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 “region” of the alkene do different atoms end up forming bonds to?). This post is about the second key theme in addition reactions of alkenes: stereochemistry.
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.
Those three reactions we’ll look at today are addition of HBr, bromination with Br2, and hydrogenation with Pd-C and H2. However, later we’ll see that each of these reactions is characteristic of a particular “family” of reactivity for addition reactions.
Remember, these are results from experiment. They are observations. 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. Later on, we will use this evidence to make hypotheses about how these reactions work. Let’s have a look.
First example: let’s take a cyclic molecule like 1,2-dimethylcyclohexene and treat it with hydrobromic acid (HBr). Here’s what we get.
In the previous post I said to ignore all the dashes and wedges, because we’d deal with them later. “Later” is now! Examine the placement of the H and the Br that are added. 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”. The “connectivity” of each molecule is the same, but they differ in their orientation in space!
In the product on the left, the Br and H are on opposite sides of the ring. In other words, H and Br added to opposite faces of the starting alkene. 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). Our term for this relationship is “syn“.
So a feature of this reaction is that it produces a mixture of syn and anti products. They exist in roughly equal amounts in this example, but the point is that the mechanism does not specifically deliver either the syn or the anti product. 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.
“Anti” Addition Stereochemistry
Let’s look at a different reaction next. When we treat an alkene with a halogen such as Br2, (often in a halogenated solvent such as CH2Cl2 or CCl4) we obtain the following product using 1,2-dimethylcyclohexene.
Again, pay attention to the dashes and wedges. Here, notice that we observe only the “anti” product and none of the “syn” product. In other words, the reaction is highly selective for one stereoisomer over the other. We could go even further and say that because of the complete absence of the “syn” product, the reaction is stereospecific. Only one type of stereoisomer is formed.
We’ll see that this pattern is observed for other reactants similar to Br2. Again, any mechanism we propose will have to account for the fact that we only get the “anti” product and non of the “syn”.
“Syn” Addition Stereochemistry
Finally, let’s look at a third category of addition reaction. When 1,2-dimethylcyclohexane is treated with hydrogen gas and palladium catalyst (Pd-C), the result is as follows:
Notice how the only product of this reaction is the one where two hydrogens have added to the same face of the alkene (“syn” stereoselectivity). The product where hydrogens add to opposite faces is not observed. Again, this is an example of a stereospecific reaction. The mechanism will once again have to explain why we only obtain the syn product of this reaction and none of the anti product.
The key point from this post is to pay close attention to the stereochemistry of addition reactions. Different reagents can lead to very different stereochemical results – a point which is often tested for by organic chemistry instructors. Stereochemistry really is the key theme of Org 1!
In the next post we’ll start looking at some of these reactions in more detail.
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