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Regioselectivity, stereoselectivity, and stereospecificity

Reviewing the difference between regioselectivity, stereoselectivity, and stereospecificity in elimination reactions.

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  • blobby green style avatar for user Anushka Singh
    What is the difference between a cis alkene and a Z alkene?
    (3 votes)
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  • leaf green style avatar for user Daniel Ng
    At , I can see how the trans product is formed, but can't quite wrap my head around what the mechanism is in order to get the cis product. How is the cis product formed when what I assume is the only beta carbon is to the right of the alpha carbon?
    (7 votes)
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    • piceratops ultimate style avatar for user Bailan
      I think it is about the repulsion.
      In the Beta carbon, there are two hydrogens. Let's place it on the plane, one is above and other below(just like a 2D image)
      If the above Hydrogen is being protonated, double bond will be above( of the two products, in the trans double bond is above). This means, there is a hydrogen below and this can cause possible repulsion with the end carbon making the end carbon to take a geometry pointing upwards.
      In the cis, the hydrogen which is protonated is the one below( Double bond is seen below). This means the remaining hydrogen will be present above. The end carbon needs to occupy a position in space to stay away from the positive hydrogen which is above. So the end carbon accepted a cis geometry in which it points to down, away from un-protonated hydrogen.
      I think it's all about proper distribution of charges. Positive hydrogen and end carbon needs to be properly arranged to avoid the steric hindrance.
      (1 vote)
  • leafers ultimate style avatar for user gaurav.shinde
    For stereospecificity example, it would have helped if you showed the other hydrogen on the bromine carbon. Basically for anyone who didn't understand that one. In the first substrate, the hydrogen was going back and methyl was coming out, so its trans. In the second, both the methyl and hydrogen are going back so the product is cis.
    (4 votes)
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  • primosaur seedling style avatar for user Rhea Braband
    After , you say that we know what the product must be, but I am not sure how you came to that conclusion. How does the stereochemistry of the substrate determine the stereochemistry of the product? Is it based on sterics?
    (3 votes)
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  • marcimus purple style avatar for user Maria Mazuca
    how do you determine which is the major product and the minor product?
    (2 votes)
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    • piceratops ultimate style avatar for user Bailan
      Stable one will be the major product.
      In regioselectivity, the more substituted product was the major product. Because of more the substitution, more the stability.
      In stereoselectivity, trans was the major product. Because trans arrangement kept the bulky groups far away.
      In stereospecificity, E alkene kept the bulky phenol far away and hence more stable.
      You should look at the possible products and find the major product by looking at its arrangement. The arrangement which demands stable arrangement will be the major product.
      (4 votes)
  • blobby green style avatar for user anthonygeorge617
    I think I understand the difference between stereospecificity and stereoselectivity, but please confirm and correct me if I'm wrong. I understand stereospecificity is how the substrate will affect the product. Is that the case because there are 3 alkyl groups (i.e we have a trisubstituted alkene)? Thus, because there is only one possible hydrogen that can be taken by the base, we are guaranteed a specific product based on the substrate. Also, does that mean that if the alpha and beta carbon each have a hydrogen (a unsubstituted alkene), then stereoselectivity will take place because then the conformation that allows for the more stability (trans/E Conformation) will be chosen since there isn't only one hydrogen but rather two hydrogens that can be taken away by the base? I'm trying to determine the factor that causes stereospecificity and stereoselectivity to take place. Thank you!
    (2 votes)
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    • leaf red style avatar for user Richard
      For the stereospecific reaction, the alkene product could either be in the E configuration, or the Z (either one would be a trisubstituted alkene). However, we only observe the E product because the reaction mechanism was E2 (because of the strong base). This is because the E2 mechanism requires the leaving group (the bromine) and the extracted hydrogen to be anti to each other. This means the alkane substrate has to rotate its carbon-carbon single bond to orientate the bromine and the hydrogen in such a way. When it does this, it simultaneously places the phenyl groups anti as well. So when the alkene is formed, the phenyl groups are on opposite sides of the double bond, making it an E alkene. There’s no possibility of the Z alkene forming because of the orientation required from the substrate for the E2 reaction. So the substrate is selecting a specific stereoisomer over another due to its structure.

      The alkane substrate for the stereospecific reaction already has a hydrogen on both the α and β carbons. The reason we’re extracting the hydrogen we do in the video is that the leaving group, the bromine, is on the other carbon. There isn’t a good leaving group elsewhere in the molecule to yield a product with the double bond anywhere else.

