In other words, the reaction is highly selective for one stereoisomer over the other. Here, notice that we observe only the “anti” product and none of the “syn” product. When we treat an alkene with a halogen such as Br 2, (often in a halogenated solvent such as CH 2Cl 2 or CCl 4) we obtain the following product using 1,2-dimethylcyclohexene.Īgain, pay attention to the dashes and wedges. Addition Of Bromine (Br 2) To Alkenes Is Stereoselective, Giving “Anti” Addition Stereochemistry Any mechanism we propose for this reaction will have to be able to explain why we end up with a mixture of these two products. They exist in roughly equal amounts in this example, but the point is that the mechanism does not selectively deliver either the syn or the anti product. So a feature of this reaction is that it produces a mixture of syn and anti products. Our term for this relationship is “ syn“. Our term for this relationship is “ anti” *. In the product on the right, the Br and H are on the same side of the ring (and therefore have added to the same face of the alkene). In other words, H and Br added to opposite faces of the starting alkene. In the product on the left, the Br and H are on opposite sides of the ring. The “connectivity” of each molecule is the same, but they differ in their orientation in space! Notice how in the left hand product, the H and Br are on opposite sides of the ring, whereas in the right hand product, they are on the same side? These two compounds are not the same – they are “ stereoisomers“. “Later” is now! Examine the placement of the H and the Br that are added. In the previous post I said to ignore all the dashes and wedges, because we’d deal with them later. Addition Of H-Br To Alkenes Is Not Stereoselective, And Gives A Roughly Equal Mixture Of “Syn” And “Anti” Productsįirst example: let’s take a cyclic molecule like 1,2-dimethylcyclohexene and treat it with hydrobromic acid (HBr). Later on, we will use this evidence to make hypotheses about how these reactions work. Without prior knowledge, it isn’t possible to predict from first principles how they proceed – those who discovered these reactions in the late 1800’s and early 1900’s didn’t know what we know now. Remember, these are results from experiment. (The carbocation pathway, the “3-membered ring” pathway, and the “concerted” pathway) However, later we’ll see that each of these reactions is characteristic of a particular “family” of reactivity for addition reactions. Those three reactions we’ll look at today are addition of HBr, bromination with Br 2, and hydrogenation with Pd-C and H 2. A Collection Of Observations That Nobody Predicted Ahead Of Time Until They Did The Experiment Summary: Stereoselectivity In Alkene Addition Reactionsġ.Hydrogenation Of Alkenes With Pd-C And H 2 Is Selective For Addition Stereochemistry.Addition Of Bromine To Alkenes Is Stereoseletvie, Giving “Anti” Addition Stereochemistry.Addition Of H-Br To Alkenes Is Not Stereoselective, And Gives A Roughly Equal Mixture Of “Syn” And “Anti” Products.A Collection of Observations On Reaction Alkene Addition Stereochemistry That Nobody Predicted Ahead Of Time.We’re going to look at three key reactions of alkenes and see how they each demonstrate a different pattern of stereochemistry in addition reactions. This post is about the second key theme in addition reactions of alkenes: stereochemistry. In the last post on alkene addition reactions, we discussed one of the two key themes to look for in addition reactions: regiochemistry (in other words – what is the favored direction in which the pi-bond breaks). Stereoselectivity In Alkene Addition Reactions: “Syn” vs “Anti”
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