Reactivity of carboxylic acid derivatives

Voiceover: Here we have a representative carboxylic acid derivative with this Y substituent here bonded to the carb needle. We know that carb needles are reactive because this oxygen is withdrawing some electron density away from our carb needle carbon, making it partially positive. And for carboxylic acid derivatives our Y substituent is an electronegative atom too. So that's going to withdraw even more electron density from our carb needle carbon. So let's go ahead and write down the first effect, the inductive effect. So induction is an electron withdrawing effect. We're withdrawing electron density from our carb needle carbon. That makes our carb needle carbon more partially positive. So it's more electrophilic and better able to react with a nucleophile. And so induction increases the reactivity of carboxylic acid derivatives. So this effect increases the reactivity. We have a competing effect of induction with resonance. So let's think about resonance next. So our Y substituent with a lone pair of electrons can donate some electron density to our carb needle carbon. When we draw our resonance structure we can see that our top oxygen is going to have a negative one formal charge. And we would have a pi bond between our carbon and our Y substituent. Giving our Y a plus one formal charge. Once again, this concept of increasing the electron density from this lone pair of electrons to our carb needle carbon, that increases the electron density. That's an electron donating effect. And if you're donating electron density, you're decreasing the partial positive charge. Making it less electrophilic, and therefore making it less reactive with the nucleophile. So resonance will decrease the reactivity of a carboxylic acid derivative. So we have these two competing effects, induction versus resonance. And whichever one is going to win- we can think about this balance for helping us to determine the reactivity of our carboxylic acid derivatives. So we start with an acyl or acid chloride. With the inductive effect we know the oxygen withdraws some electron density from our carb needle carbon, and so does our chlorine. So we have a strong inductive effect. To think about the possibility of resonance, I would move these electrons into here, and push those electrons off onto the oxygen. So when we draw in the possible resonance structure, once again a negative one formal charge on the oxygen, and a plus one formal charge on the chlorine. It has only two lone pairs of electrons around it now. So if we think about this resonance structure, we have a pi bond between carbon and chlorine, and if we draw the P orbital- carbon's in the second period, so we draw a P orbital for the second period, and the thing about chlorine, chlorine's in the third period so it has a bigger P orbital. And it turns out that when you mismatch these sizes they can't overlap as well. And so poor orbital overlap means that chlorine is not donating a lot of electron density to our carb needle carbon here. So this is not a major contributor in the overall resonance hybrid. So therefore induction is going to dominate. So let me go ahead and write that here. I'll go ahead and use this color here. So induction dominates. And if induction dominates, then we would expect acyl or acid chlorides to be extremely reactive. And indeed they are. They will react with water, sometimes violently, at room temperature. So let's look at our next carboxylic acid derivative, which is an acid anhydrite. So once again we think about induction first, so this oxygen is withdrawing some electron density from this carbon. So is this oxygen. And if you think about this is your Y substituent, you have this other oxygen here which could contribute. So, once again, we have a strong inductive effect. When we think about resonance, I could move this lone pair of electrons from oxygen into here and push those electrons off. So when we think about overlapping our orbitals for oxygen and carbon, this is a better situation than before, because carbon and oxygen are the same period on the periodic table. So here we have carbon and oxygen. It's the same period, so similar sized P orbitals, so better overlap. So therefore there is more of a contribution, more of an electron donating effect, than in our previous example. So if you think about a lone pair of electrons from the oxygen increasing electron density around this carb needle carbon here, therefore decreasing the reactivity. However, the induction effect still dominates the resonance effect. One way to think about that is we have a competing resonance structure. So this lone pair of electrons can move over to here and those electrons come off onto this oxygen. So some of the electron density- not all of it is being donated to the carb needle carbon on the left. Some of the electron density is going to the carb needle carbon on the right. So resonance is not as big of an effect as induction, and so induction still dominates here. It's much stronger. So let's go ahead and write that. So, induction is much stronger than resonance. So we would expect an acid anhydrite to be pretty reactive. Because induction increases the reactivity. And that is again what we observe. Acid anhydrites are reactive with water. Something like acetic anhydrite will react with water at room temperature. So induction is the stronger effect again. Let's go to the next carboxylic acid derivative which is an ester. Once again we think about induction. So this oxygen withdraws some electron density, so does this one. We think about resonance, we move this lone pair to here, and move those electrons off onto the oxygen. We don't have a competing resonance structure this time, so the resonance effect is a little bit more important than before. So the resonance structure is a little bit more important than before, and so there's a closer balance between induction and resonance. However, induction still wins. So induction is stronger, but it's closer than the previous examples. So induction is stronger. Alright, let's move now to our final carboxylic acid derivative, which is our amide. So once again this oxygen withdraws some electron density from this carbon. Nitrogen is a little bit more electronegative than carbon, so we could think about that possibility. When we consider the resonance effect, move this lone pair of electrons into here push those electrons off onto your oxygen, and we draw the resonance structure for our amide, our top oxygen gets a negative one formal charge, and we would have our nitrogen now double-bonded to this carbon, put in this hydrogen here and then this would be a plus one formal charge on the nitrogen. And so, in this case, this is a major contributor to the overall hybrid. So this resonance structure right here- I'm going to go ahead and identify it. This is a major contributor to the overall hybrid. And we know this because the carbon-nitrogen bond has significant double-bond character due to this resonance structure. And this much more of an important resonance structure than, say, the one that I didn't draw but we can think about here, the ester. And the reason why is because nitrogen is not as electronegative as oxygen. So nitrogen is more willing to donate its lone pair of electrons than this oxygen is. And therefore this resonance structure is more of a contributor. Another way to say that is the least electronegative element is the one that's most likely to form a plus one charge. And so we're donating a lot of electron density to our carb needle carbon, therefore we're decreasing the reactivity. And since we have a major contributor to the overall hybrid here. It turns out that the resonance effect is more important than the inductive effect. So I go ahead and write here this time "resonance wins." So resonance dominates induction. And if resonance dominates induction then we would expect amides to be relatively unreactive. And that is, of course, what we observe. So we talked about induction and resonance for these four carboxylic acid derivatives and we can see a clear trend now in terms of reactivity. As you move up in this direction you get more reactive. So acyl or acid chlorides are the most reactive because induction dominates. And amides are the least reactive because resonance dominates. It's important to understand this trend for reactivity and especially if we think about biology, because in the human body there are a lot of esters and there are a lot of amides. And these are the two least reactive ones that we talked about. There are no acid chlorides or acid anhydrites, they'd just be too reactive for the human body. So this, once again, has applications in biology and in medicine.