      Hope that helps.
      (4 votes)
  • blobby green style avatar for user harshita.kumari.1102
    R there any specific types where the selectivity and will tend to or does it depend on the reaction
    (2 votes)
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  • aqualine tree style avatar for user originaldebu
    At , in the video he says they are regio-isomsers. The reason given by him was the different placement of the double bond. But then how is this different from position isomers?
    (2 votes)
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Video transcript

- [Instructor] Sometimes, definitions can be confusing, and I wanted to go through the difference between the terms regioselectivity, stereoselectivity, and stereospecificity. And I'm going to use examples that we've talked about in earlier videos, so if you want to know the details of these reactions, go back and watch those earlier videos. We'll start with regioselectivity. So the reaction I've shown here is a regioselective reaction. This alcohol gets dehydrated to form two products, the alkene on the left and the alkene on the right. These two alkenes are regioisomers. So let me write this down here. So they are regioisomers. They're isomers of each other, but they differ in terms of the region or the location of the double bond. The isomer on the left has the double bond here, and the isomer on the right has the double bond here. This is a regioselective reaction. One regioisomer is favored over the other, and in this case, the tri-substituted alkene, the one on the left here, is the major product, whereas the di-substituted alkene, the one on the right, is the minor product. So that's regioselectivity. Let's compare that to stereoselectivity. So for this next example, this alcohol, it's another dehydration reaction, reacts with sulfuric acid to give us two alkenes. The mechanism for this reaction, first we protonated the OH to form water as a good leaving group so water left and that gave us a carbocation with a plus one formal charge on this carbon, this benzylic carbon. So we have a benzylic carbocation in our mechanism. Let me just go ahead and draw that in here. So here's our benzylic carbocation, so plus one formal charge on this carbon in magenta. And from this carbocation, we had a choice of which proton we wanted to take. There are two hydrogens on this carbon, and depending on which proton we took, we got one of these different products. So this would be the trans product, just let me write that down here, this is the trans product, and this would be the cis product. These are stereoisomers, so let me write that down. These two are stereoisomers. That's one term that you could use to describe them. And this reaction is stereoselective. One stereoisomer is favored over the other. In this case, the trans products, right, this is the most stable product, so this is the major product, this is favored over the cis. So, this would be the minor product. Now, let's look at stereospecificity. In a stereospecific reaction, the stereochemistry of the substrate determines the stereochemistry of the product. And the E2 reaction can be a good example of a stereospecific reaction. On the left, we have our substrate, and we have these two phenyl groups here. We have a bromine, but notice the stereochemistry at this carbon. You have a methyl group coming out at us in space and a hydrogen going away from us. When our strong base takes our beta proton, we end up with the E alkene, so there's stereochemistry in our product. We would have the two phenyl groups on opposite sides of the double bond. Look at this reaction now. We have the phenyl groups, we have our bromine. Those are all the same. The difference is the stereochemistry at this carbon. Now, we have a hydrogen coming out at us and a methyl group going away from us. Our strong base takes our proton, our beta proton in the mechanism, but this time we get the Z alkene. So the stereochemistry of the substrate determined the stereochemistry of the product. There's no choice because of the mechanism. You could also think about that going backwards. If you look at this product here, the Z alkene, because you know the product is a Z alkene, you know the stereochemistry at this carbon, it must have this particular stereochemistry. Same thing for the other reaction. We form only the E alkene, and because we form only the E alkene, we know the stereochemistry at this carbon. So the stereochemical information is kept in a stereospecific reaction. Finally, let's directly compare stereoselectivity with stereospecificity. We just said in a stereospecific reaction, the stereochemistry of the substrate determines the stereochemistry of the product, so the stereochemical information is preserved because of the mechanism. That's not the case in this stereoselective reaction. If we look at the stereochemistry of this OH here, at this carbon, the stereochemistry is not preserved in our products. This stereochemical information is lost when we formed our carbocation. For example, we could have started out with, let me go ahead and draw this in here, we could have started out with the OH on a dash and we would have ended up with the same products. So the stereochemical information in the substrate is not preserved. Let's think about that concept going backwards again. So for this E alkene, because this is an E alkene with the phenyl groups on opposite sides, we know the stereochemistry at this carbon in this stereospecific reaction. But if we look at the products here, if we look at this trans product and the cis product, that does not tell us the stereochemistry of our substrate. We do not know, we do not know what the stereochemistry is at this carbon. Is the OH on a dash or is the OH on a wedge? So this is not a stereospecific reaction. Instead, think about a stereoselective reaction as being selective for, in this case, the trans isomer, the more stable isomer.