Metal Ion Catalysis: Water Activation - Online Tutor, Practice Problems & Exam Prep

Alkali metals, such as lithium, sodium, and potassium, react with water in a single displacement reaction, producing hydrogen gas (H₂) and a metal hydroxide (M(OH)). This reaction exemplifies metal ion catalysis, where metal hydroxide complexes, typically with +2 or +3 charges, activate water similarly to the hydrolysis of carboxylic acid derivatives. The nucleophilic acyl substitution mechanism is employed, where water acts as a nucleophile, leading to the formation of carboxylic acids or carboxylate anions through proton transfer or metal ion transfer, enhancing reaction rates.

Metal Ion Catalysis: Water Activation Concept 1

Hey everyone. In this series of videos, we're going to talk about metal ion catalysis in terms of water activation. But first, let's take a look at alkali metal reaction with water. We're going to say that our alkali metals, remember those are your group 1a metals on the periodic table, react with water in a single displacement reaction that uses radicals. Now, the alkali metals that we talk about are these in red.

So we're talking about lithium, sodium, and potassium. Here, if we're taking a look at the reaction with water, we're going to have our metal, our alkali metal, one of these three reacting with water. As a result of this reaction, we have the proliferation or creation of hydrogen gas, so we have H2 gas, plus we're going to create a metal hydroxide. Remember, these are all group 1a metals, so they're going to be plus 1 in charge. And hydroxide is OH−1.

The numbers in the charges are the same, so when they combine they just cancel out. So we create M OH which is our metal hydroxide. Here, this does not represent the balanced equation. Here, we're just focused on what is the equation from reactants to products. We know here that when an alkali metal reacts with water, we would create hydrogen gas plus a metal hydroxide.

Alright. Click on the next video, and let's do one with one of these alkali metals where we have to write the products formed and also balance it. Alright. So again, click on the next video, and we'll see how to work one out.

Metal Ion Catalysis: Water Activation Example 1

Hey, everyone. So here it says, complete and balance the following reaction. Here we have potassium solid reacting with liquid water. So we have an alkaline metal reacting with water. We know that one of the products formed would be hydrogen gas.

So we have H₂ gas and then we create a metal hydroxide. Potassium is in Group 1A so it's +1. Hydroxide ion is -1. When they combine, we get KOH aqueous. So we have our reaction, but now we have to remember to balance it.

We're going to say here, on this side, we have K, H, and O. Now on this side, K, H, and O. We have 1 potassium, 2 hydrogens, 1 oxygen. Here, we have 2 hydrogens, oh actually, 3 hydrogens, 1 potassium, 1 oxygen. Gotta balance this out. So here, these numbers are not the same.

One is 2 and one is 3, but they're in different locations. So it becomes hard to determine exactly how we would balance this out. So what I'm going to do here is I'm going to throw a 2 here. That 2 gets distributed to the potassium to give us 2, to give us 2 oxygens, then we have 2 hydrogens here and 2 here so that's 4. Let's go to the other side.

So if we go on the other side, I could throw a 2 here. That'll give me 4 and 2. Cool. And then I will throw a 2 here. So now both of our lists are the same.

We have 2, 4, and 2 in relation to potassium, hydrogen, and oxygen. And so this will represent our complete reaction as well as a balanced reaction. Alright. So this would be our final answer.

Metal Ion Catalysis: Water Activation Concept 2

Now, let's take a look at the substitution involving water activation, and we'll see its similarities to the hydrolysis of a typical carboxylic acid derivative. Now, here we're going to say that metals, typically those with plus 2 or plus 3 charges, form metal hydroxide complexes when reacting with water. This, we already talked about. Now, we're going to say that the metal hydroxide complex reacts in a similar way to the hydrolysis of carboxylic acid derivatives. Now, recall that carboxylic acid derivatives can react with a water molecule via the nucleophilic acyl substitution reaction or NAS.

So, if we take a look here, we're going to say our uncatalyzed reaction represents a typical hydrolysis reaction of a carboxylic acid derivative. Because it's uncatalyzed, it would work slower in terms of rate. When we use the activated water molecule, we're going to say that we're dealing with a catalyzed version of the same reaction. And because it's catalyzed, it'll be faster. So, let's just take a look at the first one.

