process when we're talking about the esterification of our starting carboxylic acid with this methanol molecule. This is the process that would happen. And this acidic carboxylic acid group is helping to have this happen. Okay. So it's helping to basically catalyze this reaction so that we can get to this ester product at the end.

Intramolecular Acid-Base Catalysis Concept 2

Everyone, in this video, let's take a look at intramolecular general base catalysis. Here we're going to say, basic groups can catalyze hydrolysis reactions by deprotonation of water. This would result in the formation of hydroxide ions which could help facilitate and catalyze the basic hydrolysis process. Now, here we're going to say, in the first one, we have this ester and we just have regular water. Hydrolysis here would split this into its alcohol component and its original carboxylic acid component.

We can say that the relative rate here would just be normal. It'd be 1. And then for the next one, we still have our ester, but now we have water and we've introduced a small amount of base, a weak base. This would help to deprotonate the water and a portion of that would help to create a hydroxide ion which would help to catalyze this reaction even more. We create this alcohol and this carboxylic acid at a faster rate.

So, if we were to say we could say that this here is the slowest because it's not being enhanced. This would be moderate. The last one will be the fastest. And it'll be fastest because the basic group is on the same structure as the ester. So, this is an intramolecular view of this process.

So, our basic group is present on this structure which means that we could have the splitting of the ester bond to create our alcohol and our carboxylic acid even faster. So, we'd say hydrolysis, this would be the slowest. Introducing some weak base would help to change this into hydroxide, a portion of it. That'd be faster, so we put that in the middle. This last one will be the fastest one because the basic group is on the same molecule of the ester.

We would create our alcohol and our carboxylic acid at the fastest relative rate when comparing these 3. Alright. So this is the way we look at it when it comes to this intramolecular, base hydrolysis process.

Intramolecular Acid-Base Catalysis Example 2

Everyone, in this example, it asks to write a plausible mechanism for the following hydrolysis reaction. Here, we have our carboxylate anion that's attached to the same structure as the ester, and at the end, we're going to split the ester bond to create our alcohol and our carboxylic acid. Now, step 1 deals with deprotonation and nucleophilic attack. Here we're going to say that the carboxylate anion deprotonates water to produce our hydroxide ion.

So, this oxygen will deprotonate the water. This oxygen holds on to the electrons. The hydroxide ion attacks the carbonyl group to form a tetrahedral intermediate. So, it would use its lone pair to attack here, keeping this bond here. As a result, what are we going to have?

We're going to have the hydrogen that got added and then we have this OH attaching itself here. We'd still have this methyl group here, and that carbonyl oxygen now becomes this negatively charged oxygen. And so, we have this structure here. Now, we're going to say the leaving group and intramolecular proton transfer are step 2.

Here we're going to say the C-O oxygen kicks out the phenoxide ion. And we're going to say that an intramolecular hydrogen ion transfer takes place as the phenoxide is kicked out. So what's going to happen here is we have this structure; oxygen uses its lone pairs to grab this hydrogen which causes this to be kicked to this oxygen. This happens because this oxygen decides to make a double bond here, which causes this movement to the oxygen here. So what I think we're going to get is, here's our benzene.

Here is our carboxylate anion that we have. We have this oxygen here, grabbing that hydrogen, so it's an OH, plus we have this carbon now becoming a carbonyl carbon again. It is connected to this methyl and it is connected to this OH. So this would give us our final products and how we got to them through steps 1 and step 2 of this whole process. Alright.

These are the steps you need to take, going from step 1 to step 2, to get to our final end results here.

Which of the following compounds should undergo hydrolysis faster at neutral pH? Write mechanism for the hydrolysis of that compound.

• A.
• B.
• C.
• D.

The following compound, when heated in a solution, undergoes hydrolysis without an acid catalyst. Write a plausible mechanism for the hydrolysis reaction.

Hey, everyone. So in this practice question, it says, the following compound when heated in a solution undergoes dehydration without an acid catalyst. Write a plausible mechanism for the reaction. Right. So here we're going to redraw this structure in order to show what happens. We're going to show our carboxylic acid here. It's connected to this carbon, which is connected to this carbon with 2 methyl groups and a hydroxyl group. What's going to happen here is that this hydroxyl group will deprotonate the carboxylic acid. Oxygen holds on to the electrons. What we wind up creating is our negative oxygen, so our carboxylate anion is formed, and we create water. Remember, water is a good leaving group. So this water molecule will leave, creating a Tertiary Carbocation. Now, this tertiary carbocation needs an additional bond. So what happens here is we're going to get a loss of this proton to fall down here to make a double bond. In the process of this, this is really a proton transfer to that carboxylate oxygen that's negatively charged to give us a neutral compound at the end, so our neutral carboxylic acid at the end. So, what we just created is a conjugated carboxylic acid. Remember, conjugation leads to greater stability and we created a neutral carboxylic acid in the process. This would be the mechanism that we would use without the use of an acid catalyst. Alright? So this would be our final answer.