a. When a carboxylic acid is dissolved in isotopically labeled...

Question

a. When a carboxylic acid is dissolved in isotopically labeled water (H₂O₁₈) and an acid catalyst is added, the label is incorporated into both oxygens of the acid. Propose a mechanism to account for this.

Chemical reaction showing isotopically labeled water and acid catalyst incorporating label into carboxylic acid oxygens.

Accepted Answer

All right. Hi, everyone. So for this question, it states that acetic acid is dissolved in isotopic labeled water, H2o 18 with an acid catalyst, draw the labeled products formed. And here I've drawn the structure of acetic acid on the screen. So here what's happening right is that we're reacting acetic acid, which is a carboxylic acid with water. So what that means is that oxygen 18 is somehow going to be involved or incorporated, so to speak in the structure of our new carboxylic acid. So in order to understand that right, let's go ahead and go through the mechanism. So we can see what our product is going to look like. And because the question mentions that there's an acid catalyst, our first step of the reaction is going to involve our acid, right? So after reacting with our acid, what happens is that our carbonyl here is going to get Proin. So here I'm drawing acetic acid once again, but now the carbonyl oxygen has an additional proton that gives it a positive charge. So now this is where H2O 18 comes in because now our isotropic labeled water can go ahead and attack the carbon or carbon. So it's going to do exactly that and that forces the pi electrons of our carbon to shift upwards towards oxygen giving it a neutral charge once again. Right. So now we have this tetrahedral intermediate, right? We have two neutral o age groups from our original acetic acid. But now oxygen 18 has attached itself to the carbonyl carbon, right? So here's oxygen 18 and it still has both of its protons, right? Giving it a positive charge now, so that of course, is problematic, right? Because oxygen is not a fan of having a positive charge. So this is where another molecule of water is going to come in and it is going to go ahead and take a proton away from our oxygen 18 here. And so now, oh, almost drew the arrow in the wrong color. But as I was saying before, right now that a proton has been removed, we have three neutral O H groups on this carbonyl carbon, right? But one of them contains are oxygen 18. So what's happening here? What's happening up next is that our car bono has to reform, right? Because we know that we're going to end up with a carboxylic acid at the end of this process. So this is where our acid in solution comes in handy once again, right? Because our acid and our next step is going to go ahead and proton, one of the 20 H groups from our original acetic acid. And why is that? Right? Well, the reason that's the case is because recall that an O H group is a pretty terrible leaving group on its own. Right. It's a pretty terrible lead group. So it's important to somehow modify it in order to make this reaction a little bit easier. And one such method is by proton, our O H group to make O H two plus. So now that we've made O H two plus, the other age group from acetic acid can go ahead and donate a pair of electrons between itself and the carbon it's bound to. Therefore, kicking out our O H two plus. So on our next step in our next step, we're going to have a positively charged car bottle once again because recall that our oxygen here still has that hydrogen attached. So it's a car bottle, but it's still going to have that positive church. And of course, here's oxygen A T, all right. So here you'll notice how in the first step at the very beginning of this reaction, we had to proton our carbon first before oxygen 18 can go ahead and attack, right? But here our carbon is already proin, right? It already has a positive charge. So at this point is when another molecule of oxygen is going to go ahead and attack. So now our second molecule is going to go ahead and attack and the pi electrons of our carbo are going to be pushed towards the oxygen that has a positive charge. So here we have yet another intermediate right? We have another intermediate in which we have 20 H groups and 10 H two plus. So here our second molecule of h2o is going to have its two protons giving it a positive charge, right? So this is where another molecule of water is going to come for the rescue, so to speak, and it's going to detonate R h2o to give our oxygen 18 a neutral charge. So here, right, in our next step, we have 30 H groups on carbon, but two of them are going to contain oxygen 18. Mhm. So now our carbona is going to form, but this time, one of our oxygen 18 is going to go ahead and create that pi bo in between itself and carbon. Actually let me backtrack because there's something crucial that I forgot to mention earlier. So I apologize for that. But before oxygen 18 can donate its pair of electrons, we have to proton the O H group that we want to leave, right? Because recall that O H A hydroxy group is a terrible leaving group on its own, right? So first, we're actually proton the O H group that we want to kick out, so to speak. And now oxygen 18 can donate its pair of electrons between itself and carbon, which is therefore going to displace a molecule of water containing our regular run of the mill oxygen. Right? So now here we're almost done, I swear. But here we have a car bottle once again, but it contains oxygen and it has a positive church, right? So that has to be fixed using a molecule of water because water can deproteinate our oxygen 18. And at long last we've arrived at our final answer, right? Our resulting product is a carboxylic acid that contains two atoms of oxygen 18. Right? And so this is going to be your final answer. So here oxygen 18 will end up on both the O H group and the carbon of our carboxylic acid. So if you stuck around to the end, thank you so much for watching. I appreciate it. And I hope you found this helpful.

a. When a carboxylic acid is dissolved in isotopically labeled water (H₂O₁₈) and an acid catalyst is added, the label is incorporated into both oxygens of the acid. Propose a mechanism to account for this.

Key Concepts

Carboxylic Acid Structure and Reactivity

Isotopic Labeling in Chemical Reactions

Acid-Catalyzed Reactions

Watch next

a. When a carboxylic acid is dissolved in isotopically labeled water (H2O18) and an acid catalyst is added, the label is incorporated into both oxygens of the acid. Propose a mechanism to account for this.

Key Concepts

Carboxylic Acid Structure and Reactivity

Isotopic Labeling in Chemical Reactions

Acid-Catalyzed Reactions

Watch next

a. When a carboxylic acid is dissolved in isotopically labeled water (H₂O₁₈) and an acid catalyst is added, the label is incorporated into both oxygens of the acid. Propose a mechanism to account for this.