Adding H and X, Markovnikov

We will now explore the details of adding a hydrogen halide (HX) across a double bond. Here are two examples  

In order to understand the regiochemistry and stereochemistry of HX addition, we must analyze the mechanism. When adding HX across a double bond, there are two key steps involved in the mechanism  

In this step, a proton is transferred to the alkene, which generates a carbo-cation. This carbocation is then attacked by the halide in step 2  

The overall result is the addition of H and X across the double bond. We have specifically used a starting alkene that avoids issues of regiochemistry or stereochemistry we will soon see other examples in which we must explore both of those issues. For now, focus on the curved arrows used in both steps. It is absolutely critical to master the art of drawing curved arrows properly. Let s quickly practice  

Answer In the first step, there are two curved arrows  

PROBLEMS Predict the products for each of the following reactions. In each example, make sure to determine whether or not yon are forming two stereocenters. If not, then the stereochemistry will be irrelevant. 

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Markovnikov s rule was empirical that is, it was based on observation only. At the time it was proposed, the concept of organic reaction mechanisms had not yet been imagined so it was impossible to provide a theoretical basis for why the H added to one carbon and the X to the other. Now that we know the mechanism for the reaction, it is easy to understand why these reactions are regiospecific. In fact, if we write the mechanism for the additions to give both possible products, we can predict the preferred product simply on the basis of what we already know about carbocations. It is much better to make predictions based on the mechanism of the reaction because cases that are exceptions to the empirical rule will be readily apparent. 

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