Explain why the half-life (the time it takes for one-half of t...

Question

Explain why the half-life (the time it takes for one-half of the compound to be metabolized) of Xylocaine is longer than that of Novocaine.

Accepted Answer

All right. Hi, everyone. So this question says that when used as local anesthetics, it takes a longer time for one half of etidocaine to be metabolized than tetracaine. Explain why. And here on the screen, we have the structures of tetracaine and Eido cane respectively. So recall first and foremost that when both of these drugs enter our system, they're metabolized by their reaction with water. Both of these compounds contain a carbonel group in which the carbon atom of that carbonel is electron deficient due to its bond with the electron oxygen atom. Oxygen takes electron density away from carbon which therefore leaves it electron deficient. So because of this water is going to behave as a nucleo file and it's going to go ahead and install itself onto the carbon carbon of both compounds. That is how these drugs get metabolized. Now, the difference between these car bal groups is that in tetracaine, it's part of an ester, whereas in Tito cane, it is part of an amide. So to understand the trend that we see in the metabolism of these compounds, we have to understand their resonance structures since both ester's and amides participate in resonance with that car bal. So starting off with the Esther right, the oxygen atom that's single bonded to the carbon or carbon can go ahead and establish a pi bond between itself and carbon, which is therefore going to push the pi electrons of the car bottle upwards to oxygen. So we get a resonance structure where the oxygen atom that was previously single bonded to the carbonyl carbon now has a positive charge and the car bal oxygen has a negative charge. So let me just go ahead and actually draw the rest of the molecule. There we go. So here is the resonance structure or the other resonance structure of tetracaine. So now let's do the same thing with a teto cane. Now, etidocaine is or contains an amide functional group. So when it comes to depicting its resonance, the lone pair of electrons on nitrogen is going to create a pi bond in between itself and the car bal carbon which once again pushes the pi bonds of the car bal upwards to oxygen. So here we can already see a difference between their resonance structures aside from their general structural differences because this time, in the case of Tito came, its resonance form is going to have a positive charge on nitrogen, but the carbon oxygen is still going to have a negative charge. Now, going back to our discussion earlier of the metabolism of these compounds when they come into contact with water, recall that the carbonyl carbon is the primary electro file for that particular reaction. No, because both of these compounds exhibit residents that positive charge is delocalized. It's spread out across a greater area instead of being concentrated on that carbon carbon. Now, what that does if you recall is actually reduce the electro hylic of that carbon carbon because resonance makes that specific electro pilic site less reactive overall. So if it's less reactive, right, the reaction itself is going to take longer to proceed, which means that more time is going to pass before one half of that particular drug gets metabolized. So to understand the trend that's described in the beginning of this video, we have to understand or rather compare these two resonates forms to each other because recall also that there's a correlation between stability and reactivity when a molecule or some kind of intermediate is more stable, it's considered less reactive. So in this case, notice how the amide of etidocaine results in a positive charge on nitrogen. Whereas in the ether of tetracaine, there's a positive charge on oxygen. Now, oxygen is more electron than nitrogen, which means that oxygen is destabilized to a greater extent when it has a positive charge compared to nitrogen. So the residents form of the amide in which nitrogen has a positive charge is actually more stable because nitrogen is better able to accommodate a positive charge compared to oxygen because it is less electron. So therefore, just to reiterate the point that we've been making previously, the resonance form of the amide of etidocaine is more stable than the resonance form of the ether and tetracaine. This is because nitrogen is better able to accommodate a positive charge than oxygen. So because etidocaine has a more stable resonance form, it as a whole becomes less reactive. So it takes longer to be metabolized compared to tetracaine. And with that being said, we've arrived at our final answer, which you can see here on the screen after I scroll down somewhat. So the final answer is as follows, both tetracaine and etidocaine exhibit resonance. But the resonant structure of etidocaine is more stable because of the better ability of nitrogen to hold a positive charge than oxygen. This makes etidocaine less reactive and longer to metabolize. So with that being said, if you watch this video all the way through, thank you so very much. And I hope you found this helpful.

Explain why the half-life (the time it takes for one-half of the compound to be metabolized) of Xylocaine is longer than that of Novocaine.

Key Concepts

Half-Life

Metabolism of Drugs

Chemical Structure and Properties

Watch next