Markovnikov And His Anti-Brother

By now, you are probably settling into your organic chemistry class.

And you have most likely met Markovnikov…and his brother anti-Markovnikov. It can sometimes be hard to tell them apart.

       Well – that is the whole point of this post. To show you that there is a simple way to see the difference between the two.

Here…let’s look at the following reactions.

Now, these reactions are adding water across the alkene. The fancy name for this reaction is hydration of an alkene.However, the water is not adding in the same way in both reactions – the top reaction adds the OH to the more substituted side and the bottom reaction adds the OH to the less substituted.

Well, the alkene is the same in both cases…the +/- breakdown is the same. That doesn’t change.

Well…then…what does?

It’s all in the hydrogen!

The placement of the hydrogen in the structural formula is changing. Now – how does this make a difference?

If you find the hydrogen hanging out in the front of your formula, it will be positively charged. If it wants to hang out in the back of the formula, it will be negatively charged.

Lets’ check out how this looks in the mechanism

In the first reaction, we can see the double bond attacks the positively charged hydrogen. In the second reaction, the double bond attacks our boron (positive) and the hydrogen (negative) attacks the partially positive side of the alkene.

It is here we can easily see the difference – the hydrogen is getting attacked (electrophile) in one case and attacking (nucleophile) in the other.

Leave a Reply

Your email address will not be published. Required fields are marked *