The ribonucleosides that make up ribonucleic acid (RNA) a...

Question

The ribonucleosides that make up ribonucleic acid (RNA) are composed of D-ribose (a sugar) and four heterocyclic “bases.” The general structure of a ribonucleoside is shown here.

The four heterocyclic bases are cytosine, uracil, guanine, and adenine. Cytosine and uracil are called pyrimidine bases because their structures resemble pyrimidine. Guanine and adenine are called purine bases because their structures resemble purine.

a. Determine which rings of these bases are aromatic.

Accepted Answer

All right. Hello everyone. So this question says that ribonucleic sides which constitute ribonucleic acid or RNA A consist of four heterocyclic bases and D ribose of sugar. The four heterocyclic bases are guanine, adenine, cytosine and Uracil. Guanine and adenine are referred to as purine bases due to due to their structural similarity to purine cytosine and uracil are referred to as Purim bases due to their structural similarity to Purim, which of these heterocyclic bases has an aromatic ring. So up on the screen for reference, I've gone ahead and pre drawn the structures of both purine on the left and pyrimidine on the right. With these structures in mind, it's easier to see the similarities of each base to their respective model, right. But let's talk about aromatic recall that there are a few different criteria that need to be met before a compound can be considered aromatic. The first of which is that the molecule has to be cyclic, it has to contain a ring. The second requirement is that the ring itself must be cleaner or flat. The third requirement is that the ring itself must contain a fully conjugated system of pi electrons. This means that every atom inside of the ring has to have P orbitals. And lastly, the fourth requirement is what determines aromantic only after the first three are proven to be true. And that is the four N plus two rule. Essentially, if you take the number of pi electrons in a given ring and make that equal to four N plus two, if you get a whole number for N, then that system is aromatic. So by contrast, if the first three criteria are met, but four N applies instead, that means that the molecule is actually anti aromatic and quite unstable. If any of the first three are not met for whatever reason, then the molecule is simply non aromatic. So with all this in mind, let's go ahead and consider the four structures. Just for a second. Looking at the structures, we can see already that the 1st 2nd and 3rd requirements are true for all four bases because all four bases contain ring structures. They are planar and they contain conjugated pie systems. In some cases, it's not always the most apparent considering that py bonds or at least all of the py bonds aren't always visible in the structures that were given because of that. It is worth mentioning that nitrogens inside of the rings that have hydrogens attached to them generally are going to contribute their lone pair of electrons to the ring system via resonance. So they are considered in the count of pi electrons. So with this in mind, let's go ahead and count all of the pi electrons that are present in all four bases. Now keep in mind that for both adenine and guanine, it is the six member ring that will be counted for the number of pi electrons. So starting off with guanine, here we have two pi bonds already depicted inside of the ring, which means that now we have a total of four pi electrons. However, in addition to that, we also have this nitrogen inside of the ring that is going to donate its loan pair of electrons, which means that it would be counted. That means we have six pi electrons in total for Guan. And the same is true for adenine. Adenine also has six pi electrons. This time, we can see all four pi bonds depicted here in the ring moving on to cytosine, we can see two pi bonds in the structure of cytosine inside of the ring. And we have another nitrogen that's going to contribute its lone pair, which means that we also have six pi electrons. Now uracil is a little bit different in the sense that we can only see one pi bond inside the ring at least how uracil is currently drawn. However, here we have two nitrogens that both contain hydrogens that will contribute their lone pairs towards the ring system via resonance. So one pie bond and two lone pairs from each nitrogen, one from each nitrogen to be precise, gives a total of six pi electrons for all four bases. So from here, let's take our expression four N plus two and set that equal to six, which is the total number of pi electrons in all four compounds doing this, I would subtract two by both sides. And eventually, after I solve for N N is equal to one, which is a whole number because I got a whole number for N according to four N plus two, all of these compounds are aromatic, which means that all of them have aromatic rings and there you have it. So our answer here is spell that wrong. Our answer here is that guanine, adenine, cytosine and uracil all have aromatic rings. And so with that being said, thank you so very much for watching. And I hope you found this helpful.

Key Concepts

Aromaticity

Pyrimidine and Purine Bases

Resonance Structures

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