Draw Newman projections along the C3―C4 bond to show the most ...

Question

Draw Newman projections along the C3―C4 bond to show the most stable and least stable conformations of 3-ethyl-2,4,4-trimethylheptane.

Accepted Answer

Hello, everyone. Let's work through this problem together. It says using Newman projections show the most stable and least stable confirmations of three S four S five s four ethyl, 35 dimethyl octane. The molecule should be viewed along the carbon 32 carbon four bond. So the question asking us to show the most stable and least stable confirmations is technically asking us for something else, right? We need to know what it means to be the most stable and least stable confirmations for a new projection. So what are those recall that the most stable confirmation is anti? So that is when the bulkiest groups have a 180 degree dihedral angle, right? They are 180 degrees from each other and the least stable confirmation is going to be eclipsed, right. So that is when those bulkiest groups are right on top of each other, they have a zero degree dihedral angle. OK. So first let's draw the bond line structure of this compound, then we will convert it to the Newman projections. OK. So dimethyl octane, so octane is going to be our parent chain. That is an eight carbon chain 12345678 and 35. So let me number my chain 35, dimethyl tells us we have a methyl on carbon three and a methyl on carbon five four, ethyl tells us we have an ethyl group or a two carbon alkyl chain group on carbon four. But notice we are also given the configuration three S four S five S for these three chiral carbons. I'm not going to go into too much detail here about how to determine whether it's R or S configuration. We have a bunch of other videos on that, but I will show how we label these specific carbons. So carbon three, if we have this methyl group wedge, that means there is a hydrogen dashed on that carbon as well. Hydrogen would be fourth priority. Methyl group is third priority. Carbon four. That group is highest priority. Carbon two. That group is second priority connecting 12 and three, we are going counterclockwise four is dashed. So that is S configuration, the ethyl is going to be dashed. There is also going to be a wedge. Hydrogen there we know hydrogen is always lowest priority than carbon five. That group is highest priority carbon three. That group is second highest priority. Ethyl group is third highest priority connecting groups 12 and three goes clockwise, but hydrogen is wedge. So that is S configuration and lastly on carbon five, our methyl group is going to be dashed, which means our hydrogen is wedge hydrogen is lowest priority. Our methyl group is going to be third priority. Carbon number for that group is one priority and first priority and carbon six is second priority, connecting groups 12 and three that goes clockwise, but we have a wedge hydrogen. So that is s configuration. So that is how we can confirm that our structure looks like the following 12345678, a wedge metal on carbon three, a dashed ethyl on carbon four and a dashed methyl on carbon five. OK. And our Newman projection should be viewed along carbons 324. OK. So let me put my eyeball on the left side of the molecule, right? Because we have carbon three first, then carbon four. And when we put the eyeball in this side, technically, if we picture ourselves standing where that eyeball is, anything that is wedged is going to be on our right hand side and anything that is dashed is on our left hand side, right? Also, if something is pointing up or down on our bond line structure, we will stay true to that in the Newman projection. So let me show you what I mean. So let's draw our first new projection. I see that the methyl is wedge and it's pointing down. So that means it's going to be down and to the right, right, we said wedges to the right and then that hydrogen is dashed and down to the left. So we will have our hydrogen down to the left and then pointing straight up, we are going to have carbon four and the rest of that chain, right? So I'm sorry, that would be the back carbon, the front of the, we're on the front carbon, right? The third group is CH two CH three. So carbon two and carbon one, ok. Even get even I get confused sometimes guys. So don't be so hard on yourselves if you don't get this right away. OK. Now, looking at the back carbon, so carbon four through eight, let me draw that wedge hydrogen. So we don't get lost here. So we have that wedge hydrogen which is going to be pointed to the right, right. We said, let me draw it on the right side. So that's less confusing. So that hydrogen is wedged. So it's going to be on our right hand side and it is pointing up our ethel group is dash and pointed up. So that is CH two CH three and pointing down on that back carbon is going to be the rest of our chain, which is CHCH three CH two CH two CH three. OK. And we actually have just drawn one of the Newman projections that we need. Do you think this is the most stable confirmation or the least stable? Well, I noticed that the bulkiest groups on each carbon, the Chch three CH two CH two CH three and on the back carbon and on the front carbon, ch two CH three, they are 180 degrees from one another, right? So they are anti, which means this is the most stable Newman projection confirmation. OK. So now let's draw the least stable one. And we said that is the eclipsed form where all of the groups are on top of one another. So see how the groups are in the windows of one another on the most stable confirmation. Well, now we want to draw our three bonds right in front of each other on the front and back carbon. OK, slightly off, so that we can decipher the writing. So how about we keep the groups on the front carbon the same and then we will rotate the groups on the back carbon. So the front carbon we have in the top position ch two CH three, then moving clockwise, we have CH three on the bottom, right, and hydrogen on the bottom left. Now moving to the back carbon, the bult group is that Chch three ch two ch two ch three. So we wanna put that on the top bond right behind that ethyl group, right? So let's write that up there. And then we want to keep the order of all the groups the same, right when we rotate. So that would put the ethyl on the back carbon right behind the methyl group on the front carbon and the hydrogens are aligned with one another. OK. So this is the eclipsed or least stable Newman projection. So that is the correct answer for this problem. I hope this video was helpful and thank you for watching.

Draw Newman projections along the C3―C4 bond to show the most stable and least stable conformations of 3-ethyl-2,4,4-trimethylheptane.

Key Concepts

Newman Projection

Steric Hindrance

Conformational Analysis

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