This mechanism is familiar to us because we covered it under the chapter dealing with carboxylic acid derivatives. Here, we have a typical ester. Our water molecule would undergo nucleophilic attack, so it would come in here and hit this carbonyl carbon, kicking the pi bond up to oxygen. Oxygen becomes negatively charged, and here goes our water molecule that we just attached. Now, here we could do a proton transfer where this hydrogen would transfer to this OR group making it a better leaving group.

Here, oxygen's making 3 bonds, so it's positively charged. This oxygen, deciding it wants to remake pi bonds, does so, and in doing this and creating this pi bond once again, we can kick out this leaving group. We have now created a carboxylic acid. And if we want to do a proton transfer through the use of a water molecule, that would give us a carboxylate anion at the end. Now, we know the hydrolysis of esters from previous chapters.

When we're dealing with water activation, it's the same similar mechanism. The only difference now is that one of the hydrogens that was part of the water has been substituted out with our metal to make our metal hydroxide complex. So here it is right here. We'd say the metal here would be positive in terms of its charge. We'd say that this oxygen is still partially negative.

We have this dotted bond between the metal and the hydroxide oxygen. What's going to happen here is the same as before. We'd have a nucleophilic attack, so this will come in and attach right there, lifting this bond up. Oxygen becomes negative again.

And now we're going to say we're attaching this activated water molecule, so oxygen is still attached just like we have up above. And we have this metal attached still. Alright, so we have this dotted line with the metal. This here would be partially positive, and this would be partially positive or actually fully positive.

We had a proton transfer up above and now we're going to have a metal ion transfer. Remember, we said that these metal ions can behave like protons. So this metal here would transfer over to this OR group making it a better leaving group. Alright. So now we're going to have here our dotted line to this metal.

It's positive. And here this will be partially positive. And this is still negative. Just like up above, this oxygen decides it wants to remake its pi bond, so it does. And it'll kick this whole thing out as a leaving group.

Just like up above, we have a loss of leaving group. At this point, what do we have? We have a carboxylic acid just like up above. And if we want, we can do a final proton transfer or a deprotonation where we create a carboxylate anion at the end. So, although we're talking about water activation and we're looking at its mechanism, a lot of it should be very familiar to you.

Here, we're dealing with water activation, dealing with hydrolysis of an ester, a similar type of mechanism that you saw earlier in chapters dealing with esters and the hydrolysis of them back into the parent carboxylic acid. And if we continue with deprotonation, our carboxylate anion. Right? So just keep that in mind. Although this is a new topic, a lot of it is built on old materials talking about the hydrolysis of carboxylic acids or derivatives of carboxylic acids, in particular, the ester.

Metal Ion Catalysis: Water Activation Example 2

Hey everyone. So here it says, provide the mechanism for the Zinc 2+ ion or Zinc ion catalyzed hydrolysis of methyl acetate. Alright. So Methyl Acetate. Acetate means we have a methyl group connected to a carbonyl, and methyl would just be OCH₃ here.

So this is our ester. We know that we're dealing with zinc-catalyzed hydrolysis, so zinc will be the metal connected to our hydroxide ion. This will be partially negative. This is positive. What happens first is a nucleophilic attack where this gets attacked here, kicking the bond up to oxygen.

So we'd have the negative oxygen here. We have our OCH₃ here. We've attached our oxygen which has an H and the zinc attached. And then we'd say here this is partially positive. Next, we have a metal ion transfer.

So the zinc would move to the OCH₃ group, making it a better-leaving group. Being partially positive and this is OH. Oxygen decides it wants to remake a double bond so it does, causing a loss of leaving groups, so this would leave. So what we have now is our carboxylic acid, and then the final step is a proton transfer which we would make our carboxylate anion. So we've made our acetate ion as the final product here based on this reaction.

Name the carbonyl containing product formed when propyl-3-methylpentanoate reacts with Mg²⁺ in an aqueous solution. 

• A. 5-methylbutanoate
• B. 3-methylpentanoate
• C. sec-butylpentanoate
• D. propyl pentanoate

Given below are the pK_a values of metal ions after binding to water. If the greater the acidity of a metal cation then the more easily the complex forms, which of the following ions would facilitate the fastest basic hydrolysis of ethyl propanoate? 

• A. Al³⁺ (pK_a = 4.85)
• B. Cu²⁺ (pK_a = 7.52)
• C. Fe³⁺ (pK_a = 2.20)
• D. Ni²⁺ (pK_a = 10.60)