Aromatic compounds do not normally react with hydrogen in the presence of a palladium catalyst but will if very high pressures (200 atm) and high temperatures are used. Under these conditions, toluene adds three molecules of H₂ to give an alkane addition product. What is a likely structure for the product?

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All right. Hi, everyone. So this question says that aromatic compounds can be hydrogenated at high temperatures and pressures, draw the structure of the all cane products formed when three molecules of H two are added to paraxylene in the presence of Rh catalyst at 1000 P si and 100 °C. Now, here we have four different answer choices which we will discuss in more detail. But for now, I just want to point out that some of them or rather all of them have metal groups in different positions, whereas one of them simply has no methyl substituents on the rink. No. Before we talk about our choices here, I just want to talk about our starting material given which is Pera or P Xy. Recall that xylene refers to a benzene ring or an aromatic ring with two methyl substituents. Now, the positions of those methyl substituents can depend. So that's why the letter P is placed in front of xy. Because paraxylene or P Xylene implies that the methyl groups on the benzene ring are directly opposite each other. So if I take my benzene ring here, right, then, then the two methyl groups are going to be attached to carbons inside of the ring that are on opposite sides of each other. So now recall that when an aromatic ring like paraxylene here reacts with three molecules of hydrogen gas, then each molecule of H two is going to add on to two carbons in each double bond, right. So because we have three double bonds inside of the benzene ring here, then three hydrogen gas molecules are required to add on to each of the double bonds, right. So that in the presence of an Rh catalyst and a remarkably high temperature and pressure is going to yield an all can product, right. So instead of having an aromatic ring in our final product, we're simply going to have a cyclohexane ring instead. However, the two metal groups are going to remain on opposite sides of each other. And we can add hydrogens to each of the carpets right? To showcase this aromatic molecule has been reduced to an canne using hydrogen gas. This means that our final product is going to be 14 dy methyl oops cyclohexane. So if I scroll up here to an to consider my answer choices, then option B corresponds to the structure of 14 dimethyl cyclohexane. But notice how based on the name alone, write paraxylene as my starting material, I could have eliminated my answer choices right away because option A for example, does not have any methyl substituents. So the aromatic starting material to yield product. A would not have been xylene because xylene require methyl substituents. Option C on the other hand, has the two metal groups on adjacent carbons, namely carbons one and two, that if you recall is the Ortho designation. So the starting material needed to yield product C would have been Ortho xy. Whereas option D here has the two metal substituents placed on carbons that have one carbon in between them, right? So the methyl groups are on carbons, one in three of the ring. And that is the meta position or the meta designation. So the starting material that would have yielded option D here would have been meta Z. So once again, our answer here is option B in the multiple choice and there you have it, right? So with that being said, thank you so very much for watching and I hope you found this helpful.

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Key Concepts

Aromatic Compounds

Hydrogenation

Reaction Conditions