Determine compounds A and B ...

Question

Determine compounds A and B from the following reaction sequence.

Reaction sequence for Eglinton Reaction showing compounds A and B with reagents.

Accepted Answer

So the practice problem states determine compounds a and b from the following reaction sequence. We're starting out first with what's called a Vicinial Dihalide. Remember, in a vicinial dihalid we have neighboring carbons each possessing a halogen. They are in the same vicinity of one another. What we're doing here is we're reacting our vicinol dihalide with excess strong base, so sodium amide, followed by water. Remember this is an early organic chemistry reaction. Anytime we're reacting a vicinol dihalide with excess base in particular the amide ion that means we're trying to form a triple bond. So we're trying to make an alkyne in the first step. So what happens here, if we wanna see what happens we have our vicinial dihalide. Now, chlorine is a leaving group. So either carbon could represent our alpha carbon. Remember, the alpha carbon is the carbon possessing the leaving group. If one carbon is the alpha, the other one would be the beta by default. I just choose this to be alpha. But the other carbon could have been alpha as well. So since that's Alpha, I'll make this one Beta. Our Beta Carbon has Hydrogens. So what happens here is we have 1 mole of Sodium amide. So this amide comes in, removes a beta hydrogen, the bond breaks, falls here to make a double bond, and kicks out the cl. So at this point we're gonna have this structure, but this is not a, or it's not done yet. Alright and we still have a hydrogen here and we technically had a hydrogen here the whole time as well. Now a second mole of sodium amide comes in. Now, only this carbon here has a halogen so now it has to be the alpha carbon and then this has to be beta. So now, this comes in, removes this H here, falls here to make a triple bond and kicks this out. So compound A is this terminal alkyne. Now we use water as a follow-up because we're using excess base here. That excess base is supposed to, in a sense, wash off any excess base that's still lingering lingering around because remember terminal alkynes are acidic so if we have any base remaining it would deprotonate that terminal alkyne and remove this hydrogen. So you can think of water as, just helping to make sure that we have a terminal alkyne at the end. Alright. So that's our compound a. And now we know from what we've seen previously that this here is important for the eglinton coupling reaction where we have 2 identical terminal alkynes reacting together. Remember, 2 identical terminal alkynes. That would mean that this reaction that we've seen up to this point happened to 2 of these. Okay. So there's 2 of these that will now react together in the eglinton coupling reaction to give us compound b. So, here goes what we saw we created. And then I'll just do an exact copy of it. Okay. So remember, this happened twice because we need 2 identical terminal alkynes. And then, for simplicity sake, we know that these hydrogens will be lost. And then the terminal alkyne ends will connect together to give us So they're connected together. Oh. That looks horrible. Let's just clean that up guys. So we have that. So that would be our final answer as compound b. Now, we know that there's much more involved and then than just these hydrogens being lost because we've gone over the mechanism together. But just for saving time and not having go through the whole mechanism, we know that what's gonna happen is those 2 hydrogens are lost and then the 2 end carbons are gonna connect together and we get our our final compound b. So hopefully you guys are able to follow along. This question was great because it brings back things that you might have learned a long time ago in terms of how do you change a vicinol dihalide into a terminal alkyne and that's by using excess base. So from there, the rest of it is just coupling reaction where we're just combining the 2 terminal alkynes together to give us compound B.

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Determine compounds A and B from the following reaction sequence.

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Eglinton Reaction practice set