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Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

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18. Reactions of Aromatics:EAS and Beyond

EAS:Ortho vs. Para Positions

In general, we refer to the products of an EAS o,p-director as a mixture – but there are some patterns we can learn. 


Ortho, Para major products

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in this video, we're going to go into a little bit more depth on the major products of Ortho Para directed reactions. So in general, whenever we have an E s reaction on an Ortho pair director, we're just going to say that the product is a mixture of both the Ortho and the pair of position. So if you want to play it safe, you could always just say that it's a mixture. Okay, And that's the way that you know, most textbooks teach it okay. However, there are a few patterns that might be helpful for us to learn, especially later on when you start talking about specific groups called Blocking Group. So let me tell you about this difference to the Ortho position in the para position. Well, basically, these positions are in competition with each other because they both have advantages. Look, let's say you have an electron donating group. Okay? That means it's pushing electrons into the ring, and it's probably it's an Ortho pair director, right? Well, the Ortho position has an advantage, a clear advantage right off the right from beginning, because notice that it has two positions instead of one. The pair of position. There's only one para, but there's to Ortho positions. So you might think, Well, we're gonna get more Ortho product because you have twice as many positions that could react. Okay, that's one way to think about it. There's two times as many positions. But on the other hand, the pair of position also has an advantage, which is that the pair position is usually the one that's less hindered because it's furthest away from the donating group. So it's actually easier toe add to that pair of position, even though there's less of them. Okay, and in general, when you kind of pair these two together, when you compete them against each other, number versus Ciric Hindrance, Starik Hindrances usually gonna win. So usually there's gonna be a slight advantage for the pair of position over the Ortho positions. Okay, so if in a question you're asked to supply one major product instead of a mixture, if it says specifically which ones the major product here we can see a sulfa nation on tall Ewing. Okay, so you can see we've got our s 03 we've got are fuming h two s 04 and look what we're gonna get. What we are going to get is a major product of Pere. Okay, so you would say that your parasol phonic acid is going to be the major product and your Ortho oh, is gonna be the minor. Okay, Even though there's more Ortho positions, the Starik hindrance, the benefit of stare ICS is going to make the pair of the major products okay. And basically, that's going to be the rule that we always follow. We're going to say that pere wins out over Ortho. Okay, if asked to supply one major products now, there is one noted exception, and that one exception is if the final product can hydrogen bond with itself. Okay, so if the final product can Haider in bond because of the orientation of the groups, then we would say that the Ortho will slightly win. So here you guys can see the nitrate shin of fennel. Notice that here I have my nitric acid. I have my sulfuric acid. We know that we're going to generate What is it? It's gonna be the nitro knee. Um cat ion, Right? My Trone iam ion. And we know that o h is an Ortho pair director, right, because it has a lone pair. So that means that it's an electron donating. I mean, it has to. Lone parents were saying it has a least one lone pair, right, so we know that it's going to direct, you know, either next to it, which is Ortho or at the bottom, which is pere Okay. But it turns out that in this case, Ortho was slightly better. Why? Because if you make it or throw, then you could get a hydrogen bond between the funeral and the Nitro group. If you make it pere, it's less hindered, so it's easier to add that way. But it can't interact with itself, so it's not going to be quite a favorite now. This ratio actually isn't as big of a difference as you might think. It's actually only going to be a 60% to 35% ratio, so meaning that it's not even winning by that much. But still, this would be the only exception where we would expect more Ortho okay, but for everything else if supply one major product, I mean you can always say it's a mixture, but if you're asked to supply one. You're usually gonna go with the pair of product because it's the one that's less hysterically hindered. Alright, guys. So believe me, you don't need to know these percentages. I'm just giving them as a teaching moment. So you guys can see actual examples in real life and you guys can understand how it plays out. Okay? But in general you can just go with the rules that I'm telling you. Alright, guys, let's move on to the next video.

There is one exception to this rule – which occurs if the final product can hydrogen-bond with itself. Then, we would expect more ortho, than para product.