Write structural formulas for compounds that meet the followin...

Question

Write structural formulas for compounds that meet the following descriptions:

b. An alkene with a chemical formula of C₁₀H₁₂, that has cis–trans isomers and contains a benzene ring.

Accepted Answer

Welcome back, everyone. Let's take a look at our next problem. It says, draw the structural formula of an alkene with a chemical formula of C nine H 10. Its longest chain has three carbons can exhibit cyran isomerism and contains a benzene ring. Well, the easiest way to start here is with our known benzene ring. So we'll draw that in, we're supposed to give the structural formula. So I've written out each of the carbons and hydrogens. We've got our three alternating double bonds. Now, to think about what this should look like the rest of it. I know my molecular formula C nine H 10, the formula of a benzene ring is C six H six, except in this case, my benzene ring is connected to something. And so it'll be missing one of those hydrogens since it needs to make a bond with one of the carbons on the ring. So I'm going to subtract six carbons and five hydrogens from the molecular formula because those have been used up in benzene. So that will give us three carbons left to account for and five hydrogens. So that will help us make sure that we're accounting for everything properly. So we know we need a three carbon chain, which makes sense because we have three carbons left to put on. And then we need to have it exhibit cyst trans isomerism. So let's look at our possibilities here. So we can add a carbon and then a double bond to another carbon and then a last third carbon here. So double bond and then single bond does this give us the correct number of hydrogens? We have a three carbon chain with a benzene ring onto it. And our carbon attached to the benzene ring has three bonds already. So it has just one hydrogen, our carbon in the middle, there has three bonds already. So just another hydrogen and then our terminal carbon needs three hydrogens on it that gives us five hydrogens and the correct molecular formula. So I've satisfied all but one of my conditions here. Now, I just need to look at whether this can exhibit cyst trans isomerism. So let me redraw the structure a little bit differently to make it clear what the R groups are around the double bond. So let's draw our double bond like that sort of the central feature here. We have a methyl group ch three and a hydrogen on our terminal carbon. On the other double bond, we have a hydrogen and a benzene ring. So, rather than write out all the CS and HS, I'm just going to draw that as a line structure, my benzene ring. So when we look at our molecule this way, it's clear that this does have cyst trans isomerism, we could draw it, this would be the trans arrangement where our benzene ring and our methyl group are opposite from each other. But we can draw it in a cis arrangement as well. So this molecule could also be drawn with both the benzene ring and the methyl groups on the same side of the double bond. So this structure would fulfill all of our conditions. However, note that we could arrange that double bond in our three carbon chain a little bit differently. So I'm going to draw an alternative arrangement. We have the double bond next to the carbon that's attached to the benzene ring, but it could be on the next carbon out. So let's draw that I'm not going to put out all the hydrogens in the benzene ring just for purposes of looking at this structure. So I'll draw my carbon attached and then I'm going to draw this with a single bond. I'll put ch two C and then a double bond to a final carbon. Now know that this terminal carbon will only have two hydrogens because it has a participates in a double bond. So imagine two hydrogens coming out of there. And last, we need to add one more hydrogen to the other carbon in our double bond. Well, when we look at this molecule, we can see that we have two R groups going to note this in red, that are the same. So we have two hydrogens on the other side of our double bond. So this has no cyran isomerism. So this is not a possibility for the molecule we're asked for. Then finally, we have one more arrangement where we have our benzene ring, draw that in again, and we have the carbon attached to it. And then we have a methyl group on that same carbon where the benzene ring is and then a double bond to another carbon. So here's our three carbons with an alkene. Does this give us enough hydrogens or the right amount of hydrogens, we'd have ch two, which we should draw a different way to look at our cis trans isomerism that does give us five hydrogens and three carbons. So correct molecular formula. But again, we have the issue because this terminal carbon has two hydrogens. This has the same R groups. So the same issue where there's no cyran isomerism with this structural formula. So in the end, we have just one arrangement of our three carbon chain that shows says trans isomerism. So it's the, we have the benzene ring with the first carbon attached to it and then a double bond, another carbon that has a methyl group because it's the presence of the methyl group. And then a hydrogen at the very end, that allows the possibility of cyst trans isomerism. See you in the next video.

Write structural formulas for compounds that meet the following descriptions:
b. An alkene with a chemical formula of C₁₀H₁₂, that has cis–trans isomers and contains a benzene ring.

Key Concepts

Alkenes

Cis-Trans Isomerism

Benzene Ring

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